Exp 6 help - CEM 221/222 Organic Chemistry Laboratory...

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: CEM 221/222 Organic Chemistry Laboratory Manual 2009-2010 Academic Year Written/edited by Daniel Berger Table of Contents REQUIRED MATERIALS FOR CEM 221/222 LABORATORY......................................................................... 4 EXPECTATIONS FOR CEM 221/222 LABORATORY ........................................................................................ 4 PREPARATION AND ATTENDANCE.............................................................................................................................. 4 Preparing for the laboratory................................................................................................................................ 5 LABORATORY NOTEBOOKS........................................................................................................................................ 6 REPORTS ................................................................................................................................................................... 6 Stylistic considerations for lab reports ................................................................................................................ 8 Sample experimental sections from the chemical literature................................................................................. 9 Treatment of error.............................................................................................................................................. 10 Evaluation of reports ......................................................................................................................................... 10 Submission of reports ......................................................................................................................................... 11 Lab report grading rubric for CEM 221/222 ..................................................................................................... 12 MOLECULAR STRUCTURE AND YOUR MOLECULAR MODEL KIT ....................................................... 14 STRUCTURE AND NOMENCLATURE ......................................................................................................................... 14 Using the Molecular Visions model kit .............................................................................................................. 14 Structure and nomenclature ............................................................................................................................... 16 Conformation and Newman projections ............................................................................................................ 19 CHIRALITY .............................................................................................................................................................. 22 Chiral centers..................................................................................................................................................... 22 Fischer projections ............................................................................................................................................ 24 Rules for manipulating Fischer projections ....................................................................................................... 26 INTRODUCTION TO MOLECULAR MODELING USING ARGUSLAB ............................................................................. 27 CALCULATIONS FOR ORGANIC SYNTHESIS ............................................................................................... 31 Chemical amount and mass ............................................................................................................................... 31 Chemical amount and volume ............................................................................................................................ 31 Theoretical yield ................................................................................................................................................ 32 Relative yield ...................................................................................................................................................... 33 LABORATORY PROCEDURES ........................................................................................................................... 34 SAFETY RULES ........................................................................................................................................................ 34 WASTE DISPOSAL .................................................................................................................................................... 35 SOME DEFINITIONS RELATED TO LABORATORY SAFETY .......................................................................................... 36 REFERENCE SOURCES ............................................................................................................................................. 37 FIRE EXTINGUISHERS AND INFLAMMABLE MATERIALS ............................................................................................ 38 Requirements for a fire ...................................................................................................................................... 38 Types of fires ...................................................................................................................................................... 38 Types of fire extinguishers ................................................................................................................................. 39 TECH 700: PRACTICING SAFETY IN THE ORGANIC CHEMISTRY LABORATORY ...................................................... 41 CEM 221 LABORATORY FINAL EXAMINATION ....................................................................................................... 43 EXPERIMENTAL TECHNIQUES ........................................................................................................................ 44 TECH 701: MEASURING THE MELTING POINTS OF COMPOUNDS AND MIXTURES................................................... 45 BIOSYNTHESIS OF ETHANOL FROM MOLASSES ......................................................................................................... 47 Pre-laboratory worksheet for Biosynthesis of ethanol ....................................................................................... 51 ISOLATION OF CAFFEINE FROM COFFEE OR TEA ....................................................................................................... 53 Pre-laboratory worksheet for Isolation of caffeine from tea.............................................................................. 57 RESOLUTION OF A RACEMIC MIXTURE: Α-METHYLBENZYLAMINE .......................................................................... 59 Finding optical purity and mole fraction of enantiomers ................................................................................... 62 Schedule for “Resolution of a Racemic Mixture” .............................................................................................. 63 Pre-laboratory worksheet for Resolution of a racemic mixture, week 1............................................................. 65 Pre-laboratory worksheet for Resolution of a racemic mixture, week 2............................................................. 67 THIN LAYER CHROMATOGRAPHY ............................................................................................................................ 69 Acetanilide, Phenacetin and Caffeine: a study using ArgusLab ......................................................................... 72 Pre-laboratory worksheet for Thin Layer Chromatography .............................................................................. 75 ANALYSIS OF FATTY ACID COMPOSITION OF LIPIDS .................................................................................................. 77 Fatty acid composition of some common oils ..................................................................................................... 80 Pre-laboratory worksheet for Analysis of fatty acid composition of lipids ......................................................... 81 INTRODUCTION TO TWO-DIMENSIONAL NMR SPECTROSCOPY ................................................................................. 83 Using 2-dimensional NMR to solve structures ................................................................................................... 83 VIRTUAL SPECTROSCOPY......................................................................................................................................... 87 QUALITATIVE ANALYSIS: IDENTIFICATION OF UNKNOWN ORGANIC COMPOUNDS .................................................. 88 IDENTIFICATION OF AN UNKNOWN ESTER................................................................................................................. 90 Pre-laboratory assignment ................................................................................................................................. 91 REACTION STUDIES ............................................................................................................................................. 92 FACTORS AFFECTING THE REACTIONS OF ALKYL HALIDES ....................................................................................... 93 Pre-laboratory assignment ................................................................................................................................. 95 1 H NMR ANALYSIS OF KETO-ENOL TAUTOMERISM .................................................................................................. 96 Pre-laboratory assignment ................................................................................................................................. 97 KINETICS OF THE ESTERIFICATION OF TRIFLUOROACETIC ACID .............................................................................. 98 Pre-laboratory assignment ................................................................................................................................. 99 SYNTHETIC EXPERIMENTS ............................................................................................................................. 100 DEHYDRATION OF AN ALCOHOL............................................................................................................................. 101 Pre-laboratory worksheet for Dehydration of an alcohol ................................................................................ 104 REDUCTION OF A KETONE ...................................................................................................................................... 106 Pre-laboratory worksheet for Reduction of a chiral ketone ............................................................................. 109 SAPONIFICATION: BIODIESEL SYNTHESIS AND SOAP-MAKING ............................................................................... 111 Pre-laboratory worksheet for Saponification ................................................................................................... 115 MULTISTEP SYNTHESIS OF TRIPHENYLMETHANOL ................................................................................................ 117 Pre-laboratory assignment for week 1: Esterification ...................................................................................... 123 Pre-laboratory assignment for week 2: Grignard reaction .............................................................................. 125 SYNTHESIS OF AN ETHER USING PHASE-TRANSFER CATALYSIS ............................................................................ 127 Pre-laboratory assignment ............................................................................................................................... 129 ALCOHOL SYNTHESIS BY HYDROBORATION/OXIDATION....................................................................................... 130 Pre-laboratory assignment ............................................................................................................................... 132 THE DIELS-ALDER REACTION................................................................................................................................ 133 Pre-laboratory assignment ............................................................................................................................... 134 DIELS-ALDER REACTION, ALTERNATE VERSION..................................................................................................... 135 Pre-laboratory assignment ............................................................................................................................... 136 WITTIG SYNTHESIS OF 1,4-DIPHENYLBUTADIENE................................................................................................... 137 Pre-laboratory assignment ............................................................................................................................... 138 OXIDATION OF A BIFUNCTIONAL ALCOHOL ............................................................................................................ 139 Pre-laboratory assignment ............................................................................................................................... 141 THE ALDOL CONDENSATION ................................................................................................................................. 142 Pre-laboratory assignment ............................................................................................................................... 143 THE PERKIN REACTION .......................................................................................................................................... 144 Pre-laboratory assignment ............................................................................................................................... 145 A SYNTHESIS USING MELDRUM’S ACID ................................................................................................................ 146 Pre-laboratory assignment ............................................................................................................................... 147 Required materials for CEM 221/222 laboratory You are required to have the following materials for CEM 221/222 laboratory: • This laboratory manual, which contains the experiments we will perform this year. • Zubrick’s The Organic Chem Lab Survival Manual, which contains invaluable descriptions of glassware and procedures. • A permanently-bound laboratory notebook with permanent page numbering. The bookstore carries the Hayden-McNeil notebook which is strongly suggested for this course. You must turn in copies of your lab notebook pages with your lab reports, and the H-McN notebook makes duplicates as you write. • Two lab modules from Chemical Education Resources: o o • TECH 700 (“Safety in the organic chemistry laboratory”). TECH 701 (“Measuring the melting points of compounds and mixtures”). A molecular model kit. I strongly suggest the Molecular Visions kit sold by the bookstore as it combines cheapness and versatility. You will be expected to identify models constructed with the Molecular Visions kit, and to build a molecule that cannot be built with any other single kit I know of, because of its large number of carbon atoms. Expectations for CEM 221/222 laboratory Preparation and attendance We will meet for laboratory almost every week of the academic year. You are expected to be present for every lab period unless the professor has excused you; however, an excused absence does not excuse you from completing the required work. Time will be available for students who need to make up laboratory work; see the laboratory schedule for the current semester, handed out with the course syllabus and available on the course website. Students must turn in a pre-laboratory assignment before being admitted to the laboratory, for each week except the first week of each semester and the fall laboratory final exam. Time spent doing the prelab during lab time will not be made up, even if everyone else is allowed to run overtime. Pre-laboratory assignments are found at the end of each lab procedure. Failure to complete all laboratory work, including an acceptable lab report for each and every experiment, will lower your final grade for the semester. 1 If one lab exercise is incomplete, your semester grade will be lowered “half-a-grade,” that is, from B- to C+ or C to C-. If two lab exercises are incomplete, your final grade will be lowered by a full letter grade, for example from a B- to a C-. If three or more labs are incomplete, you will fail the course. SOME EXPERIMENTS MAY REQUIRE MORE THAN ONE WEEK TO COMPLETE; failure to complete a multiweek experiment may result in the penalty for missing the same number of one-week experiments. For example, not completing a two-week experiment could result in your grade being lowered a full letter grade, rather than the half-a-grade penalty for a singleweek experiment. 1 4 Emergencies will require some other arrangement, but you must make those arrangements in cooperation with the professor. The goals of any laboratory course are to help you understand material from the lecture and, as much as possible, to introduce you to what goes on in real chemistry laboratories. The goals of this particular laboratory course are to introduce you to basic techniques used in organic chemistry. To get maximum benefit from the laboratory, you need to be free to learn by doing and not hobbled by constant, slavish reference to an unfamiliar text. Therefore, you are expected to familiarize yourself with the procedure for each experiment before coming to the lab. To encourage this, you have an excellent laboratory textbook, and lab procedures will lack some details. You are expected to interact with the professor and the lab textbook if you have questions on how to perform a procedure. This is a three-hour laboratory, and the experiments–usually but not always excluding instrument time– should take no longer than three hours. While time extensions may be granted at the professor’s discretion, you may be cut off at the end of three hours. If your work is not finished, you will receive a zero for the experiment. This is a standard procedure at most universities and colleges. Some of the experiments will involve the use of instruments to analyze your product. You should take the time to familiarize yourself with the principles behind the instrument and the analysis to be performed. If your text does not provide adequate preparation in this, you will be pointed toward good reference books in the Science Department lobby. The professor will provide training in the use of the instrument. Often, you will be asked to perform instrumental measurements outside your regular laboratory time. The protocol for doing this will be explained to you. Laboratory reports or other paperwork are required as part of each experiment. You will be penalized 10% of your report score for each violation of safe laboratory procedure 1 AND for each experiment in which you fail to clean up after yourself. Violations that are judged to be sufficiently serious will result in further penalties up to and including being asked to leave and receiving a zero for the experiment. Laboratory conduct will under most circumstances not be taken into account except for adjusting borderline grades; however, egregious violations will be penalized as noted above, or otherwise as the professor deems appropriate. Preparing for the laboratory 1. Be sure you are familiar with the hazards to be expected for each experiment. CEM 221 students must complete the prelab, which is based on lab safety, to be admitted to the laboratory. 2. Use The Organic Chem Lab Survival Manual. It discusses just about everything you need to know about doing organic chemistry lab. 3. Use the resources in the Shoker Science Center lobby. We don’t keep textbooks and laboratory manuals there for show. We intend that you use them to prepare for the laboratory, so that you have a better idea of what to expect. See also the section on reference materials, below. 4. An introduction to the experiment should be written in your laboratory notebook before coming to the laboratory. Procedural steps are to be written as or after they are performed (see the next section). 1 For example, since mercury is a hazardous substance, breaking a mercury thermometer is a lab safety violation, which will be penalized by deduction of 10% of the grade for that experiment. Failure to follow handling and disposal guidelines given in the lab procedures are also lab safety violations. 5 Laboratory notebooks Laboratory notebooks must be permanently bound and have permanently and consecutively numbered pages. An appropriate notebook is one of the required course texts. You will hand in copies of your lab notebook pages and of any instrument readouts (not the originals!) with each lab report. Notebooks will also be inspected at random intervals, during lab time, to ensure that you are keeping your notebook properly and writing in it as you work. For guidance in writing your laboratory notebook, you are expected to follow Chapter 2 of Zubrick. The touchstone for a good laboratory notebook is this: someone who has never performed the experiment should be able to reproduce your procedure and understand both your results and the purposes of the experiment. But here are a few rules, based on standard practice: • It is strongly recommended that you diagram your experimental equipment in your notebook, as shown in Zubrick Chapter 2. This will help you remember how to set up and run the experiment. • Laboratory notebooks are kept in ink, not in pencil. • Strikeouts are performed by drawing a single line through the material to be deleted, like this. This allows an auditor to know that you haven’t been cheating. • All lab notebook entries must be dated. If you make several entries on the same day (as you will during lab), you need not date every separate entry. But each page must be dated, and if you make entries on the same page on two different days, the page must show the date of each entry. • All lab notebook entries must be signed or initialed by you. Normally one signature per day is sufficient, but if you have more than one date on a page, each day’s entry should be initialed. • Manipulations and observations are written in your notebook as you work. In real-world research labs, notebooks are updated every time there is a break in the procedure being performed. Develop the habit of writing everything in your notebook as soon as you do it; for example, take your notebook with you to weigh something. If you are caught writing data (for example, the weight of something) on a separate sheet of paper, it will be taken away from you and destroyed. • All spectra and other printouts of instrument data must be fastened into your notebook. It should be made clear to the reader where to find the printouts, if they are not on the pages immediately following the experiment. • A conclusion should be written as soon as all data are in. Reports Reports will be entirely typewritten or word-processed 1 and patterned on the format for a paper submitted for publication in the chemical literature; see the ACS Style Guide, on reserve in Musselman Library. 2 1 Pictures and figures may be neatly hand-drawn, but for a neater look see the note on available free drawing software, below. 2 Please note that the style for the chemical literature is somewhat different than for the physics literature, so that the report format expected for organic chemistry is different than that for physics. 6 Except for the abstract, the report will be double-spaced and will have numbered pages. You may use ALL CAPS, italics or boldface type for titles; it is strongly suggested that you take advantage of the “Heading” styles in Microsoft Word. Reports that do not use the appropriate format will be returned to you for rewriting. Pay attention to the instructions in your lab procedure and given to you orally; some reports will not require all of the sections specified below. Each full report will have • A cover sheet, with the title of the report, the author’s name, the due date, the class and professor, and a single-spaced, left-justified Abstract 1 of the contents of the report. See the course website for a link to “How to write an abstract.” • • A body–often, but not always, in two parts: Results and Discussion–in which the work performed is discussed and the results evaluated. This does not consist in simply rehashing the contents of the Experimental section, unless such is relevant to a discussion of the principles guiding the work, or of the results. Do not forget to discuss topics requested in the lab procedure. • A Conclusion, in which the results and their interpretation are summarized. • An Experimental section, in which details of the experimental or other procedure are given. This is normally placed either after the conclusion or after the introduction. • A list of References, placed as endnotes and executed in ACS format. See the ACS Style Guide, which is on reserve in the library. • Copies of the relevant pages from your lab notebook. These are NOT part of the report, but a check on how well you are keeping your notebook. Do not refer to anything in the lab notebook pages in the report unless that item or information is reproduced in the report itself (for example, do not refer to “Figure 1” if “Figure 1” exists only in your notebook pages). • 1 An Introduction, in which the problem is presented and relevant background information is given. Copies–not originals, which belong in your notebook–of any spectra or other instrument output obtained during the experiment. These must be labeled as Figures and referred to by The abstract must contain: • The purpose of the experiment (“Anhydrous thanotalamine was prepared”) • The results of the experiment (“the unknown was identified as bat guano” “a 0.1% yield”) • Anything else relevant. The purpose of the abstract is to whet the reader’s appetite (so s/he will read the rest of your work) and, as a last resort, to give the gist to the reader who simply doesn’t think s/he has time to read the whole thing. 7 number in the body of the report. 1 You may also include structural drawings or chemical equations as Figures or Schemes. All figures and schemes should be numbered in the order in which they are cited in the text. Figures and schemes are numbered separately (e.g. Figure 1, Scheme 1–not Figure 1, Scheme 2). Chemical structures and other drawings • ISIS/Draw, ACD ChemSketch and KnowItAll are standard chemical structure drawing programs, available to students for free download (to find them, do a Google search). ChemSketch is installed on the Science Department computers. If you have problems finding the programs, ask the professor. These programs will help you generate more professionallooking structure drawings but do have a bit of a learning curve! Because of abuses in the past, you are not permitted to copy pictures into your report even if full credit is given. Doing so will result in your report being returned to you ungraded, and late-report penalties will be assessed. All chemical structural drawings and reaction mechanisms must be either neatly hand-drawn or generated using a chemistry drawing program such as the three programs listed above. (You may, of course, use photographs if credit is given. But I can’t see why you would want to.) This is to give you practice; you will have to generate such structures on examinations!! Stylistic considerations for lab reports Reports are expected to be concise. You are not expected to have a certain number of pages, but your discussion must be complete. If you examine the chemical literature, you will find that all articles are written in the third person, passive voice. Rather than “I determined the pH” or even “the experimenter determined the pH,” the correct form is “the pH was determined,” or better yet, “the pH was [for example] acidic.” Copies of American Chemical Society journals such as the Journal of the American Chemical Society or the Journal of Organic Chemistry are available online via OhioLink’s Electronic Journal Center, so that you may look at the format and style used for chemical papers. In the chemical literature, the Experimental section is normally found either after the Introduction or after the Conclusion. Very long Experimental sections are almost always placed after the Conclusion. “COOKBOOKING” IS NOT PERMITTED. “Cookbooking” means a blow-by-blow account of what you did in the laboratory. An example of cookbooking: I measured 5.0 mL of concentrated HCl and weighed out 1.0 g of NaOH. I dissolved the NaOH in 100 mL of water in an Erlenmeyer flask. I added the HCl and mixed. I determined the pH using litmus paper. The way it should be written: 5.0 mL of concentrated HCl were mixed in a 125-mL Erlenmeyer flask with a solution of 1.0 g NaOH in 100 mL of water. The resulting solution was acidic to litmus paper. Most common procedures can be disposed of by one or two phrases. For example, “the solid product was recrystallized from 10% v/v ethanol/water, with a yield of 5.1 g (80%).” “A solution of 10 g 1-naphthol in 50 mL benzene was added, dropwise, over 30 minutes and the resulting mixture was refluxed for one 1 8 You must discuss spectral features that pertain to the identification of your compound(s)! hour.” “Solvent was removed by evaporation and the product (b.p. 80-85º/100 torr) was purified by distillation under reduced pressure.” In Experimental sections, shorter is usually better as long as no important information is omitted. Normally, the experimental section also includes a resumé of results: • The absolute and relative yields are usually given. 1 This will be required in this class. • Results of identifying measurements (melting/boiling points, spectroscopy, and other tests) are summarized. This will be required in this class. All information not obtained by you during the course of the experiment will be properly referenced. See the ACS Style Guide, on reserve in the library. Sample experimental sections from the chemical literature Tetraphenylborate Salt of Protonated 1,8-Diazabicyclo[5.4.0]undec-7-ene (TPB-DBUH+). 2 Into a 50mL round-bottomed flask were placed 1.0 mL (6.69 mmol) of DBU and 10 mL of absolute ethanol. An excess of HCl gas was bubbled through the solution. The ethanol was then removed at reduced pressure. To remove residual hydrochloric acid, the residue was dissolved in distilled water and then lyophilized to give a white solid. The chloride salt was dissolved in 10 mL of distilled water and mixed with an aqueous solution of NaBPh4 (2.29 g, 1.0 equiv, in 10 mL of distilled water). The resultant white precipitate was collected and dried in a drying pistol under vacuum over P205 using refluxing ethyl acetate. Mp: 198.5202.0º C dec. 1H NMR (CD3CN, 250 MHz): δ 7.43 (s, N-H, 1H), 7.26 (m, Ph-H, 8H), 7.00 (t, Ph-H, 8H), 6.85 (t, Ph-H, 4H), 3.45 (m, NCH2, 2H), 3.38 (t, NCH2, 2H), 3.19 (m, NCH2, 2H), 2.51 (m, CH2C=N, 2H), 1.91 (m, CH2, 2H), 1.68 (m, CH2, 6H). 13C {1H} NMR (CD3CN, 62.5 MHz): δ 167.0, 164.8 (q), 136.7, 126.6, 122.8, 55.2, 49.3, 39.1, 33.9, 29.4, 26.9, 24.3, 19.8. Anal. Calcd. for C33H37N2B: C, 83.89; H, 7.89; N, 5.93. Found: C, 82.30; H, 7.87; N, 6.03. Potassium Hydrogen 4-Sulfo-3-hydroxybenzoate Monohydrate (15). 3 To a magnetically stirred solution of 14 (20 g, 145 mmol) in concentrated H2SO4 (27 mL) heated to 90º C was added dropwise 30% SO3 in concentrated H2SO4 (3 mL). After 12 h of stirring, a precipitate formed and stirring became difficult. The mixture was heated at 90º C further while mechanically stirred for another 1 h. The reaction was then cooled to room temperature and H2O (100 mL) was added to dissolve the reaction mixture. The mixture was stirred while 25% KOH (32.5 mL) was added dropwise. The resulting precipitate of the title compound was collected by filtration and recrystallized from H2O (29.36 g, 79%): mp >250º C; 1H NMR (DMSO-d6, 400 MHz) δ 10.5 (s, 1H), 7.58 (d, J = 8.2 Hz, 1H), 7.4 (d, J = 6.9 Hz, 1H), 7.3 (s, 1H), 3.85 (s, 1H); 13C NMR (DMSO- d6, 100 MHz) δ 166.98, 153.38, 134.56, 133.36, 127.77, 119.80, 117.56. Anal. Calcd for C7H5O6K·H2O: C, 30.65; H, 2.55; S, 11.68. Found: C, 31.09; H, 2.51; S, 11.9. Reaction of 6-bromohexanoic acid (7) with boron trichloride. 4 To a stirred solution of 6-bromohexanoic acid (1.2043 g, 6.17 mmol) in dry CH2Cl2 (10 mL) at –78°C was added BCl3 (1 M solution in CH2Cl2, 6.50 mL, 6.50 mmol). The clear solution was warmed to 0°C and stirred for 0.5 h. 1 See “Calculations for Organic Synthesis,” below. Please do NOT use the phrases “absolute yield,” “relative yield” or “percent yield.” You have been warned. 2 Doering, W.von E.; Zhao, D. J. Am. Chem. Soc. 1995, 117, 3432-3437. 3 Venkatesan, H.; Davis, M.C.; Altas, Y.; Snyder, J.P.; Liotta, D.C. J. Org. Chem. 2001, 66, 3653-3661. 4 Dyke, C.A.; Bryson, T.A. Tetrahedron Lett. 2001, 42, 3959–3961. 9 The reaction mixture was cooled to –78°C and excess methanol (3.00 mL, 74.06 mmol) was added via syringe. The solution was warmed to rt, then diluted with excess ether (25 mL) and washed subsequently with sat. Na2CO3 (50 mL), and brine (50 mL) then dried with MgSO4. The excess solvents were removed under reduced pressure to give the methyl ester 4a as an oil (97%). IR (neat): 2944, 2862, and 1739 cm−1; 1 H NMR (300 MHz, CDCl3): δ 1.40-1.59 (m, 2H), 1.61-1.69 (m, 2H), 1.81-1.90 (m, 2H), 2.31 (t, 2H, J=8.75 Hz), 3.39 (t, 2H, J=6.97 Hz), 3.65 (s, 3H); 13C NMR (CDCl3): δ 24.3, 27.9, 32.6, 33.7, 34.0, 51.8, 174.1; m/e 210, 177, 74 (base), 59. An example of an experimental section for this course Six tea bags were placed in approximately 150 mL of water, then boiled for 15 minutes. After cooling the solution and squeezing all excess liquid from the tea bags, the solution was extracted three times with 2025 mL portions of dichloromethane. The dichloromethane was evaporated on a steam bath, leaving behind 0.1206 g of light green crude caffeine (m.p. 225-226º C). The crude caffeine was purified by sublimation, yielding 0.0105 g of white feathery crystals (9%), m.p. 234-235º C (lit.1 235-237º C). References 1. Windoltz, M., Budavar, S., Stroumtsos, L.Y., Fertig, M.N., Eds. The Merck Index, 9th Ed.; Merck: Rahway, NJ, 1976. Treatment of error Be sure to distinguish between human error—i.e. knocking over a flask—and experimental error—the uncertainties inherent in any act of measurement, or the failure of a reaction to run. If your reaction fails you must discuss what might have caused it to fail. 1 However, vague speculation is discouraged and will be penalized. If you don’t know, come talk to the professor! Reports will not be docked for honesty in reporting a laboratory goof, except as applicable for safety violations. However, if you report something that you did not actually perform in the lab, and the professor remembers otherwise, your report will be appropriately penalized. Evaluation of reports Reports will be marked for grammar, style, and so forth as well as content. 2 Clarity is NOT a style issue, and writing which is not sufficiently clear will result in the presumption that you don’t know what you are talking about. Reports should be written for an audience similar to that for a scientific paper. The audience for chemical journals is assumed to be sophisticated in chemistry in general–including common experimental techniques such as recrystallization, distillation, obtaining spectra, and so forth–but unfamiliar with the specifics of the work described. You should write your report accordingly: assume that the reader knows how to carry out laboratory procedures in general, but is unfamiliar with the specific experimental sequence and with the theory behind the experiment. 1 You are wasting your time trying to explain “low” yields unless you lost a significant fraction of your product to a spill. Typical student yields for these experiments range from 15% to 100%, and experienced chemists may not get yields over 60% or even 40% on some experiments. 2 Some guidelines are available at http://www.bluffton.edu/~bergerd/classes/writing.html. See also the rubric on the next page. 10 In other words, you don’t need to detail the assembly of a distillation apparatus–although you should draw the apparatus in your notebook!–but you do need to tell me that you distilled something. You don’t need to tell me how long you waited for your product to crystallize, but you do need to tell me that you purified it by recrystallization, and from what solvent. On the other hand, the length a reaction is allowed to run is usually important; see the procedures quoted above. Submission of reports Reports in the above format will be submitted to the black tray outside my office, after completion of each experiment. Reports are due at 5:00 PM on the due date, which is the day of your lab section unless otherwise specified. They will be graded according to the rubric below, and returned to you as soon as possible. Late reports will be penalized 10% per calendar day, up to a maximum of 25% per week or 50% total penalty. 1 This is a strategic decision; you may decide to delay handing in a poor report for a day so you can polish it up. In such a case a late penalty might be a net gain. Unacceptable reports 2 will be returned to you and will be subject to late penalties. Such penalties will begin on the day you are informed that you must rewrite the report. If a report is returned to you for rewriting, the experiment is counted as incomplete until you return an acceptable report to the instructor. Reports that are obviously unacceptable for formatting reasons (e.g. not double-spaced; improper cover sheet) will be returned to you immediately. There is no guarantee that the content will be found acceptable when you return a properly formatted report. 1 For example, a report that is 8 calendar days late will be penalized 35% (25% for a week + 10% for a day). 2 “Unacceptable” means that the report does not fulfill the requirements set out above, in the lab procedures and in the grading rubric. 11 Lab report grading rubric for CEM 221/222 Abstract (5 points) The Abstract should be brief but complete, including a statement of what you were trying to do. All important results should be summarized (for example, relative yield; purity; identification of an unknown.) Introduction and Discussion (50 points) The Introduction must discuss the expectations for the experiment. What were the goals? This is also the place to present a summary of the general theory or principle(s) behind the experiment. Brevity is not essential but will be appreciated by the grader! The Discussion must address each result in enough detail to present the theory (if any) behind the finding and to tell whether the desired results were obtained. Evidence obtained by your own experimental work must be used. Spectra obtained should be analyzed in some way, and reactions involved (if any) should be presented and discussed briefly. CEM 222 students MUST discuss the mechanism of the reaction. What goes in the Introduction and what goes in the Discussion is flexible, and you need not duplicate explanations or background material. Please do not include a “calculations” section. Conclusion (15 points) The Conclusion must summarize the important results and address each point raised in the introduction if relevant to the experimental results. That is, you should not say “the principles of chromatography allowed us to separate the mixture” but if one of the goals was to isolate clove oil, you should say whether you did or did not! Experimental (7 points) The Experimental section will concisely present the operations actually performed in the laboratory; for guidance, see the laboratory manual and consult consult issues of ACS journals such as the Journal of Organic Chemistry via the OhioLink electronic journal center. Normally this section will include all measurements performed (i.e. starting mass, absolute and relative yield, melting or boiling points, spectral data) during the experiment. “Cookbooking” (defined in the laboratory manual) will result in a score of zero for this section. But we walk a fine line; failure to present (concisely) all the important operations will also result in a lower score. Yield and appearance of product (8 points) Yields will be independently calculated by the grader, from the data recorded in your notebook. It will be graded on a 0-5 scale with the largest yield getting 5 points. Failure to properly calculate the yield will result in a grade of zero. If the grader cannot reproduce your yield using data from your notebook pages, you will receive zero points of five for yield. Appearance of your product will be graded on a 0-3 scale. These points will be awarded for correct identification of unknowns in those experiments in which unknowns are to be identified. 12 Style (10 points) The report must be well written in ACS standard style with scoring on this scale: 0 Standard English is trampled on 2 4 6 8 10 Concise, well-written and well-edited Points to consider include not only clarity but spelling, proper use of paragraphs, and so forth. Good writing style will not make up for poor or nonexistent content. For each violation: Information obtained elsewhere than the laboratory is not properly referenced: subtract 3 points. Standard ACS format not followed in references: subtract 2 points. Spectra or reaction schemes are not properly referenced in the text: subtract 2 points. Standard report format not followed: subtract 5 points. This includes a cover sheet with all of the required items. Inclusion of phrases such as “percent yield” or anything similar: subtract 2 points. See “Calculations for Organic Synthesis” in your manual. No report will receive a grade lower than zero. Notebook pages (5 points) 0. Notebook pages are illegible (including “too faint to be read”) or not present; not all experimental measurements and procedures recorded in notebook; something in the notebook is “crossed out” in such a way that it cannot be read; notebook is not kept in ink; notebook pages are not dated; notebook pages are not signed or initialed. 1. Notebook pages pass inspection for items in (0) but are missing introduction, conclusion, or instrument readouts (TLC plates, GC trace, spectrum or spectra obtained); observations show signs of having been rewritten (e.g. “too neat”). 2. Notebook pages pass inspection for items in (0-1) but are not sufficiently clear. 3. Better than (2) but not perfect. All items listed in (0-1) are included. 4. Better than (3) but not perfect. All items listed in (0-1) are included. 5. Notebook pages are easy-to-follow and proper conclusions are drawn. Grader is confident of being able to exactly reproduce what you did in the lab. All items listed in (0-1) are included. Scaling Because not every report will have every item listed above, all reports will be scaled to 100 points. 13 Molecular structure and your molecular model kit Structure and Nomenclature This manual is available at www.bluffton.edu/~bergerd/Models/. For a short tutorial on using online models, go to www.bluffton.edu/~bergerd/classes/jmol.html. Thinking in three dimensions is one of the most important skills in organic chemistry. Most organic molecules (especially biological molecules) function through and because of their particular threedimensional shapes. You must be able to translate flat pictures of molecules into three-dimensional models in your mind in order to do well in either organic or biochemistry. In order to develop this skill, you have been asked to purchase a molecular model kit. This series of exercises, aided by online models, will help you learn to use your model kit. Using the Molecular Visions model kit The Molecular Visions model kit was chosen for its combination of low price and ability to represent a great many molecules; many professional chemists prefer it as a research model kit. However, like the line-drawing method of representing molecular structures, it is highly stylized. Three resources will help you learn to use your model kit: the kit manual, which has many color pictures; demonstration by the instructor; and online molecular models, which will allow you to compare the model you build to a more conventional representation. Demonstrations and online models will be provided throughout the course; reading the manual is, of course, up to you! Some of the laboratory periods will be devoted to learning to use your model kit, and you may be required to identify molecular models during examinations. Your kit contains several different types of pieces, but the ones that concern us at present are those which allow us to represent organic molecules. 1 These are the tetrahedral pieces, the gray double-bond endpieces which go with the gray or red double bond pieces, and the triple bonds. There are also several colored balls. Tetrahedral pieces should not be confused with the gray double-bond end-pieces! There are black, red and blue tetrahedral pieces, which are the standard (or CPK) colors for carbon, oxygen and nitrogen respectively. To assemble a tetrahedral center, take two tetrahedral pieces (usually of the same color) and snap them together. A tetrahedral center represents an atom with four groups attached. Remember from VSEPR 2 that a group can be either a bond or a lone pair of electrons. Assemble tetrahedral centers from tetrahedral pieces of C N each color. Notice that the four arms or “bonds” are arranged at angles of 109.5º. The black center represents a carbon atom, with four bonds. The blue center black blue represents a nitrogen atom, with three bonds and a lone pair of electrons. The red center represents an oxygen atom, with two bonds and two lone pairs. O red 1 The gray trigonal pieces will be used later, to represent certain reactive carbon species. Chemistry majors will find the gray trigonal and linear pieces, and the pink pieces, useful when they study trigonal bipyramidal, square planar and octahedral inorganic and organometallic molecules. The short pink pieces are “bond extenders” used for inorganic and organometallic compounds, which often have longer bonds. 2 Valence Shell Electron Pair Repulsion, which says electron groupings (bonds or lone pairs) distribute themselves evenly around a central atom. See your textbook. 14 To represent methane (CH4), you may use four white balls (representing hydrogen atoms) attached to the black tetrahedral center; ammonia (NH3) and water (H2O) can be represented by using three and two white balls, respectively. However, larger molecules will have too many hydrogens for this method to be practical, given your limited supply of white balls. One alternative is to represent a lone pair of electrons by a colored ball, and let blank ends represent hydrogen atoms! Also, divalent oxygen (as in water) can be represented by a single red tetrahedral piece. Build models of methane, ammonia and water. Compare them to the online models (www.bluffton.edu/~bergerd/Models/vision2.html). Now build ethane (CH3CH3), methanamine (CH3NH2) and methanol (CH3OH); again, compare your models to the online models (www.bluffton.edu/~bergerd/Models/vision3.html). Double-bond pieces include several types: end-pieces, which are gray; gray double H bonds; and half-bonds which are either gray or red. The gray pieces are used to H CC represent carbon atoms and carbon-carbon double bonds; in order to represent a carbon-carbon double bond, snap a gray end-piece into each end of one of the gray H H double bonds. ethene Build a model of ethene, 1 and compare it to the online model. The “bonds” are arranged at normal trigonal planar angles of 120º. Notice that the online model does not show the double bond! (www.bluffton.edu/~bergerd/Models/vision4.html) However, you can deduce the presence of a double bond from the trigonal planarity of its two carbon atoms. Carbon-oxygen double bonds - carbonyl groups - may be represented using the gray and red half-bonds. To represent a carbonyl group, first snap together one red and one gray half-bond; then snap a gray endpiece into the gray end of the double bond. Carbon-nitrogen double bonds are rarer in organic chemistry, and the fact that H CO the nitrogen's valence is not filled means that a group may be bonded to nitrogen. To represent a carbon-nitrogen double bond, take a gray double- H bond piece. Snap a gray end-piece into one end to represent carbon, and a methanal blue tetrahedral piece into the other end for nitrogen. Build models of methanal 2 and methanal imine, and compare them to the online models (www.bluffton.edu/~bergerd/Models/vision4.html). H CN H H methanal imine Triple bonds are represented using the gray triple-bond pieces. Each piece represents TWO carbon atoms with a triple bond between them and one open valence for each atom. Notice that triple bonds have a normal bond angle of 180º. Carbon-nitrogen triple bonds are also possible (such bonds are referred to as cyano or nitrile groups), but there is no distinct way to represent them with this HCCH model kit. Build a model of ethyne 3 and compare it to the online model (www.bluffton.edu/~bergerd/Models/vision4.html). 1 Commonly known as ethylene. 2 Commonly known as formaldehyde. 3 ethyne Commonly known as acetylene. 15 Structure and nomenclature Use your models in conjunction with the instructions and the online models. Build the models as you read each description so that you can see what is being discussed! Alkanes can be represented entirely with the black tetrahedral pieces from your model kit, while alkenes and alkynes require double- and triple-bond pieces to be included. In this module, you will explore the relationship between all-atom representations, line drawings, and molecular models. Alkanes, alkenes and alkynes www.bluffton.edu/~bergerd/Models/structure.html Review the sections in your texts concerning alkane nomenclature. Using your model kit, build methane, ethane, propane and butane and compare them to the online models. Rearrange the model of butane to its constitutional isomer, 2-methylpropane (isobutane). This demonstrates that constitutional isomers use the same atoms, differently connected. In the table below, line drawings have been used for butane and 2-methylpropane. Line drawings cannot be used for methane and are almost never used for ethane or propane. But line drawings make higher alkane structures much easier to read! methane ethane H H H C H H H H CC propane butane 2-methylpropane H H H3C CH2 H CH3 Alkenes (www.bluffton.edu/~bergerd/Models/struc2.html), of course, must have at least two carbon atoms since the central feature of an alkene is a carbon-carbon double bond. Build ethene (ethylene), propene (propylene) and 1-butene and compare them to the models online. Now reconnect your model of 1-butene to represent its constitutional isomer 1 2-methylpropene (isobutylene). ethene H 1-butene 2-methylpropene H CC H propene H H3C CH CH2 Cycloalkanes (http://www.bluffton.edu/~bergerd/Models/struc3.html) are constitutional isomers of alkenes. 2 Since you need at least three vertices to form a closed figure in geometry, at least three carbon atoms are required for a cycloalkane. You should use the odd-colored flexible pieces in your model kit to form small (3- and 4membered) rings, as the strain is likely to break the normal, less flexible pieces. See your kit manual for guidance. 1 Constitutional isomers have the same formula (for example, C3H7NO) but have different “skeletons” (arrangement of atoms along the “main chains”). 2 Both have the general formula CnH2n. 16 Build models of cyclopropane and cyclobutane using the flexible pieces to form the rings. Build cyclopentane and cyclohexane using the normal tetrahedral pieces. Notice, from the models you have built with your model kit, how the rings get “floppier” as they get larger. The molecules are shown below as line drawings, in which each vertex represents a carbon atom, and hydrogens are automatically assumed to exist at unfilled carbon valences. You will see that your Molecular Visions kit works very much like a line drawing. cyclopropane cyclobutane cyclopentane cyclohexane Alkynes (www.bluffton.edu/~bergerd/Models/struc4.html) contain carbon-carbon triple bonds. An important feature of alkynes is their rigid linearity; four atoms are held firmly in a straight line. Build ethyne (acetylene), propyne, 1-butyne and 2-butyne. Why is there no other triple-bonded isomer of butyne? ethyne propyne HCCH H3C 1-butyne CCH 2-butyne H C C CH2 CH3 H3C C C CH3 You probably noticed that, while we built 1-butene and 2-methylpropene, we left out one of the isomers of C4H8. 2-butene can be represented in several ways: H H H H H C C C C H H H3C CH CH CH3 H None of these indicates any particular geometry; the squiggly line in the third drawing makes that explicit. However, if you build a model of 2-butene you will quickly notice that there are two possible geometries. Stereoisomers: Cis-trans diastereomers (www.bluffton.edu/~bergerd/Models/struc6.html) The two possible isomers of 2-butene are cis-2-butene and trans-2-butene; cis indicates that substituents are arranged on the same side of the double bond, while trans indicates opposite sides. These are stereoisomers; the atoms are connected in the same way, but arranged differently in space. cis-2-butene H H H CC H3C trans-2-butene CC or CH3 CH3 H3C or H For alkenes, there is a more general way of indicating cis/trans orientation: E/Z nomenclature. Z stands for the German word zusammen (together) and corresponds to cis; E stands for entgegen (opposite) and corresponds to trans. The use of these designations is discussed in your text and will be covered in 17 lecture; for our present purpose it is sufficient to point out that another way of naming the two diastereomers 1 of 2-butene is Z-2-butene H E-2-butene H CC H3C H CH3 CC or H3C CH3 or H In a similar fashion (www.bluffton.edu/~bergerd/Models/struc7.html), cycloalkanes which have two substituents on the ring can have them arranged on the same face of the ring (cis) or on opposite faces (trans). Look at your model of any of the cycloalkanes discussed above, and notice that the hydrogen atoms project both above and below the “plane” of the ring. Now put two methyl groups on the ring at adjacent positions. Is what you have built cis or trans? 2 1,2-dimethylcyclopropane 1,2-dimethylcyclobutane cis cis trans CH3 CH3 CH3 CH3 CH3 CH3 CH3 trans CH3 1,2-dimethylcyclopentane 1,2-dimethylcyclohexane cis cis trans CH3 CH3 CH3 CH3 trans CH3 CH3 CH3 CH3 1 Diastereomers are stereoisomers which are not mirror images of each other. 2 Z and E are NEVER appropriate for cycloalkanes!!! 18 Conformation and Newman projections (www.bluffton.edu/~bergerd/Models/newman.html) Use your models in conjunction with the instructions and the online models. Build the models as you read each description so that you can SEE what is being discussed! Look at your model of ethane. Notice that the central carbon-carbon bond rotates freely. This means that the hydrogens on the adjacent carbon atoms can be either alternating or all lined up; the appropriate terms are staggered and eclipsed. Since hydrogen atoms have size and thus can interfere with each other, the staggered conformation of ethane is lower in energy than the eclipsed conformation. The eclipsed conformation has what we call torsional strain because of the interactions of eclipsed hydrogen atoms. The way we emphasize conformation about a particular bond is with a Newman projection. To understand a Newman projection, imagine that you are sighting down the carbon-carbon bond in ethane (take your model and do so now!) The carbon atom behind is represented by a large circle; the hydrogens attached to each carbon can be clearly seen in the projection. staggered ethane H H H C C H H H H H H H H H eclipsed ethane H H H HH H C C H H H H H H In the same way, propane can be staggered or eclipsed about either of the carbon-carbon bonds. staggered propane H CH 3 H H eclipsed propane H HCH 3 HH H H H However, butane has another bit of conformational information: when you look down the central carboncarbon bond, the staggered form can have the methyl groups either adjacent (gauche) or opposite (anti). The anti conformation is lower in energy than the gauche. While this makes little difference in most of the chemistry of butane, conformational considerations become important when we consider cycloalkanes. anti butane H H CH 3 H H CH 3 gauche butane H H H H3 C H CH 3 19 Unsubstituted cycloalkanes (www.bluffton.edu/~bergerd/Models/newman2.html), if planar, would not only have significant angle strain (caused by abnormal bond angles in the ring) but also considerable torsional strain (from eclipsing of adjacent hydrogens). Cyclopropane and cyclobutane cannot avoid having large amounts of strain in their structures, but cyclopentane and cyclohexane can easily adopt conformations in which not only the angles have normal values but eclipsing (and thus torsional strain) is minimized. Planar cyclopentane has almost-normal bond angles of 108º. Nevertheless, to avoid torsional strain the molecule bends into the so-called envelope conformation, in which one of the carbon atoms is bent out-of plane. This staggers the hydrogens on that atom relative to those on adjacent carbon atoms. H HH or H If cyclohexane (www.bluffton.edu/~bergerd/Models/newman3.html) were planar, there would be considerable angle strain as the angles in a planar hexagon are 120º (vs. 109.5º). However, when you use your model kit to build cyclohexane you will see that the ring is really rather “floppy.” This floppiness allows both angle and torsional strain to be relieved, and cyclohexane has two main conformational types: boat (in which two opposite carbons are bend out-of-plane in the same direction) and chair (in which two opposite carbons are bent out-of-plane in opposite directions). Boat cyclohexane has no angle strain, but there are several eclipsed interactions between neighboring hydrogens. The actual “boat” conformation is the so-called twist boat, in which the eclipsed interactions are alleviated by twisting around some of the carbon-carbon bonds. Nevertheless, the boat conformation is still relatively high in energy because of the unavoidable flagpole interactions between the hydrogens on the insides of the “prow” and “stern.” The Newman projection below emphasizes the eclipsed interactions along the “gunwales” of the boat. H H H H H H H H The flagpole hydrogens are shown in boldface. or H H H H The twist-boat conformation is very difficult to draw; however, it can be seen in the online models (www.bluffton.edu/~bergerd/Models/newman3.html) and you should be able to reproduce it using your model kit. Chair cyclohexane has neither angle strain nor eclipsed interactions! In fact, it has zero strain energy. If you build a model of chair cyclohexane, you will notice that there are two types of hydrogens, depending on whether they point up or down with respect to the ring, or point along the ring “plane.” These types are called, respectively, axial and equatorial, and are color-coded in the on-line version of this tutorial. The Newman projection emphasizes that all hydrogens are staggered; if you examine your model you will see that this is true all the way around the ring. H H H H H H H H H H H H H H H H H H H H 20 Mark one axial and one equatorial position on your model with colored balls. Now “ring-flip” to the other chair conformation. The colored balls have changed places: the one which was axial is now equatorial, and vice versa. For unsubstituted cyclohexane, the two chair conformations (or conformers) have the same energy. But when we begin to put substituents on the ring, this quickly comes to a halt: because of axial-axial steric 1 interactions, most substituents prefer to be in an equatorial position. Very large substituents, such as tbutyl groups, lock the ring into a particular conformation! CH 3 CH 3 Examine the online models of methylcyclohexane and t-butylcyclohexane in space-filling mode. (www.bluffton.edu/~bergerd/Models/newman4.html) Notice the steric interactions. When more than one substituent is present, the ring will take on whichever conformation is lowest in energy; if possible, all substituents will be in equatorial positions. However, as you will see, this is not always possible in a chair conformation. Build models of the following molecules, and report whether one chair conformation will be favored over the other (www.bluffton.edu/~bergerd/Models/newman5.html): Cis-1,2-dimethylcyclohexane Trans-1,2-dimethylcyclohexane Cis-1,3-dimethylcyclohexane Trans-1,3-dimethylcyclohexane Cis-1,4-dimethylcyclohexane Trans-1,4-dimethylcyclohexane Now build cis-1,4-di-t-butylcyclohexane. Do you expect it to be a low-energy molecule or a highenergy molecule from steric considerations? 1 Size-related. 21 What is observed is that this molecule tends to exist in a boat form because this is the only conformer which allows both t-butyl groups to be equatorial! Chirality This manual is available at www.bluffton.edu/~bergerd/Models/chiral.html. Use your model kit in conjunction with the instructions and the online models. Build the models as you read each description so that you can see what is being discussed! Chirality 1 is an attribute of objects that makes it impossible to superimpose them on their mirror images. Examples of chiral macroscopic objects include hands, feet, screws, automobiles and so on. Molecules can also be chiral. Ways of measuring chirality are explained in your text and will be explored in the laboratory; the purpose of this module is to explore chirality using molecular models. Chiral centers For a single carbon atom to be chiral, there must be four different substituents attached. Such a carbon atom is called a chiral center. 2 Chirality may be illustrated by considering a series of substituted methanes. Methane itself (www.bluffton.edu/~bergerd/Models/chiral2.html) is obviously achiral (not chiral). It is easy to see that methane can be superimposed on its mirror image; nevertheless, you may want to test this. The same is true for chloromethane. H H C H H H H HC Cl or CH3Cl Bromochloromethane, with three different substituents on carbon, may be more difficult to see; but if you build models of the two “different” molecules below you will find that they can be superimposed. H H Br C Cl Br H HC Cl However, a methane with four different substituents, such as bromochlorofluoromethane, is chiral (www.bluffton.edu/~bergerd/Models/chiral3.html). Build models of the two different molecules below. 1 “Handedness.” 2 A chiral center need not be a carbon atom, as long as there are four different groups attached. For example, it is possible to have chiral ammonium ions or chiral silanes (compounds of silicon). In neutral nitrogen, a lone pair is formally able to serve as a fourth group but, because of the very low inversion barrier for amines, “chiral” neutral nitrogen compounds usually exist as racemic mixtures. Phosphines (phosphorus compounds analogous to amines) can be chiral because the inversion barrier at phosphorus is very high. 22 You will see that they are mirror images and cannot be superimposed! Such molecules are enantiomers 1 of each other. H Br FC F Br HC Cl Cl Use your models to explore the method explained in your text for determining whether configuration is R or S. Which of the two molecules above is R? Which is S? Other molecules can be thought of as “substituted methanes.” Those with four substituents – like 2bromobutane and 2-butanol – are also chiral. Build pairs of enantiomers for 2-butanol and 2-bromobutane. Draw the R and S configurations of each. Of the two molecules shown below, which is R and which is S? H H3C C H H C CH3 H3C or Br Br H C H C CH3 or Br H Br Molecules with more than one chiral center will obviously have more than two stereoisomers; in general, a molecule with n chiral centers will have 2n stereoisomers. However, this is a maximum and is not always the case, as we will see. For example, 3-bromo-2-butanol, with two chiral centers, will have 22 = 4 stereoisomers. If each chiral center can be either R or S, obviously the stereoisomers will be RR, SS, RS and SR. These four are shown below; notice that each horizontal pair of isomers is a pair of enantiomers. HO H C H3C C CH3 H Br H OH H3C C C CH3 H Br OH or Br OH or Br H OH H3C C C CH3 Br H HO H H3C C C CH3 Br H OH or Br OH or Br Assign the correct RS designation to each of the chiral centers in the molecules above, and name the molecules correctly. Remember that stereoisomers which are not mutual mirror images are called diastereomers. The top two molecules are diastereomers of the bottom ones. When both chiral centers in a molecule have the same substituents, the molecule as a whole may or may not be chiral. If one half of a molecule is the mirror image of the other half, the molecule contains a plane of symmetry and cannot be chiral even though it may contain chiral centers. 1 Enantiomers are stereoisomers which are mirror images of each other. 23 Consider 2,3-dibromobutane. Like 3-bromo-2-butanol it has two chiral centers and therefore four (22) possible configurations: RR, SS, RS and SR. However, if you examine models of the four molecules below you will see that the bottom pair are identical! Molecules which contain chiral centers but are not themselves chiral are called meso, and we refer to them as (for example) meso-2,3-dibromobutane. H Br H3C C C CH3 H Br Br H3C C Br H3C or CH3 H Br Br Br Br H C Br C C Br H CH3 H Br H Br H3C C C CH3 Br H or Br or Br or Br Assign R or S configuration to each of the chiral centers in the molecules shown above. Which molecules contain a plane of symmetry? Fischer projections www.bluffton.edu/~bergerd/Models/chiral6.html Around 1890-1900, organic chemists were getting used to the idea that organic molecules are threedimensional things, and that different arrangements of substituents in space give different molecules. But they were not at all used to thinking in three dimensions (in fact, the wedge-dash system of “perspective” drawing did not come into general use for about another 50 years!) Emil Fischer devised the system of Fischer projections, which allows a 3-D molecule to be correctly represented by a plane figure. There are two conventions associated with Fischer projections, but only the first is essential: 1. All vertical lines represent bonds going away from the viewer; all horizontal lines represent bonds coming toward the viewer. 2. The main chain of carbon atoms is laid out vertically by convention. The two enantiomeric bromochlorofluoromethanes shown above can be represented thus: H Br FC H Cl F Br HC Cl 24 Br Br C F H F Cl Br F Cl Br C Cl H F H Cl Satisfy yourself that the Fischer projections shown below correspond to the 2-bromobutanes, 3bromo-2-butanols and 2,3-dibromobutanes shown in Section A. CH3 CH3 H Br Br CH2CH3 CH2CH3 CH3 CH3 H H Br HO H H OH CH3 H Br H CH3 CH3 CH3 CH3 Br Br H H OH H Br HO H CH3 CH3 CH3 CH3 H H Br Br H Br Br H Br H CH3 R,R-2,3-dibromobutane CH3 S,S-2,3-dibromobutane CH3 meso-2,3-dibromobutane One advantage of a Fischer projection is that it becomes immediately obvious when a molecule is meso. 25 Rules for manipulating Fischer projections www.bluffton.edu/~bergerd/Models/chiral7.html The rules for manipulating Fischer projections as given in your text may be incomplete. The full rules are as follows. 1. Projections, as single units, may be rotated 180º only (not 90º), and only in the plane of the paper. 2. Any three groups may be rotated, with the fourth remaining fixed, and the result will be a molecule identical to the starting molecule. 1 If we take the Fischer projection of one of our 2bromobutanes and perform such a rotation, we can easily see that it is S-2-bromobutane. (Remember that, in the projection on the right, the H atom is pointing away from us.) H CH3 H Br CH2CH3 Br CH3 CH2CH3 Using your model kit or the online online models, you should satisfy yourself that the two Fischer projections above represent the same molecule. 1 This is equivalent to rotating around a single bond. 26 Introduction to Molecular Modeling using ArgusLab ArgusLab is a free molecular modeling package that runs under Windows (and only under Windows). It is installed on all public computers in Shoker Science Center (an icon should be on your desktop), and you may also download it for personal use from www.arguslab.com/downloads.htm. (Of course, you can’t install it on a public computer at Bluffton University; you don’t have permission for that!) By the end of this tutorial, you will know how to build molecules in ArgusLab an atom at a time, or using template structures; how to change atom and bond types; and how to use previously-saved structures as starting points for building new structures. NOTE: ArgusLab may freeze, or may even freeze your computer, unless you turn off video hardware acceleration on your computer. To do this, 1. Right-click on your Windows desktop and select “Properties”. 2. Select the “Settings” tab and click the “Advanced” button at the bottom right. A new window will open. 3. Select the “Troubleshoot” tab in the new window. Move the slider labled “Hardware acceleration” to “None”. 4. Click “OK,” and click “OK” to make the “Properties” window go away. In practice, you will probably find that you can have video hardware acceleration turned on at a low level, but be aware (if things freeze up for you) that this is what is happening. Chirality, chiral molecules and chiral centers Any object that is not identical to its own mirror image is chiral. Two easy macroscopic examples are your hands: your right hand is the mirror image of your left (unless you’re missing fingers), but your right hand is not identical to your left (you can’t easily get it into a left glove, for example). Therefore your hands are chiral objects. Molecules can be chiral too; we will look at a few examples in this exercise. Within molecules, individual atoms can be chiral centers if they have four different substituents; this can include lone pairs, as we will see. Molecules with chiral centers have stereoismers that differ only in the arrangement of groups around each chiral center. We will build the following chiral molecules and examine their stereoisomers: • 2-chlorobutane • Alpha-methylbenzylamine (that is, 1-phenylethanamine) • Tartaric acid, which has two chiral centers. A note: you should create a separate directory on your H-drive labeled “ArgusLab” or something similar. Save all ArgusLab files to this directory. You want to do this because ArgusLab computations create extra files that you may later want to delete, and having them all in a single directory makes this easier. 27 2-chlorobutane. Open ArgusLab and click the “Create New Molecule” button at the upper left. Cl CH3 ArgusLab defaults to “no bonds drawn,” but that’s not the way we want it. So H3C click the button just to the left of the dumbbell-shaped buttons in the second 2-chlorobutane row. (On mouseover it should say, “Automatic bonds are OFF. Click to turn ON.”) Now right-click on the molecule window; an atom will appear (the default is carbon). Move the mouse a little and right-click again; another atom will appear, bonded to the first atom. Add two more atoms to the chain in this way. (If you forgot to turn on automatic bonds, you can get bonds by left-clicking on the first atom to highlight it, then left-clicking on the atom you want the bond to go to. Continue to create bonds in this way until all bonds have been formed.) When you have drawn four atoms, bonded in a straight (or zigzag) line, click the yellow arrow button in the second row. This turns off the builder mode. Now look for the “H” buttons in the top row. The leftmost “H” will say “Add hydrogens” when you mouse over it. Click this button; hydrogens will appear on your molecule. Now click the pliers button (“Clean geometry”). Now we will convert our model of butane into 2-chlorobutane. Right-click on one of the hydrogen atoms on the second carbon and select “Change atom” and then “Cl [s], Cl chlorine.” Now clean the geometry again by clicking the pliers button. Before the cleanup is performed, ArgusLab will ask you to save your file. Name it “2-chlorobutane.” You have just built your first chiral molecule! But… is it R or S? To find out, select the “Label” menu and click “Atom label settings.” Select the “Chiral centers” radio button at the bottom, then click “OK.” Now, from the Label menu, select “All atoms.” Your chiral center will now be labeled either (R) or (S). Convince yourself that the label is correct by using the “steering wheel” method. Now open a new molecule window. Click “Create new molecule” and under the “Window” menu, select “Tile.” In the new window, build the mirror image of your first molecule. Set the labels to show chiral centers and turn on labeling in your new molecule. Does it have the opposite (R/S) designator as your first molecule? If so, satisfy yourself that the two are indeed mirror images. (Ignore variations in methyl group conformations; your chiral centers should be mirror images!) Close the two molecule windows. 28 Alpha-methylbenzylamine. Open a new molecule window. On the left-hand side of your screen will be three tabs; select “Rings” and click on the benzene ring (see the structure of αmethylbenzylamine and select the ring that looks like that.) Right-click in the molecule window to make the ring. The ring will be a bit odd-looking, and all atoms will be selected; to make it look more normal (and allow us to continue to build), click on the yellow arrow button. Now right-click on one of the hydrogen atoms and “Change atom” to “C [sp3], C_3 tetrahedral.” CH3 NH2 α-methylbenzylamine Select the “pencil dot” button, and left-click on your new carbon atom to select it. Turn on automatic bonds if you haven’t already, and left-click away from the selected atom to create another carbon atom. Now click the “add hydrogens” and “clean structure” buttons. Save your file as “a-methylbenzylamine.” Right-click one of the hydrogens on your CH2 group, and change it to “N [sp3], N_3 tetrahedral.” Add hydrogens and clean the structure again. Save it. Now create a copy by selecting “Save as…” from the file menu. Create a new window. Rather than build the mirror-image from scratch, we will make it from the copy of our first α-methylbenzylamine model. Under the File menu, select “Open” and choose one of your methylbenzylamine models. (If you choose the one that is already open, you will get an error message. If you do, just open the other file.) Now select “Window”, then “Tile” so you can see them both onscreen. To give yourself more room onscreen, you may want to turn off the “pencil benzene” button (“Turn off the build menu”). If you do this, you will need to tile your windows again. For each window, select the “four-arrows” button in the top row (“Center the molecule in the window”). For one of your models, do the following sequence: 1. Select the “H eraser” button (“Delete hydrogens”). 2. Right click on the nitrogen atom and delete it. 3. Select the left-hand “H” button (“Add hydrogens”). 4. Turn your molecule so that it’s a mirror image of the other (except for the missing nitrogen). Then change a hydrogen into a nitrogen to make it a true mirror image, and click “Add hydrogens” again. 5. Clean the structure and save. Using the “steering wheel” method, assign each window “R” or “S”. Now turn on chirality labels in both windows. Were you correct? 29 Tartaric acid. This molecule has two chiral centers; see whether you can identify them in the drawing. To build our first tartaric acid model, we begin by building a 6-atom chain. This is to HO accommodate the four carbons, with one oxygen on each end. Build a chain of six carbon atoms, and select the yellow arrow button to turn off the build mode. Build the carbonyl groups. Right-click an end atom, and change the atom to “O [sp2] > O_2 trigonal, non-aromatic.” Do the same with the other end atom. HO O O HO OH tartaric acid Right-click the carbon next to each of our new oxygen atoms, and change it to “C [sp2] > C_2 trigonal, non-aromatic.” Now right-click the bond between carbon and oxygen and select “Double.” You should now have a chain that looks like this: O=C–C–C–C=O. Click “Add hydrogens.” (If you get two hydrogens on one of your end carbons, it means you forgot to change its type. Click “Delete hydrogens” and change the carbon atom as directed in the last paragraph.) After adding hydrogens, change each of the end hydrogens to an oxygen “O [sp3] > O_3 tetrahedral” and add hydrogens again. Clean the geometry and save your structure as “butanedioic acid” because that’s what it is. HO HO O HO O O HO HO OH R,R-tartaric acid HO O O HO OH S,S-tartaric acid HO O HO OH R,S-tartaric acid There are three isomers of tartaric acid. Build each isomer from your model of butanedioic acid by changing two hydrogens to OH groups; use chiral labeling to confirm that you have made what you think you have made. Compare the three isomers in three windows. Two of them are mirror images of each other; one is not. Which is the odd structure out? Which structure has a mirror plane such that half of it is the mirror image of the other half? To hand in: Name each structure file with the correct chiral designation and e-mail your structure files to the instructor for grading. 30 Calculations for Organic Synthesis 1 In any experiment, it is the relationship between chemical quantities (moles or millimoles) which is important, rather than that between physical quantities such as mass or volume. Because there are no “molemeters,” we are forced to use scales and volumetric glassware to measure the quantity of a substance. This should not be allowed to obscure the fact that it is chemical quantities that are fundamental. Only by knowing the chemical amounts of the reactants involved in a synthesis can you recognize the stoichiometric relationships between them or predict the yield of the expected product. It is helpful to regard a chemical calculation as a “conversion,” in which a given quantity is “converted” into the required quantity. This can be accomplished by multiplying the given quantity by a series of ratios used as unit or dimensional conversion factors. Unit conversions are carried out by conversion factors whose quotient is unity. (For example, 454 g = 1 pound, so 454 g ÷ 1 lb = 1 and we use a conversion factor of 454 g/lb or 1 lb/454 g.) Dimensional conversions convert between different dimensions, such as mass, volume and chemical amount. For example, the density of a substance may be regarded as a conversion factor linking mass and volume, and is used to convert a given mass to units of volume, or vice versa. You should already be familiar with some dimensional conversions. All calculations should explicitly contain all units involved, and should be checked to make sure that units cancel, leaving only the desired units in your answer. This does not ensure that your answer is correct, but if the units don’t cancel properly your answer is sure to be wrong. The following examples illustrate some fundamental types of calculations that you will encounter. Chemical amount and mass The chemical amount (in moles or millimoles) of a pure substance is converted to its mass by multiplying by its molar mass, a unit conversion factor. Remember that the molar mass of a substance is obtained by simply appending the unit g/mol to its molecular weight, which is a dimensionless quantity. For example, the mass of 15.0 mmol of butyl acetate (MW = 116) is 15.0 mmol × 1 mol 116 g × = 1.74 g 1000 mmol 1 mol Note that the chemical amount in millimoles must be converted to moles before the conversion factor is applied; otherwise the units will not cancel. Mass can be converted into chemical amount by inverting the conversion factor(s) before multiplying: 1.74 g × 1 mol 1000 mmol × = 15.0 mmol 116 g 1 mol Chemical amount and volume The chemical amount (in moles or millimoles) of a pure liquid is converted to volume by multiplying by its molar mass and the inverse of its density. For example, the volume of 25.0 mmol of acetic acid (MW = 60.1; d = 1.049 g/mL) is 25.0 mmol × 1 1 mol 60.1 g 1 mL × × = 14.4 mL 1000 mmol 1 mol 1.049 g This is based on a similar section in Lehman, J.W. Operational Organic Chemistry, 3rd Ed., Prentice-Hall: Upper Saddle River, NJ, 1998; pp. 771-773. 31 The volume of a solution needed to provide a specified chemical amount of solute is calculated by multiplying the number of moles required by the inverse of the solution’s molar concentration. 1 For example, the volume of 6.0 M HCl (which contains 6.0 mol of HCl per liter of solution) needed to provide 18 mmol of HCl is 18 mmol × 1 mol 1L 1000 mL × × = 3.0 mL 1000 mmol 6.0 mol 1L Note that concentrations expressed in mol/L and in mmol/mL have the same numerical value (since 1/1 = 1000/1000). Thus a 6.0 M (6.0 mol/L) solution also has a concentration of 6.0 mmol/mL; using these units simplifies the calculation considerably: 18 mmol × 1 mL = 3.0 mL 6.0 mmol Theoretical yield The maximum amount of product that could be obtained from a reaction is called the theoretical yield of the reaction. Theoretical yields can be calculated using stoichiometric factors – ratios derived from the coefficients (expressed in moles) of the products and reactants in a balanced equation for the reaction. For example, the stoichiometric factors relating the chemical amount of product to the chemical amounts of reactants in the following reaction O O O 2 PhCH + CH3CCH3 benzaldehyde (B) NaOH acetone (A) PhCH CHCCH CHPh + 2 H2O dibenzalacetone (DBA) are 1 mol DBA/1 mol A and 1 mol DBA/2 mol B. Suppose you were trying to prepare DBA (MW = 234.3) starting with 5.00 g of B (MW = 106.1) and 2.0 mL of A (MW = 58.1; d = 0.792 g/mL). 2 You can calculate the maximum amount of product that could be formed from each reactant by converting the given quantity to moles, then applying the appropriate stoichiometric factor: 5.00 g B × 2.0 mL A × 1 mol B 1 mol DBA × = 0.0236 mol DBA 106.1 g B 2 mol B 0.792 g A 1 molA 1 mol DBA × × = 0.0273 mol DBA 1 mL A 58.1 g A 1 mol A Since there is only enough benzaldehyde to produce 0.0236 mol of DBA, it is impossible to obtain more than that from the specified quantity of reactants. Once that much product has been formed, the reaction mixture will have run out of benzaldehyde and the excess acetone will have nothing left with which to react. Therefore, benzaldehyde is the limiting reagent on which the yield calculations must be based. The theoretical yield of DBA, in grams, is then 1 Remember that the symbol “M” stands for “moles per liter.” 2 Molecular weights and densities may be found in references such as the CRC Handbook, the Merck Index or the Aldrich Catalog. Molecular weights can, of course, also be calculated from the molecular formula. 32 0.0236 mol DBA × 234.3 g DBA = 5.53 g DBA 1 mol DBA Remember that the limiting reagent is always the one that would produce the least amount of product, not necessarily the one present in the lowest mass. In this example, benzaldehyde is the limiting reagent, although there is three times the mass of benzaldehyde present as there is acetone. Relative yield O It is seldom, if ever, possible to obtain the theoretical yield from an organic synthesis. The reaction may not go to completion; there may be side reactions PhCH CHCCH3 that reduce the yield of product; and there are always material losses when the benzalacetone product is separated from the reaction mixture and purified. For example, in the reaction of benzaldehyde with acetone, some benzalacetone may be formed as a byproduct, reducing the yield of dibenzalacetone. The relative yield of a preparation compares the actual yield to the theoretical yield as defined here: relative yield = actual yield ×100% theoretical yield If you prepared 4.09 g (the actual or absolute yield) of DBA using the reaction conditions in the previous section (theoretical yield 5.53 g), the relative yield of your synthesis would be relative yield = 4.09 g DBA ×100% = 74.0% 5.53 g DBA Please, PLEASE do not report that “the percent yield was 74.0%” 1 or even that “the relative yield was 74.0%.” 2 Always report that “the yield was 74.0%” or in the experimental section, “the yield was 4.09 g (74.0%).” Reporting the yield with “gram” units means it’s an absolute yield; reporting with a percent sign means that it’s a relative yield. Don’t overspecify! Outside the experimental section, only relative yields are to be reported because the relative yield is (in principle) quantity-independent. 1 Say it out loud to yourself to see why this is a problem! 2 And then there are the students who insist on reporting that “the percent yield was 74.0.” WRONG. 33 Laboratory Procedures Safety rules Minimum safety standards and Disposal guidelines are given in most experimental procedures and you will, of course, follow them. You are responsible for the contents of “Practicing Safety in the Organic Chemistry Laboratory” (TECH 700) and you must hand in the contract from it before you are allowed into the laboratory after the first week. You are also expected to follow these general laboratory rules: 1. Proper eye protection, including side shields, will be worn at all times in the laboratory. This is a state law. The only exception is when no chemistry or other potentially hazardous work is going on in the area. You are allowed one warning per week; second offenses will draw the 10% penalty for a lab safety violation. 1 Persistent failure to wear safety glasses will result in permanent expulsion from the laboratory and consequent failure of the course. 2. Nothing between the shoulders and the knees will be exposed in the laboratory. 2 Open footwear (e.g. sandals or clogs or sneakers with holes in them) will NOT be worn in the laboratory. Students have been sent away from lab to put on appropriate footwear; such lost time will NOT be made up. It is recommended that you wear an overgarment such as a lab coat or apron, and that you wear rubber gloves when handling reagents or solvents. Lab coats are available for you to borrow. Dishwashing gloves from the grocery store provide adequate hand protection and do not interfere with dexterity; get a pair that fits snugly. Disposable nitrile gloves are provided (they are expensive; please use no more than one pair per week!) but do not protect as well as a set of dishwashing gloves because they tear more easily. Disposable latex gloves are NOT recommended as organic liquids will pass right through them, 3 though they are OK if you are only performing aqueous chemistry. After some experiments, your clothes will stink. You should not wear your best clothes to the lab! 3. You will be assigned a bench, a hood and a shared bench. You will be held responsible for cleaning up after yourself. Failure to do so will result in a 10% report penalty. Students will be assigned responsibility for cleanup of common areas (balance tables, instrument areas and so on) in rotation; failure to clean up these areas will result in the 10% penalty for that student. To avoid upsetting your lab mates, remember that your mother doesn’t come to lab with you and clean up after yourself! 4. Any chemical that you do not want to be treated as waste must be CONTAINED (placed in a closed container–this includes stoppered flasks), and LABELED with the name of the chemical, the name of the student, and the date. It should be placed either in a hood or in the drying cabinet, which is vented into the hood exhaust system, unless you are instructed otherwise. 1 If you catch the professor in the laboratory without appropriate eyewear during an experiment, you will be awarded 5 points out of 100 on your next lab report. 2 Short sleeves are allowed; short pants are permitted but not recommended. See also Zubrick, Chapter 1. 3 A chemistry professor at Dartmouth was killed by wearing only latex gloves while handling a highly-toxic substance; see list.uvm.edu/cgi-bin/wa?A2=ind0202d&L=safety&D=1&P=12811 and other listings at www.google.com/search?q=dartmouth+chemistry+mercury+death. 34 Any chemical not contained or properly labeled will be disposed of; this will usually make you unable to complete your work, resulting in a zero for the experiment. 1 The benchtops and drying cabinet will be inspected after each laboratory period to ensure this! 5. You may obtain melting points, micro boiling points, or perform gas chromatography or spectroscopy without the presence of the instructor as long as there is a science professor in the building (indirect supervision). You must ensure that a professor knows you are in the lab, especially at night. All other operations except minor cleaning require direct supervision. This means that a professor must be in the laboratory with you. 6. No food or drink may be brought into the laboratory. 7. Be careful with glassware and other equipment; it costs money, and replacing it drives up your tuition. Breaking it will drive down your grade, which is partly based on good laboratory technique. While some of these rules will seem draconian, they are fairly mild compared to the penalties levied by law for improper laboratory procedure. One high school chemistry teacher in California (ca. 1998) came within a whisker of doing prison time for washing a quantity of copper sulfate–used to kill algae in ponds and lakes–down the sink. He was let off with a fine and a stern warning because he was found to have acted in good faith. He had not checked state regulations… Waste disposal A section on appropriate disposal of wastes from each experiment is included in the instructions for that experiment. In general “mixed wastes” (such as reaction mixtures and solvents from crystallizations) are treated according to standards for the most noxious substance they contain, unless other specific guidance is provided. • Paper towels used to wipe up chemical spills will be placed in the hood until volatile chemicals have evaporated, then disposed of in an appropriate manner according to what is on them. Be sure to use something to hold the towel down, or it may be sucked up the flue. • Chemical waste will be placed in appropriate waste containers. What constitutes an “appropriate” container will be spelled out for each waste substance in the Disposal section for each experiment. o Never mix halogenated with non-halogenated waste. Disposal costs are much higher for the former, and any amount of halogen makes all the waste “halogenated”! • Glassware will be properly cleaned and placed (normally upside down) on paper towels next to or in your hood. Substantiated complaints from the next person to use your glassware will result in the 10% cleanup penalty. • Odor control is an important part of waste disposal for this laboratory. Many measures you are directed to take are for odor control. For example, o 1 Disposal procedures may direct you to “flush” something down the sink. You must run water continuously for at least five minutes so that nothing remains in the sink trap. Whether this results in the usual grade penalty for incomplete lab work is up to the instructor. 35 o Sinks, especially sinks that are not in the hood, must be washed carefully after lab to eliminate any chemical odors. Some definitions related to laboratory safety Autoignition temperature The temperature at which the substance will spontaneously burst into flame, given a supply of oxygen. Such a fire can occur if a lid is removed from an overheated vessel, as may happen when oil is heated in a covered frying pan. Carcinogen The substance has been found to cause cancer in animals or humans. We will not use known carcinogens. Caustic The substance is a strong base. Caustic substances cause burns to human flesh and eat holes in clothing. Corrosive The substance is a strong acid. Corrosive substances cause burns to human flesh. They also break down certain fibers (including cellulose, the polymer found in cotton). Doseresponse curve When plotting dose versus physiological effect, the result is normally not a straight line. Instead, at low doses there is often no effect at all (or a therapeutic effect), while at sufficiently high doses the effect is typically toxic. See LD50. Flammable Inflammable. Flash point The temperature at which the vapor above a liquid forms an explosive mixture with air. Ignition results in a flash (or explosion) rather than a flame. Many organic liquids (such as gasoline) are used at temperatures well above the flash point; this is relatively safe because the liquid’s vapor pressure is so high that oxygen is displaced from the air above the liquid. 1 And it still takes a spark to start things off. So don’t smoke around gas pumps! “Flush down the sink” Pour down the drain, rinsing thoroughly with water. Run water after the material for 5 minutes or more, depending on how much reagent was discarded and how concentrated it was. Inflammable The substance will burn. See also flash point and autoignition temperature. Irritant The substance will irritate skin, eyes or mucous membranes. The fumes may cause you to sneeze. LD50 The dose that caused death in 50% of a test group (usually of rats). The dose is expressed in mg/kg, milligrams of substance per kilogram of test animal. The dose-response curve typically rises sharply at this point, so that amounts smaller than the LD50 are often innocuous, and larger amounts are definitely toxic. Mutagen The substance causes genetic damage (mutations) to cells with which it comes in contact. Mutagens in the bloodstream can adversely affect germ cells, causing genetic defects in offspring. Normally we will not use known mutagens. Pyrophoric Pyrophoric materials ignite spontaneously in air below about 45º C. They will also ignite 1 Fuel-air explosives, and internal-combustion engines, are carefully calibrated to mix just the right amount of fuel with air. Too much fuel in the air and the bang is MUCH smaller. 36 nearby inflammable materials. Smelly The substance has an objectionable odor. Smelly substances should be handled only in the hood; this includes cleaning glassware contaminated with them. If they need to be weighed, they must be weighed in closed containers. Teratogen The substance causes developmental abnormalities in unborn babies; therefore pregnant women should not be exposed to it. We do use some suspected teratogens; if you think you might be pregnant you should inform the instructor immediately. Toxic The substance is poisonous. Normally we will not use “highly toxic” substances (LD50 < 50 mg/kg). “Mildly toxic” is used to describe substances with LD50 > 500 mg/kg. Reference Sources Physical data of organic compounds may be found in the following sources. All are available in the Shoker Science Center lobby. • The CRC Handbook of Chemistry and Physics • The Merck Index • ChemFinder online database, accessible from the course web page. Caution: mistakes have been found in this database by your instructor! • The Aldrich Catalog Handbook of Fine Chemicals; copies of this are also kept in the lab. • Aldrich Library of IR Spectra • Aldrich Library of NMR Spectra Chemical safety data may be found in the following sources. All are available in the Shoker Science Center lobby. • The Merck Index • MSDS online database, accessible from the course web page. These are among the few reliable online data; they are reprints of published MSDS forms. Information here may be out of date, so be sure to check the date of the MSDS you are reading! • The red MSDS binders in Shoker contain an MSDS for every substance stored in the building. Discussions of typical laboratory procedures may be found in Zubrick, or any of several organic chemistry laboratory texts in the Shoker Science Center lobby. It is highly recommended that you use these references before coming to lab! 37 Fire extinguishers and inflammable materials When dealing with inflammable materials, fires do occasionally happen. To prevent and (God forbid!) contain fires, you need to know two things: • The inflammability of the materials you are working with, as measured by the flash point and autoignition temperature, as well as the chemical reactivity; and • How to fight a fire involving the materials you are working with. This section contains basic information about fires and fire extinguishers. 1 Requirements for a fire Burning is a surface phenomenon, and occurs at the surfaces of objects. Small particles, which have a large surface area per weight, will burn well even if made of “fire-resistant” materials like iron; large objects, like tree trunks, may be hard to light even though they are “inflammable.” A fire requires three things to sustain it: a heat source, a fuel supply, and a source of oxygen. They also require the ability to sustain the combustion reaction, which is a free-radical chain process. • Heat Source. Fire is an exothermic reaction, but it requires an input of energy to start. For example, you need to use friction to start a match; a match to start a Bunsen burner; a spark to start a butane lighter. Normally a fire provides enough heat to sustain itself, but a fire in the presence of something that absorbs heat efficiently (e.g. water) will usually die out. • Fuel Supply. Obviously a fire must have something to burn. This can be something quite unlikely; for example, iron will burn if it is granulated or powdered. • Source of Oxygen. For most fires, this is air. Cut off the air and you cut off the fire. However, Class D inflammable materials (see below) are highly reactive and will extract oxygen from water and even carbon dioxide. Magnesium flares, for example, will burn underwater. • Fires are free-radical chain reactions. If something interferes with the chain, the chemical reaction of combustion can be shut down. Some fire extinguishers, notably halon and dry chemical extinguishers, contain “radical scavengers” which disrupt the chain reaction by destroying free radicals. Types of fires There are four types of fires, from Class A, which are easy to put out, to Class D, which are NOT. A. Paper/trash/wood and other more-or-less organic solids. “Ordinary combustibles.” B. Inflammable liquids such as gasoline or paint thinner. C. Electrical fires, with electricity still flowing to the burning equipment. D. Burning reactive metals, such as sodium, magnesium, titanium, and so forth. Such metals not only burn at high temperatures but can chemically extract oxygen from water and even carbon dioxide. Pyrophoric materials 2 such as organoboron, organolithium and organomagnesium 1 For weblinks, see www.bluffton.edu/~bergerd/classes/CEM221/fire.html. 2 Pyrophoric materials will burn spontaneously in the presence of air. 38 (Grignard) compounds are also in Class D because they react violently with water and carbon dioxide. Some reactive metals, such as potassium, are pyrophoric. Types of fire extinguishers Fire extinguishers are normally classified according to the type of fires they are able to handle. You should be familiar, in any situation in which a fire may arise, with (a) the locations of nearby fire extinguishers and (b) the types of nearby fire extinguishers. No fire extinguisher can remove the fuel source; an extinguisher can only remove heat or oxygen, or interfere with the chemistry of combustion. Some extinguishers do more than one of these. Type A Extinguishers Type A extinguishers are normally colored silver and are suitable only for Type A fires. They are pressurized-water extinguishers and work by removing heat. The fire's heat goes into heating and evaporating the water, which has a very high heat capacity, and soaking the burning materials with enough water will cool them to below the combustion point. However, all burning materials have to be soaked down or the fire will restart. These extinguishers CANNOT be used for Type B fires because burning organic liquids will float on water while continuing to burn. They CANNOT be used for Type C fires because of the risk of electrical shock, NOR for Type D fires because water will support the combustion of Type D materials. To extinguish a fire with a Type A extinguisher, aim at the base of the fire and soak the burning materials well with water. CO2 Extinguishers, Type BC A high-pressure CO2 extinguisher removes oxygen and, to a small extent, heat. The expanding CO2 cools, sometimes enough to produce dry ice snow, but the main effect is to blanket the burning material with a heavy gas that cannot support combustion. CO2 extinguishers leave no residue and so are especially suitable for extinguishing Type C fires, which often involve delicate equipment. They are also suitable for Type B fires. CO2 extinguishers are NOT suitable for use on Type A fires because the extinguished materials usually retain enough heat to re-ignite when the CO2 dissipates; NOR for Type D fires because CO2 will support the combustion of Type D materials. CO2 extinguishers are normally red and have large nozzles. To use a CO2 extinguisher, “spritz” the burning material with CO2 until the fire is out. Watch the materials for several minutes in case they re-ignite. Halon Extinguishers, Type ABC Halon extinguishers normally contain bromochlorodifluoromethane, a non-toxic, very heavy gas (much heavier than CO2). This not only displaces oxygen from around the fire but chemically interferes with combustion. 1 Halon extinguishers, like CO2 extinguishers, are especially suitable for Type C fires and delicate equipment, but because they chemically interfere with combustion they are also good for Type A and Type B fires. However, they are NOT suitable for Type D fires because most Type D combustibles react exothermically with bromochlorodifluoromethane. Halons are being phased out because of the damage chlorofluorocarbons do to the ozone layer. Halon extinguishers are red, and halon is used in the Shoker fire-suppression system. 1 Bromochlorodifluoromethane decomposes into chlorine and bromine radicals, which scavenge hydrogen and oxygen radicals essential for keeping combustion going. 39 To use a halon extinguisher, “spritz” at the base of the burning material until the fire is out. Watch the materials for several minutes in case they re-ignite. Dry Chemical Extinguishers, Type ABC The ABC dry-chemical extinguisher, which is suitable for Type A, Type B and Type C fires, is filled with ammonium dihydrogen phosphate (NH4H2PO4). This interferes with combustion in a manner similar to halon extinguishers. It also melts at about 350° C, forming a crust that insulates Class A fuel from oxygen. Burning Class D materials can liberate oxygen from the phosphate ion and so this type of extinguisher should NOT be used for Type D fires. Dry-chemical extinguishers are normally red, with small nozzles (and sometimes short hoses). To use a dry chemical extinguisher, envelop the flames in a cloud of powder by spraying at the base of the fire. Dry Chemical Extinguishers, Type BC The BC dry-chemical extinguisher is suitable only for Type B and Type C fires. These extinguishers are filled with sodium or potassium bicarbonate. The bicarbonate salt interferes with combustion in a manner similar to halon extinguishers. However, enough heat can convert bicarbonate into CO2 gas and so this type of extinguisher should NOT be used for Type A or Type D fires. This type of extinguisher is better for Type C fires because the residue is much easier to clean up than the ABC dry chemical. Dry-chemical extinguishers are normally red, with small nozzles (and sometimes short hoses). To use a dry chemical extinguisher, envelop the flames in a cloud of powder by spraying at the base of the fire. Type D Extinguishers The cheap version is a bucket of sand, which isolates the Type D combustible from oxygen. Sand is silicon dioxide, which is too stable to liberate oxygen even under extreme heat. Commercial Type D extinguishers are called Metal-X extinguishers and contain a propellant as well as sand. These extinguishers are NOT suitable for any other type of fire. To use a Type D extinguisher, cover the burning materials completely. Which type(s) of fire extinguishers are in Shoker Science Center? In your dormitory? Elsewhere? 40 TECH 700: Practicing Safety in the Organic Chemistry Laboratory This page is a placeholder. Insert your copy of TECH 700 here. 41 42 CEM 221 Laboratory Final Examination A laboratory final examination is given in CEM 221 only. The lab final is a multiple-choice/true-false examination in three parts: A. Fire safety. This section is based on “Fire extinguishers and inflammable materials.” It tests your ability to identify the proper fire extinguisher for a variety of situations, and your knowledge of the basics of fire prevention and fire fighting. B. Chemical safety. You have been receiving chemical safety instructions in your lab procedures and discussions all term (including the first lab period, when you signed a safety contract). This section is based on those instructions and tests your ability to identify appropriate procedures for handling a variety of substances and situations. C. Laboratory equipment. This section tests your ability to identify the appropriate reagents and apparatus to use for a number of different laboratory tasks. You must pass Sections A and B with a perfect score to complete this course. You may retake the examination as often as necessary for this; but only your first exam score will count toward your course grade. 43 Experimental Techniques The following experiments are intended primarily as practice in necessary techniques in the organic chemistry laboratory. The indispensible reference for these experiments is Zubrick’s Organic Chem Lab Survival Manual. Not all of these experiments will be used in any given academic year. 44 TECH 701: Measuring the Melting Points of Compounds and Mixtures This page is a placeholder. Insert your copy of TECH 701 here. 45 46 Biosynthesis of ethanol from molasses Introduction Sugar cane is the most useful source of ethanol for biofuels, because of its high sugar content. Molasses (the byproduct of sugar refining) is typically processed into alcohol rather than the refined sugar itself, which is more valuable. Alcohol is obtained from molasses by fermentation of its sugars by yeast, and is isolated by distillation. Other crops that contain sugar can be used to make ethanol; corn, for example, or sugar beets. But the yield of alcohol, and therefore the efficiency of the process, is dependent on sugar content, because sugar is what yeast convert into ethanol. Energy yield estimates indicate that sugar cane yields ten times the energy required to process it into ethanol, while corn yields very little more than its energy input. Fermentation vats normally have “fermentation locks,” U-tubes with water filling the curve. These serve to permit the exit of carbon dioxide (a by-product of fermentation) and prevent the entrance of air. Oxygen in the air would further oxidize ethanol to acetic acid. Techniques used: glassworking; gravity filtration; distillation. Use the index in Zubrick to find descriptions. Minimum Safety Standards for this experiment 1. Hot glass looks the same as cold glass! Before picking up a piece of glassware, be sure to check that it is cool enough to handle. 2. The ethanol we will produce is no more toxic than any other distilled liquor, but it is not likely to taste very good, and it’s being generated in glassware that is not cleaned to food-quality standards. So don’t try to drink it. Disposal All wastes and products produced in this experiment may be thrown in the trash or flushed down the sink, with one exception: Glass waste is to be placed in the glass disposal container provided. If it is hot, sure to let it cool before disposing of it! The disposal container is plastic-lined. Procedure 1. You will be making your own tubing for this experiment. Cutting, bending and polishing glass tubing will be demonstrated for you by the instructor. 2. Each student will need to cut one piece of rubber tubing about two to three inches (five to seven centimeters) in length. Each student will need to cut and fire-polish two pieces of glass tubing, one about one to two inches long (two to five centimeters) and one about four to five inches long (ten to twelve centimeters). 3. In the burner flame, carefully bend a right angle about 1½ inches (3 centimeters) in from one end of your long piece of glass tubing. Anneal the bend in the flame, as demonstrated by the instructor. Do not allow your bend to collapse! 4. After your glass tubing has cooled, use glycerine as lubricant to insert the short end of the bent tube into a one-hole rubber stopper that fits the top of a 250-mL Erlenmeyer flask. Connect the two pieces of glass tubing using the piece of rubber tubing. 5. Into the 250-mL flask, place 70 mL of molasses and 70 mL of deionized water. Add about 0.5 g of bakers’ yeast to the flask. Swirl gently until everything is well-mixed. Label your flask properly. 47 6. Fill a test tube about half-full with water. Stopper your flask, and place the other end of the tubing in the water. This is a fermentation lock: carbon dioxide can get out, but air cannot get back into the fermentation flask. Allow your flask to stand for at least a week, checking periodically to ensure that the water in the test tube is not getting too low. 7. Open your flask and gravity-filter your fermentation mixture into your 250-mL round-bottomed flask. Set up a simple distillation, being sure to leave enough room under your still pot for a Bunsen burner and a ring support with metal gauze. Be sure to clamp your apparatus properly. 8. After the instructor approves your still setup, distill your mixture fairly rapidly (one or two drops per second) and stop collecting just below the boiling point of water. 9. If a fractional distillation is to be performed, set up a fractional distillation with your initial distillate in a clean still pot, and heat carefully; try to keep the distillation rate at or below a drop per second. Record the temperature range for each fraction collected, stopping the distillation at 97°C. Fractions are identified by a rapid change in temperature and a concurrent increase/decrease in distillation product (see Zubrick). 10. Determine the density of each fraction 1 by massing a portion that is precisely measured using a volumetric pipet. Use the table below to determine the alcohol content in each fraction. 11. Record the total volume of “pure” ethanol collected and show your product to the instructor for an appearance grade. Dispose of your product by flushing it down the sink. For the report Report the volume and ethanol content of your distillate(s). Calculate how much total ethanol you have made, excluding any remaining water. Extra credit: Investigate the different types of yeast used for ethanol fermentation and their efficiency compared to bakers’ yeast. 1 Or the density of the product of the simple distillation if you did not fractionate. 48 Aqueous Ethanol (EtOH) content Density g/mL % EtOH by weight % EtOH by volume g EtOH per 100mL Density g/mL % EtOH by weight % EtOH by volume g EtOH per 100mL 0.989 5 6.27 4.95 0.856 75 81.30 64.17 0.982 10 12.44 9.82 0.843 80 85.49 67.48 0.975 15 18.54 14.63 0.831 85 89.48 70.63 0.969 20 24.54 19.37 0.828 86 90.25 71.23 0.962 25 30.46 24.04 0.826 87 91.02 71.84 0.954 30 36.25 28.61 0.823 88 91.77 72.43 0.945 35 41.90 33.07 0.821 89 92.53 73.03 0.935 40 47.40 37.41 0.818 90 93.27 73.62 0.925 45 52.72 41.61 0.815 91 93.99 74.19 0.914 50 57.89 45.69 0.813 92 94.72 74.76 0.903 55 62.89 49.64 0.810 93 95.44 75.32 0.891 60 67.74 53.47 0.807 94 96.11 75.86 0.880 65 72.43 57.17 0.804 95 96.79 76.40 0.868 70 76.95 60.74 0.789 100 100.00 78.9 49 50 Pre-laboratory worksheet for Biosynthesis of ethanol 1. What component of molasses is processed into ethanol by yeast? 2. Why do we need to allow gas to escape from our fermentation vessel? 3. Why do we need to prevent air from entering? 4. Why does a distillation apparatus need an opening in it, somewhere? 5. I, __________________________________, have read and understood the experimental procedure. I am familiar with the hazards and with the required disposal procedures for this experiment. (Sign your name) 51 52 Isolation of caffeine from coffee or tea 1 In this experiment, you will isolate a naturally occurring organic chemical from one of its natural sources. In so doing, you will learn some common purification techniques. Experiments of this type were the first stirrings of organic chemistry, and, indeed, 180 years ago scientists used “organic chemistry” to mean what we think of as “biochemistry” today: the chemistry of living things. The first organic chemists were concerned with the isolation and identification of substances from materials that were once living, just as you will do in this experiment. As in all laboratory procedures, you should look up any known hazards associated with the substances you will be using or preparing. You may also want to look up the structure of caffeine and think about why this procedure works, on the basis of that structure. Techniques used: extraction and washing (Zubrick Chapters 15, 37), evaporation on a steam bath (Zubrick chapters 17-18), drying an organic liquid (Zubrick Chapter 10), mixed-solvent recrystallization (Zubrick Chapter 13), vacuum filtration (Zubrick pp. 110-112) You must hand in the pre-lab to be admitted to the laboratory. Minimum Safety Standards for this experiment 1. Hot glass looks the same as cold glass! Before picking up a piece of glassware, be sure to check that it is cool enough to handle. In particular, mind the caution about allowing dichloromethane to contact hot water. 2. Reagents which have an odor or an appreciable vapor pressure may not be used outside the hood except in closed containers. 3. Look up the MSDS for each reagent used. More specific cautions and procedures are given below. 4. 6M sodium hydroxide 2 is a strong caustic. Treat it appropriately. 5. You should be careful to avoid spillage during the extraction portion of the experiment. Not only will you spoil your results, but–because of the hazard associated with dichloromethane–you should avoid contamination. 6. You will be recrystallizing from a mixture of 2-propanol (a purer form of “rubbing alcohol”) and mixed hexanes, which are hydrocarbons. These solvents present a flammability hazard, and inhalation of large quantities can cause lightheadedness; use them in the hood as far as possible. 7. Wash your hands after completing the experiment. 1 Based upon “Caffeine Extraction from Tea – A Simplified Procedure,” by Edward G. Neeland, Okanagan University College, Kelowna, British Columbia, Canada. 2 What does “6M” (stands for “six molar” or “six moles per liter”) mean? If you don’t know, consult your general chemistry textbook! 53 Disposal caffeine Discard in the wastebasket. dichloromethane The dichloromethane you use will be evaporated. Wipe up spillage with paper towels and allow the dichloromethane to evaporate in the hood. The paper towels may then be disposed of in the wastebasket. drying agents Allow to stand in the hood until dry; then throw in the wastebasket. hexanes Recrystallization solvents must be placed in the waste bottle provided. 2-propanol Recrystallization solvents must be placed in the waste bottle provided. Pure 2-propanol may be flushed down the sink. sodium hydroxide solutions Flush down the sink. tea bags Discard in the wastebasket. tea residue, including aqueous Clean with soap and water and flush down the sink. waste from the extraction Procedure In this experiment you will begin using clamps and rings to hold your glassware, something you will continue to do throughout the year. Unlike general chemistry, things will not be set up for you! Use your common sense: if something needs to be supported, support it. Use clamps to hold things by the neck; rings are better for resting larger, globular objects like separatory funnels. You may need to manipulate hot glassware with tongs. I suggest that you practice using the tongs before you spill something! 1. Bring 100-120 mL of tap water to a boil in a beaker. While it is heating, get between six and seven grams of either coffee or tea in a coffee filter. 2. When the water is boiling, remove it from the heat and add your coffee (tea) to the water. Stir occasionally, letting it steep for about five minutes. 3. While your coffee (tea) is steeping, place your coffee filter in the filter funnel provided to you. Filter the coffee (tea) when it has finished steeping. Cool the coffee to below 35ºC, using an ice bath. 4. Extract the coffee (tea) three times with 20-25 mL portions of dichloromethane. Wash the combined organic layers twice with 20 mL portions of 6M NaOH, and once with 20 mL of tap water. Remove the organic layer and dry it over the drying agent provided (calcium chloride). 5. Evaporate the dried organic layer in a beaker, using a steam bath. Recover as much product as you can from the beaker after the solvent has evaporated. Weigh your product. Obtain a melting point of your product, comparing it to an authentic sample of caffeine. 54 Although pure caffeine is white, the residue you obtain may be green or brown due to the presence of chlorophyll and tannins. Determine the mass of the crude product and find its melting point.1 How does the melting point compare with the literature value for caffeine? 6. Report your results (mass of coffee or tea, mass of caffeine obtained) to the instructor so that they may be compiled with those of other students. Compare your results to those obtained by your lab mates in your report. 7. If there is time, purify the crude caffeine by recrystallization. a. Place your crude caffeine 2 into a 25-mL Erlenmeyer flask. Heat a few mL of 2-propanol until it is almost boiling; then dissolve your caffeine in a minimum amount of hot 2-propanol. You will want to keep your flask warm in a hot water bath. b. Now allow your solution to cool to room temperature. With careful stirring, add 1 mL of hexanes and allow the mixture to stand for a few minutes. Collect the crystals of caffeine by vacuum filtration in a Hirsch funnel, and wash with small amounts of hexanes. c. Determine the mass and melting point of the purified caffeine. Did you lose much in the recrystallization step? For the report… The report will have a cover page with an abstract, and a body. No experimental section is required. The report should answer the questions asked in the procedure and pre-lab, with any additional comments you may wish to make about the experiment. All sources of information will be properly referenced. The body of the report will be a unified narrative, with a beginning, middle and end. Determine the weight percentage of caffeine in tea. 3 Answer the following question, reproduced from your pre-lab: This procedure is a modification of the standard one, in which tea bags are boiled for twenty minutes. What advantages and disadvantages do you think the procedure we used might have over the other? Does your answer to this question differ from the one you gave in the pre-lab? 1 It is not necessary to do this right away; you can set aside a sample of your crude caffeine in a melting-point capillary tube for later. 2 If you do not have at least 80 mg of crude caffeine, you should combine your crude caffeine with another student’s. 3 The approximate amount of caffeine in black tea is 25-110 mg per teabag. 55 56 Pre-laboratory worksheet for Isolation of caffeine from tea 1. This procedure is a modification of the standard one, in which tea bags are boiled for twenty minutes. What advantages/disadvantages do you think the procedure we will use might have over the other? 2. You will extract the tea three times with dichloromethane. Why not six, or ten times? Why not just once? (HINT: see Zubrick, Chapter 37!) 3. You will wash the dichloromethane solution with strong base. Explain a possible reason for this. (No, dichloromethane is not an acid. If that were so, it would be hazardous to treat it with concentrated base! Why? What is the extraction doing for you?) 4. Consider the chemicals used for this experiment. What realistic hazards are present? What safety procedures are necessary beyond wearing glasses and gloves? 5. I, __________________________________, have read and understood the experimental procedure. I am familiar with the hazards and with the required disposal procedures for this experiment. (Sign your name) 57 58 Resolution of a Racemic Mixture: α-Methylbenzylamine Since enantiomers have identical physical properties, it can be very difficult to resolve a racemate (a 50:50 mixture of enantiomers) into the two enantiomers of which it is composed. The only way this can be accomplished is by use of another chiral compound. 1 In this experiment, you will use optically pure (+)-tartaric acid to resolve a racemic mixture of α-methylbenzylamine or (±)-MBA. As a Brønsted base, MBA will react with tartaric acid to form a salt. Half of the salt will be (+)-ammonium-(+)-tartrate and the other half will be (-)-ammonium-(+)-tartrate. These two compounds are chiral, but they are not enantiomers of each other. Rather, they are diastereomers. Diastereomers are not mirror images of each other, and do not have identical physical properties. It happens that one of the diastereomeric ammonium tartrates is less soluble in methanol than the other, so the salts are separable by selective crystallization. OH OH H OH H OH O O H3C H OH O H OH OH + + H2N H H (+)-MBA-(+)-tartrate salt (+)-tartaric acid H3C H 3N O O NH2 H OH H OH CH3 + (_)-α-methylbenzylamine (MBA) O O H OH O NH3 H CH3 (-)-MBA-(+)-tartrate salt What sort of reaction is shown above? 2 Classifying the type of reaction will be helpful to you as you think about the purposes of the reagents used in this experiment. Techniques used: crystallization (Zubrick Chapter 13), vacuum filtration (Zubrick pp. 109-112), extraction (Zubrick Chapters 15, 37), evaporation under reduced pressure (Zubrick Chapter 22), polarimetry (Bruice Chapter 5). Minimum Safety Standards for this experiment 8. Hot glass looks the same as cold glass! Before picking up a piece of glassware, be sure to check that it is cool enough to handle. In particular, mind the caution about allowing dichloromethane to contact hot water. 9. Reagents which have an odor or an appreciable vapor pressure may not be used outside the hood except in closed containers. 1 See Bruice, section 5.16, for a description of another procedure for resolving a racemate. How is our procedure similar to that discussed in Bruice? How is our procedure different? 2 See Chapter 1 of Bruice! 59 10. Look up the MSDS for each reagent used. More specific cautions and procedures are given below. 11. Tartaric acid is mildly corrosive and will irritate your eyes and mucous membranes if you touch them with tartaric acid on your fingers. Treat tartaric acid with respect and wash your hands thoroughly after handling. 12. α-Methylbenzylamine is smelly, caustic and of unknown toxicity. Treat it with respect, handle only in the hood, and wash your hands thoroughly with the dilute acid solution after handling. Wipe up ALL spills immediately and keep the paper towels in the hood; they MUST NOT BE THROWN IN THE TRASH before treatment with dilute acid as described in the Disposal section below. 13. You will use a dilute (ca. 0.1 mol/L) solution of hydrochloric acid to clean and deodorize α-methylbenzylamine residues. This solution is innocuous provided you take care to avoid contact with your eyes or mucous membranes, and wash your hands immediately after use. 14. You will be using a 2M solution of potassium hydroxide during this experiment. KOH is a strong caustic, and a 2M solution is rather concentrated. Treat it with respect, and wash your hands afterwards. Be sure to remove jewelry when washing, so that you do not trap any of this reagent next to your skin. 15. Be sure to wash your hands (including rinsing with dilute acid) after mixing the initial reaction solution to remove any traces of the reagents. Also wash after handling any other substance produced in this experiment. Disposal amine/tartaric acid salt Small residues may be cleaned with soap and water and flushed down the sink. Be sure to rinse the newly cleaned item with dilute acid. methanol solution of α-methylbenzylamine Place in the waste bottle provided; the instructor will treat it with acid to deactivate the amine. methanol May be flushed down the sink. mother liquor from crystallization May be flushed down the sink; be sure to rinse the container with dilute acid. α-methylbenzylamine Residues should be cleaned with the dilute acid solution provided and flushed down the sink. Do not try to clean with acetone or water; you must use acid to remove the amine odor. All objects contaminated with this substance must be treated with LARGE quantities of the dilute acid, until the odor has vanished. potassium hydroxide solution May be flushed down the sink. residual aqueous solution from extraction Flush down the sink in the hood. Wash glassware with water, then with copious quantities of acid, then with water again. tartaric acid Residues should be cleaned with water and flushed down the sink. All glassware, bench tops and trash cans WILL be checked for odor. Failure to properly treat waste is a safety violation. If the person responsible cannot be determined, the entire section will be penalized. 60 Procedure, First Week 1. In a 500-mL Erlenmeyer flask, mix 18.0 g (+)-tartaric acid with 260 mL methanol. Heat the mixture on a hot plate until the tartaric acid dissolves, then remove from heat. Carefully add 14.5 g (±)-MBA. 1 Keep the amounts used within three or four percent of the amounts called for! CAUTION: Exothermic reaction! Boilover hazard! 2. Let the solution cool for about 15 minutes, stopper the flask, and let the solution stand for a week to allow crystal formation. Be sure to label your flask correctly. After the lab period is over, the professor may add a seed crystal to your solution to encourage it to form the “correct” crystals. 2 Procedure, Second Week 1. When crystals have been obtained, recover them by vacuum filtration, washing them with a small amount of methanol. What is the identity of the crystalline product? Weigh your crystals and report the yield, based on the total amount of MBA used. What yield would you expect if your separation were perfect, that is, if you got 100% of a single diastereomer? 2. Dissolve the crystals in 70 mL of 2M potassium hydroxide. (What is the purpose of the potassium hydroxide?) Be sure all your crystals have dissolved! 3 Extract this solution twice with 20-mL portions of dichloromethane, using the separatory funnel. Combine the dichloromethane extracts in a 100-mL round-bottomed flask. WARNING. From this point on, your product will be smelly. It must be either contained or kept in the hood at all times. Thoroughly rinse anything your product touches with the acid wash solution provided. 3. Evaporate the dichloromethane under reduced pressure on the rotary evaporator, using a warm water bath. Be sure to weigh your product when it is free of dichloromethane (use a stoppered flask for this; you may use your round-bottomed flask with the ground glass stopper, but be sure to set it in a beaker so it can’t tip over). 4. Dilute your product to a total volume of between 10 and 12 mL with methanol, and record the exact volume after dilution. Calculate the concentration in grams of MBA per milliliter of solution. Mix well with a stirring rod. 5. Use the polarimeter to examine your MBA solution. The specific rotation of MBA is 40.3°. 4 Determine whether you have made (+)-MBA or (-)-MBA, and record the optical rotation you observe. (From this you can determine the optical purity of your product.) When you have taken this measurement, put the MBA solution into the appropriate waste bottle. 1 Measure the MBA by volume, in the hood; use its density to determine the mass. 2 You may get large, prism-like, crystals (a single diastereomer) or small needle-like crystals (contaminated with the other diastereomer). You can still get an optical activity reading with the needle-like crystals. 3 If they do not dissolve completely, add more potassium hydroxide solution. Do not add more than another 25 mL, or your mixture will not fit in the separatory funnel! 4 S. Budavari, M.J. O’Neil, A. Smith and P.E. Heckelman, eds. The Merck Index, 11th Ed. Rahway, NJ: Merck & Co., Inc. (1989). 61 For the report Show all reactions involved in this experiment in your report. Report and discuss all observations, as well as the optical purity of your product and the mole fractions of (+) and (-) MBA enantiomers. Finding optical purity and mole fraction of enantiomers Optical purity Optical purity (e.e., “enantiomeric excess”) is defined as the percentage relationship between the observed optical rotation and that which would be expected if the compound were all one enantiomer. It may be found by the following equation: e.e. = [α ]D observed rotation × 100% × length(dm ) × concentration(g / mL ) where [α]D is the specific rotation of the substance. Mole fraction of enantiomers The mole fraction of enantiomers may be found from the optical purity. Remember that each molecule of the (+) enantiomer cancels one molecule of the (-) enantiomer; any rotation left over stems from the presence of one enantiomer in excess of the other. The lower mole fraction is found by 100% − e.e. ; 2 the higher mole fraction is found by adding the e.e. to the lower mole fraction. (Think about it, and you will see that this makes sense.) For example, suppose you took 10g of a compound with a specific rotation of 100° and diluted it to 15 mL. The concentration would be 0.67 g/mL. Now, using a 0.95-dm cell, you observe a rotation of +8.4°. The optical purity would be found by e.e. = 8.4 × 100% = 13% 100 × 0.95 × 0.67 The (+) enantiomer is present in excess over the (-) enantiomer (since we observed a (+) rotation). Therefore the mole fraction of the (-) enantiomer is (100-13)/2 or 43.5%; the mole fraction of the (+) enantiomer is 43.5+13=56.5%. 62 Schedule for “Resolution of a Racemic Mixture” Week 1 1. Perform the initial steps in “Resolution of a Racemic Mixture.” You will leave your product to crystallize until next week. Be sure to label your flask properly! 2. Orientation to the polarimeter. You will learn to take readings, and determine the concentration (in grams/mL) of an unknown solution of an enantiomerically pure chiral compound. Read the section on polarimetry in Bruice Chapter 5 before coming to lab. The concentration of an optically-pure substance may be determined by measuring its optical rotation, or the extent to which it rotates plane-polarized light. To determine this, you must look up the compound’s specific rotation [α]D in a standard reference, 1 and use the equation [α] = [α]D × d × c where [α] stands for the observed (measured) optical rotation of the solution, d stands for the path length in decimeters (not meters or centimeters!), and c is the concentration in grams per milliliter. 3. Making molecules, an exercise using your molecular model kit. Week 2 Perform the steps in “Procedure, Second Week.” 1 Specific rotations are always reported in the reference literature as positive numbers. However, levorotary compounds (negative rotation) will have a negative specific rotation, of course! 63 64 Pre-laboratory worksheet for Resolution of a racemic mixture, week 1 1. Why is it impossible to just distill the (+)-α-methylbenzylamine away from the (-) enantiomer? 2. What does making the tartrate salt do for you? Why can’t you just use acetic acid? 3. Why is dilute acid so effective at destroying the odor of the amine? How does it help in washing the amine away? (HINT: what chemical reaction is involved? What does the reaction do for the water solubility of the amine? 4. Consider the chemicals used for this experiment. What realistic hazards are present? What safety procedures are necessary beyond wearing glasses and gloves? 5. I, __________________________________, have read and understood the experimental procedure. I am familiar with the hazards and with the required disposal procedures for this experiment. (Sign your name) 65 66 Pre-laboratory worksheet for Resolution of a racemic mixture, week 2 1. What is the purpose of treating the tartrate salt with strong base? (HINT: think about the chemistry involved in making the salt. How will the base undo that chemistry?) 2. When you perform the extraction, which layer do you expect to be on the bottom? Why? 3. Why is it reasonable to evaporate dichloromethane under reduced pressure, without losing an appreciable quantity of the amine? (HINT: what are the boiling points of dichloromethane and α-methylbenzylamine?) 4. Consider the chemicals used for this experiment. What realistic hazards are present? What safety procedures are necessary beyond wearing glasses and gloves? 5. I, __________________________________, have read and understood the experimental procedure. I am familiar with the hazards and with the required disposal procedures for this experiment. (Sign your name) 67 68 Thin Layer Chromatography Chromatography is the most useful and widely applicable method available for separating chemical substances. There are many kinds of chromatography, including paper chromatography, column chromatography, flash chromatography, reverse-phase chromatography, ion exchange chromatography, gel permeation chromatography, high pressure liquid chromatography (HPLC), gas chromatography (GC), and so on. All varieties of chromatography operate on the same principle: the partition of material between a stationary phase and a mobile phase. A sample (such as a mixture of unknown compounds) is adsorbed on the stationary phase (sometimes contained in a column), and the mobile phase is allowed or forced to flow past the sample. Each compound spends part of its time in the mobile phase, and part on the stationary phase. The amount of time spent dissolved in the mobile phase or adsorbed on the stationary phase is characteristic of each compound, and determines how fast that compound moves through the system. Read more about chromatography in general, and about TLC in particular, in Zubrick, Chapters 27 and 28, or one of the available organic laboratory manuals in the lobby of Shoker Science Center. In this experiment, we will perform thin layer chromatography (TLC). TLC is a powerful and simple tool for identifying, determining the purity, and sometimes (preparative TLC) for purifying a sample of an organic compound. While Zubrick explains how to make your own TLC plates, we will be using commercially-prepared plates. Therefore you will not have to prepare TLC plates. In TLC, a thin layer of an adsorbant, such as silica gel or alumina, is bound to a solid support, such as a glass plate or a plastic sheet, forming the stationary phase. Silica gel is hydrated silicon dioxide (SiO2•nH2O, in which every surface oxygen atom is an OH group), and compounds that are capable of hydrogen bonding 1 will bind to it tightly. The less polar a compound is, 2 the less strongly it will bond to silica gel and the faster it will move up the plate. In a typical procedure, a spot of the sample is applied near the bottom edge of the TLC plate. The plate is then placed vertically with the bottom edge immersed in a solvent. NOTE: The spot should be completely out of the solvent pool! The solvent flows up the plate by capillary action, serving as the mobile phase. The organic compounds in the solvent will partition between the mobile and stationary phases to a degree characteristic of each, depending on * The polarity of the compound (more precisely, its tendency to form hydrogen bonds) and * The polarity of the solvent (more precisely, its ability to act as a donor or acceptor of hydrogen bonds). The more tightly the compound binds to the silica gel, the more polar the solvent must be to move it along the plate. You must hand in the pre-lab to be admitted to the laboratory. 1 Look up hydrogen bonding in Bruice. 2 Strictly speaking, binding to silica gel measures the ability to form hydrogen bonds. Even a relatively non-polar molecule will bind strongly to silica gel if it has several sites that can hydrogen-bond. 69 Minimum Safety Standards for this experiment 1. Reagents which have an odor or an appreciable vapor pressure may not be used outside the hood except in closed containers. 2. Look up the MSDS for each reagent used. More specific cautions and procedures are given below. 3. Be sure to wash your hands after handling any of the substances used in this experiment. 4. All of the solvents used in this experiment are inflammable. While no open flames will be used in this experiment, you should be alert to the possibility of spark or flame from other people working in your area. To minimize the fire hazard, volatile organic solvents should always be used in the hood. Disposal acetanilide Residues should be cleaned with acetone and flushed down the sink. Solid spills may be placed in the trash can. acetone Any quantity of acetone may be flushed down the sink. caffeine Residues should be cleaned with acetone and flushed down the sink. Solid spills may be placed in the trash can. diethyl ether Trace amounts may be cleaned with acetone and flushed down the sink. Quantities larger than about ¼ mL should be poured onto a paper towel in the hood and allowed to evaporate. hexane Trace amounts may be cleaned with acetone and flushed down the sink. Quantities larger than about ¼ mL should be poured onto a paper towel in the hood and allowed to evaporate. methanol Any quantity of methanol may be flushed down the sink. phenacetin Residues should be cleaned with acetone and flushed down the sink. Solid spills may be placed in the trash can. paper wet with solvent Allow the solvent to evaporate in the hood, and then throw in the trash can. Procedure In this experiment, you will attempt to find a good solvent system for separating three compounds by TLC: caffeine, phenacetin, and acetanilide. The solvent system may be a pure solvent or a mixture of solvents, though in this experiment you should not need to use a mixed solvent to achieve good separation. The solvents for you to choose from are, in order of increasing polarity, hexane, diethyl ether, acetone, and methanol. Like any volatile organic compounds, these solvents should be used only in a fume hood. Remember, the more polar the solvent, the faster the compounds will move. If the solvent system is too polar, all of the compounds will move with the solvent front and separation will not be achieved. If the solvent system is too non-polar, all of the compounds will remain at the origin and again, separation will not be achieved. The process of allowing the mobile phase to flow up the TLC plate is called developing the plate. Capillary action is limited in how high it can lift the mobile phase. If you wait too long the solvent front 70 will stop moving (because the solvent evaporates as fast as it advances). The compounds will not stop moving, and will pile up at the solvent front, destroying the separation. This is called overdeveloping. Overdeveloping can be partly prevented by saturating the developing chamber with the vapor of the solvent system. To do this, place a piece of filter paper along the wall of the chamber so that one end is dipped in the solvent. Then cover the chamber with a glass plate. For this experiment, a beaker will be the developing chamber and you will cover the chamber with a watch glass. When the plate has been developed, remove it from the developing chamber and lay it flat on the benchtop to dry. Be sure to mark your plates in some way so that you can tell them apart! You will look at the dry plates with an ultraviolet lamp, which will allow you to see the separation you have achieved. Outline the spots you see on the plate with pencil. The objective of this experiment is to identify an unknown sample as a mixture of two of the three known compounds. This will be done by determining the Rf values of the unknown in your best solvent system, and comparing it to the Rf values of the known compounds in the same solvent system. Of course, your best solvent system is the one that gives the largest difference in Rf between the three known compounds! The simplest way to accomplish this is to spot every TLC plate with your unknown and with each of the three knowns, so that you have four spots on every plate. You can confirm your identification by “double-spotting,” which is described in Zubrick. The Rf value is the ratio of the distance a compound traveled on a TLC plate and the distance the solvent front traveled. The Rf value is characteristic of a compound in a given solvent system. In your laboratory report, discuss the theory of thin layer chromatography, describe the most effective solvent system you found to separate the compounds, and identify your unknown. NOTE: Like all “instrument readouts,” your TLC plates, once developed and marked, should be fastened into your laboratory notebook. Do not use staples! Turn in a PHOTOCOPY of your TLC plates with your report. For the report Report the Rf values observed for each solvent system you used. Report the solvent system you think gave the best separation, and the identity of your unknown. Using the ArgusLab exercise below, correlate the structure of each compound with its observed mobility on the TLC plate. 1 BE SURE TO REPORT YOUR UNKNOWN NUMBER! 1 If a compound moves slowly, it can form more hydrogen bonds than a compound that moves quickly. What structural features of the three compounds participate in hydrogen bonding? 71 Acetanilide, Phenacetin and Caffeine: a study using ArgusLab In this exercise, you will use ArgusLab to build structures using templates, optimize geometries using molecular-orbital methods, and generate electrostatic potential surfaces that describe the distribution of electrical charge in a molecule. You will use the calculated charge surfaces to interpret the results of the Thin-Layer Chromatography experiment. You must discuss your results from this exercise in your lab report. CH3 NH NH CH3 O CH3 O O H3C O N N N H3C O acetanilide phenacetin Building acetanilide Click “Create new molecule” to bring up a new molecule window. If the “pencil-hexagon” button (“Show the Builder Toolkit”) is not selected, select it to bring up the Builder toolkit on the left side of your window. N CH3 caffeine NH CH3 O Select the “Rings” tab in the builder toolkit, and select the benzene ring (second row, far right). Right-click on the window to place a copy in the window, and left-click to deselect your new structure. Click the yellow “Selection mode” arrow, and right-click on one of the hydrogens. Use “Convert Hydrogen to Group” to change it to a “urea” group, “NHCONH2.” Right-click on the NH2 nitrogen and use “Change Atom” to change it to “C [sp3], C_3 tetrahedral.” Right-click on the dotted bonds to change them to single bonds, and then click on the “Add hydrogens” button in the top row. Clean your geometry by clicking the “pliers” button. Save your structure as “acetanilide.” We will now optimize the geometry using a molecular orbital method. MO methods allow us to calculate atomic charges, which will let us find centers of negative and positive charge in a structure. In the top row, click the “hexagon with a circle” button (“Settings for a geometry optimization calculation”). In the dialog that appears, go to the upper left corner and select “AM1.” Select “OK” in the upper right corner, and your calculation should start. (It will take a minute or two.) When the calculation finishes, you will see a message below your structure. If it does not say “Geometry optimization converged !!”, you will need to run the calculation again. To restart the calculation, look in the upper row of buttons for one that looks like a Bunsen burner (“Run a calculation”). Select it, and your calculation will restart. Repeat the calculation until your geometry optimization converges, or (if it does not converge) you have run at least six calculation cycles. Save your structure again by closing the structure window and clicking the “Yes” box. 72 Building phenacetin You may have noticed that “phenacetin” is simply acetanilide with an ethoxy group on the opposite end of the ring. We can build phenacetin from acetanilide. NH CH3 O H3C O Open your optimized acetanilide structure, and save it as “phenacetin.” Now change the appropriate hydrogen (opposite the nitrogen atom) to an OH group. Convert the OH hydrogen to a methyl group, then change one of the methyl hydrogens to another methyl group so that you have an ethyl group attached to oxygen. You should now have the structure shown above right. Clean the geometry. Set up and run an AM1 geometry optimization calculation, and repeat it until the geometry optimization converges, or until you have run at least six calculation cycles. (This will take longer than acetanilide because there are more atoms.) Save your structure by closing the structure window and clicking “Yes.” Building caffeine Building this structure will be more involved than for acetanilide or phenacetin, because there is no template in ArgusLab for a purine. N H Instead, we will use the “indole” template (bottom row, center of the “Rings” tab; see the structure of indole, above right). Add a model of indole to your window. Now refer to the structure of caffeine, below right. indole CH3 In the five-membered ring, change the appropriate carbon atom to “N [sp2] > N_R, aromatic” and delete the hydrogen atom. Then change the hydrogen atom on the other nitrogen to a methyl group. O In the six-membered ring, change the appropriate two carbon atoms to “N [sp2] > N_R, aromatic” and change their hydrogen atoms to methyl groups. H3C Change the hydrogen atoms on the two appropriate carbon atoms to “O [sp2] > O_2, non-aromatic” and change the bonds to these oxygens from single to double by right-clicking on the bonds. N N N N O CH3 caffeine Right click on each ring bond and change it to single or double as appropriate. Clean the geometry. Save the structure as “caffeine.” Set up and run an AM1 geometry optimization calculation, and repeat it until the geometry optimization converges or you have run at least six calculation cycles. (This will take longer than acetanilide because there are more atoms.) Save your structure again by closing its window and clicking “Yes.” 73 Electrostatic potential surfaces To interpret your TLC results, you need to identify points on each of the three known compounds that can act as hydrogen-bond acceptors, that is, as strong Lewis bases. They will therefore be centers of negative charge in the molecule. You can identify such points by generating the electrostatic potential surface for a molecule. In ArgusLab 4.1.0, this is extremely simple. Once a molecule’s geometry is optimized, click the “Easter egg” button in the second row of buttons (“Quick-plot ESP-mapped density surface”). Centers of negative charge are indicated in red. If you right-click on the surface and click “modify surface,” you can make it translucent so that you can see atoms underneath. It will be easier to compare the three compounds if you put them in tiled windows. Remember that compounds that are better hydrogen bond acceptors (that is, that have more negativelycharged sites) will adhere more strongly to silica gel. Order the three compounds in the order you expect them to move on silica gel, based on their calculated electrostatic potential surfaces. Is this trend supported by your results? 74 Pre-laboratory worksheet for Thin Layer Chromatography 1. Explain why you need to allow a TLC plate to dry completely after spotting, before either doublespotting or developing the plate. 2. Explain why you mark the TLC plate in pencil rather than in pen. (Zubrick should have something about this.) 3. Which type of fire extinguisher would you need to put out the most likely type of fire in this experiment? (Choose from water, CO2, ABC dry chemical, or Type D. More than one correct answer may exist.) Justify your answer. 4. Consider the chemicals used for this experiment. What realistic hazards are present? What safety procedures are necessary beyond wearing glasses and gloves? 5. I, __________________________________, have read and understood the experimental procedure. I am familiar with the hazards and with the required disposal procedures for this experiment. (Sign your name) 75 76 Analysis of fatty acid composition of lipids This procedure takes advantage of the ability of gas chromatography (GC) to separate a wide variety of compounds. It is a standard assay used in the oleochemical 1 industry, and in fact the procedure is adapted from a publication of Henkel Corporation’s Emery Group. Since lipids are typically far too large and involatile to be analyzable by GC, a lipid sample is first methanolyzed under basic conditions to yield a mixture of methyl fatty esters and glycerine, as shown. This reaction is called transesterification because it converts one ester (the lipid, a triglyceride) into another (the fatty methyl ester). Examine the mechanism of the Fischer esterification reaction in your textbook. Is there a Brønsted acid in the reaction shown below? What is the purpose of the Lewis acid, boron trifluoride? O CH2 OCR O CH2 OH CH OCR' O CH2 OCR" O BF3 CH OH NaOH CH3OH lipids + CH3OCR CH2 OH glycerine mixture of fatty esters The glycerine is removed by washing 2 the organic mixture with a saturated aqueous saline solution. The organic layer is dried,2 then analyzed by gas chromatography (GC).2 Techniques used from Zubrick, 6th Ed: microscale addition and reflux (Chapter 24), microscale extraction (Chapter 16), drying an organic liquid (Chapter 10), gas chromatography (Chapters 27, 32) Background Health effects of fatty acids Fatty acids are thought to have health effects depending on their degree of unsaturation. Saturated fatty acids (SFA) are known to raise blood cholesterol levels in humans and are thought to contribute to cardiovascular problems. Vegetable and fish oils which are high in polyunsaturated fatty acids (PFA) seem to have an opposite effect. Monounsaturated fatty acids (MFA) also lower total cholesterol and are probably at least as beneficial as PFAs. Unlike PFAs, MFAs lower total cholesterol without lowering levels of high-density lipoprotein (HDL, known as “good cholesterol”). You will analyze one type of cooking oil for the presence of various naturally-occurring fatty acids. You will report the ratio 1 From oleum (olive oil). The oleochemical industry derives its products from natural fats and oils. 2 This technique is discussed thoroughly in Zubrick. 77 MFA + PFA SFA as a rough indicator of “healthfulness” of the cooking oil you analyzed. This ratio may be compared to the results obtained by your classmates. Shorthand notation for fatty acids Most fatty acids have common names, which are used in preference to their IUPAC names because they are shorter. 1 However, the common names don’t tell anything about the structure of the fatty acid, and so a system of shorthand is in common use in oleochemical circles. The system is very simple: a fatty acid is identified by the number of carbons it contains plus the number of double bonds. For example, stearic acid is 18:0 and myristoleic acid is 14:1. Sometimes other prefixes are used to denote the geometry and position of the double bonds: thus, linolenic acid may be identified as c,c,c-9,12,15-18:3, and myristoleic acid as c-9-14:1. GC Analysis When methyl esters of saturated fatty acids are analyzed by GC, their retention times (Tr) increase with the length of the carbon chain according to the relationship log(Tr) ∝ N, where N is the number of carbons in the fatty acid. The retention times of unsaturated fatty methyl esters do not match those of saturated ones. When the GC stationary phase is polar, unsaturated esters take longer than saturated ones; when the stationary phase is non-polar, unsaturated esters show shorter Tr. The actual value of Tr depends on the number and position of the double bonds. ECL for Carbowax column 1.4 1.2 log Tr/min By comparing the retention times of a large number of unsaturated fatty methyl esters, a series of equivalent-chain-length (ECL) values has been worked out for various esters on different GC stationary phases. The ECL for a saturated methyl ester is (by definition) equal to N – for example, the ECL of stearic acid (methyl ester) is 18.0; that of myristic acid (methyl ester) is 14.0. However, the ECL of an unsaturated fatty ester will vary depending on the conditions. The graph at right shows three 18-carbon methyl esters; the C18 unsaturated esters are retained longer than the corresponding saturated ester on that particular column. 2 1 0.8 0.6 0.4 0.2 0 10.00 12.00 14.00 16.00 18.00 20.00 ECL 1 For example, linolenic acid is (Z,Z,Z)-9,12,15-octadecatrienoic acid. 2 On the column we will use, the unsaturated esters have a shorter retention time than the saturated ones. 78 22.00 Minimum Safety Standards for this experiment 5. Hot glass looks the same as cold glass! Before picking up a piece of glassware, be sure to check that it is cool enough to handle. 6. Reagents which have an odor or an appreciable vapor pressure may not be used outside the hood except in closed containers. 7. Look up the MSDS for each reagent used. More specific cautions and procedures are given below. 8. Sodium hydroxide solution is caustic; BF3/methanol complex is corrosive. Use appropriate precautions and wash carefully (including under watchbands and rings) after use. Disposal aqueous layer from separation Clean with water and flush down the sink in the hood. boron trifluoride/methanol complex, methanol solution Small spills should be carefully washed with water, a little at a time, then wiped up. Residues on glassware may be cleaned with soap and water and flushed down the sink. fatty acids Clean with soap and water and flush down the sink. glycerine Clean with soap and water and flush down the sink. heptane Residues should be cleaned with acetone and flushed down the sink. Leftover heptane solutions (more than 0.25 mL) may be poured out on paper towels in the hood, and allowed to evaporate. magnesium sulfate Flush down the sink. Solid spillage may be placed in the wastebasket. saline solution Flush down the sink. sodium hydroxide solution in methanol Clean with water and flush down the sink in the hood. Procedure 1. Obtain 1-2 drops of the fat or oil to be analyzed. Place it in a 5-mL vial with about 1 mL of 0.5 M methanolic NaOH solution and a boiling chip. Attach a condenser to the vial and heat the mixture gently until it comes to a low boil. This should take no more than 5-10 minutes. 2. Add 1-1.5 mL of methanolic BF3/methanol complex through the condenser and boil for two minutes. Add about 1 mL of heptane through the condenser, and boil for another minute. Remove the reaction vessel from the heat and allow it to cool. 3. Remove the condenser. Add 0.5-0.75 mL of the saturated sodium chloride solution to the reaction mixture and mix. Using a disposable pipet, transfer at least half of the organic layer to a small test tube and dry it over magnesium sulfate. 4. After allowing the heptane solution to dry, carefully remove ½-¾ of it from above the drying agent and place it into a small test tube. Add another 0.75-1 mL of heptane to the drying agent and mix; allow to settle for a few minutes. Again remove the liquid above the drying agent and combine it with your sample. Stopper the test tube securely. 79 5. Analyze your mixture by GC, and compare your chromatogram with that of the primary standard, which is a mixture of C-12 through C-18 saturated fatty acid methyl esters. Estimate the percent fatty acid composition of your lipid sample, including both saturated and unsaturated fatty acids. For the report… Report the estimated percent fatty acid composition of your lipid sample based on your GC results. Compare this to values taken from the table below. Calculate the “healthfulness” based on the ratio given above and report it, comparing to values obtained in your section for other samples. Fatty acid composition of some common oils 1 Fatty acid Tallow Lard Coconut oil 7% 2% 7% Lauric (12:0) 11% 8% Capric (10:0) Olive oil 3% Caprylic (8:0) Soybean oil 48% Myristic (14:0) 3% 2% 17% Palmitic (16:0) 26% 26% 8% Palmitoleic (16:1) 3% 4% Stearic (18:0) 23% 14% Oleic (18:1) 43% 43% 5% 23% 84% Linoleic (18:2) 2% 9% 3% 55% 5% Myristoleic (14:1) Linolenic (18:3) 1% 9% Other fatty acid tables may be found by a Google search for “fatty acid composition.” 1 Taken from a Henkel Group technical publication. 80 Pre-laboratory worksheet for Analysis of fatty acid composition of lipids 1. What is the function of the boron trifluoride in this experiment? 2. Why do we see a different retention time on the GC for fatty acids with different numbers of carbons? For fatty acids with the same number of carbons but different degrees of unsaturation? 3. In step 3 of the procedure, you are told to transfer the organic layer to a test tube. Which layer do you expect to be the organic layer: the top or the bottom? Why? 4. Consider the chemicals used for this experiment. What realistic hazards are present? What safety procedures are necessary beyond wearing glasses and gloves? 5. I, __________________________________, have read and understood the experimental procedure. I am familiar with the hazards and with the required disposal procedures for this experiment. (Sign your name) 81 82 Introduction to two-dimensional NMR spectroscopy The most important new tool introduced to organic chemistry in the past couple of decades is twodimensional (2D) NMR spectroscopy, although we ourselves will not be able to make as much use of it as we would with a higher-resolution NMR spectrometer. 1 2D NMR requires programming rather sophisticated experiments into the instrument. Our spectrometer is programmed for two kinds of 2D NMR: COSY (COrrelated SpectroscopY) and HETCOR (HETeronuclear CORrelation). • COSY is a 1H experiment. It correlates two 1D 1H NMR spectra, showing which peaks are coupled to which, as explained below. A COSY spectrum can be obtained in about 15 minutes, at one scan per slice; or 45 minutes, at four scans per slice. 2 • HETCOR is a 13C experiment, and thus requires much more sample than COSY. HETCOR correlates a 13C spectrum, on the horizontal axis, with an 1H spectrum, on the vertical axis, and is a way of determining which 13C peaks correspond to which 1H peaks. HETCOR is timeconsuming and troublesome to obtain, and we will use it only as a last resort. Using 2-dimensional NMR to solve structures In 2D NMR spectroscopy, signal correlation can be seen at a glance. Two 1D spectra are plotted against each other, and which peaks are coupled to which can be read off the 2D plot. For example, consider ethylbenzene. CH2CH3 Figure 1. The 1D 1H NMR spectrum of ethylbenzene. In Figure 1, we see the 1-dimensional 1H NMR spectrum of ethylbenzene. The five phenyl protons appear at 7.2 ppm, while the ethyl group is represented by the 2H quadruplet at 2.6 ppm and the 3H triplet at 1.2 ppm. The correlation of the CH2 and CH3 are obvious to those who know how to read splitting patterns, 1 At 60 MHz, 1H NMR peaks often overlap too much for good 2-dimensional resolution. 2 The more time-consuming 4-scan COSY is less prone to detecting false peaks, which are especially troublesome in the spectra of esters and other oxygen-containing molecules. 83 but there is another, easier way to establish that the hydrogens at 2.6 and those at 1.2 are vicinal to each other: the 2-dimensional COSY spectrum. In a COSY spectrum, two identical 1H NMR spectra are correlated. Spots on the diagonal from upper right to lower left represent self-correlation, the intersection points of identical peaks; but offdiagonal spots represent correlations between different hydrogens in the spectrum. In Figure 2, observe the couplings between the peaks at 1.2 and 2.7 ppm (circled). It is immediately apparent that the hydrogens represented by those peaks are vicinal to each other because their coupling produces an off-diagonal peak; unlike the 1D spectrum (Figure 1), no interpretation of splitting patterns is required. Notice that long-range coupling, such as that between the phenyl hydrogens and the ethyl group, is not seen in this spectrum; in general it is not found in 2D NMR. (The coupling peaks to 5.7 ppm are spurious.) A slightly more difficult example, at least at low resolution (60 MHz), is 2-butanone. Figure 2. The COSY spectrum of ethylbenzene. Circled peaks are correlations between the peaks at 2.7 and 1.1 ppm. O Figure 3. The 1D 1H NMR spectrum of 2-butanone. In Figure 3, it’s easy to see the triplet at 1.0 ppm (although it is obscured by sideband peaks); the integration is 3, which means that it corresponds to three hydrogens coupled to two vicinal hydrogens. But the peaks from 2.0-2.7 ppm are more difficult. Even discounting the sidebands, the large singlet from the COCH3 hydrogens overlaps the quadruplet from the COCH2, which are coupled to the methyl triplet. 84 While the integration is a tremendous aid to interpretation of this spectrum, it is possible to miss the triplet-quadruplet pattern due to the overlap of the other methyl group. But in the COSY spectrum (Figure 4), matters become easier. Notice that the large peak at 2.2 ppm (a) is not coupled to any other hydrogens, but there is a peak (b) from 2.2-2.6 that is strongly coupled (c) to the peak (d) at 1.1 ppm. 1 The 2D spectrum allows us to disentangle the coupling patterns and show that we do, in fact have independent CH3— and CH3CH2— groups. 2 Figure 4. The COSY spectrum of 2-butanone. The large peak at 2.2 ppm (a) is not coupled, but the peaks from 2-2.6 (b) and 1-1.3 (d) are coupled at (c). Figure 5. COSY spectra of 1-butanol (left) and 2-methyl-1-propanol (right). Find the correlation patterns. Don’t be confused by the OH peak; that hydrogen exchanges too rapidly for correlation, and comes around 3 ppm. 1 Notice how the triplet/quadruplet pattern is preserved in the cross peaks. 2 Note also that we must use both 1D and 2D spectra to obtain all available information: COSY doesn’t do integration! 85 Now compare the COSY spectra for 1-butanol and 2-methyl-1-propanol (isobutyl alcohol), in Figure 5. The limitations of 60 MHz resolution are immediately apparent. There is no separation between the C2 and C3 methylene groups in the spectrum at left, and so it is difficult to see the different coupling between n-butyl alcohol (Figure 5, left) and isobutyl alcohol (Figure 5, right). However, we are able to see longrange coupling between the C1 methylene and the physically close terminal methyl groups (C3) in the right-hand spectrum, while this coupling is not present in the left-hand spectrum. With the presence of two doublets in the 1D spectrum of 2-methyl-1-propanol (easily visible in the right-hand spectrum), this allows us to distinguish it from 1-butanol. As a further exercise, consider the two COSY spectra shown in Figure 6. One is 2-phenylbutanoic acid, the other 3-phenylbutanoic acid. Can you tell which is which? Figure 6. Which COSY spectrum is 2-phenylbutanoic acid, and which is 3-phenylbutanoic acid? Answer on page 89. Aryl and acid hydrogens are not included. 86 Virtual Spectroscopy 1 Student: ____________________________________ You will use the Integrated Spectral Database System for Organic Compounds (SDBS) 2 for this assignment. Compound numbers refer to the internal reference number for each compound on SDBS. You have been assigned the primary compound number ________ and five companion compounds numbered ________, ________, ________, ________ and ________. 1. Look up the primary compound on SDBS. 2. Draw the structure (given on SDBS). 3. Print the four spectra: MS, IR, 1H NMR and 13C NMR. 4. For each companion compound: a) Look up the compound on SDBS. b) Identify structural features that would be expected to spectroscopically distinguish the companion compound from your primary compound. For example, the companion may have a carbonyl group where the primary does not. c) View the spectra of the companion compound. Choose one distinguishing spectroscopic feature, and write a brief explanation of how that feature is inconsistent with the structure of your primary compound. You are expected to choose a feature that would convincingly differentiate the companion from the primary compound. For this reason, it is most likely that you will need to use several different spectroscopies for your different compounds. See the examples below. Example. The IR of compound #2428 1,4-benzodioxane shows no peak in the carbonyl region 1680-1740 cm-1. My compound #1725 benzyl formate has a C=O group which absorbs strongly at 1725 cm-1. Example. The 1H NMR of compound #1448 para-toluic acid has a methyl group at δ = 2.375. My compound does not have a methyl group and shows no peaks in that region. Example. The MS of compound #725 methyl benzoate shows the base peak at 105, which is 31 mass units from the molecular ion peak at 136. This corresponds to loss of a methoxy group. My compound does not have any methoxy groups and shows only a small peak at 105. 3 1 Kandel, M.; Tonge, P.J. J. Chem. Ed. 2001, 78, 1208-1209. 2 http://riodb01.ibase.aist.go.jp/sdbs/cgi-bin/cre_index.cgi, maintained by the Japanese National Institute of Materials and Chemical Research. 3 Lest you think this is an easy way out, be advised that all your compounds are constitutional isomers! 87 Qualitative Analysis: Identification of Unknown Organic Compounds Scenario You work for a chemical testing laboratory. A client has sent in several samples for identification, from reagent bottles that have lost their labels. All that is known about these samples is that they are organic chemicals; they must be identified before a waste hauler will take them away. You must identify the unknown organic substances from their melting or boiling points and mass, IR, and NMR spectra. 1 The experiment You will be given two organic samples identified only by number. The mass spectrum of each sample will be provided to you. You will obtain the boiling or melting point, the IR spectrum and any NMR needed 2 for full structural identification of each sample. You will identify each sample using the data you have obtained. Your report will fully discuss the identification of each of your unknowns. Week 1 of the experiment You will be given your unknowns this week, along with their mass spectral and CHO elemental analysis data. Obtain the IR spectra of your unknowns, and submit a brief, informal report outlining the functional groups found. This report (with spectra attached) is due the following week. Last week of the experiment (indicated on the lab schedule) During the scheduled laboratory periods during this week you will have the opportunity to obtain any spectra you still need. The instructor will be available to assist you in obtaining good spectra. For the report… Discuss your spectra and other data in detail as they relate to your structural assignment. It is legitimate to use standard reference sources such as the Aldrich spectral libraries or SDBS, but you should credit them in your report if you do. It is NOT sufficient to simply state that your spectrum matches an authentic spectrum! You must discuss the structural elements indicated by your spectra! Minimum Safety Standards for this experiment Since the compounds you will be dealing with are unknowns, it is not possible for you to be certain of their toxicity. Therefore, you must treat ALL of the unknowns with special precautions. This includes working in the hood when possible and avoiding contact with the liquids and their vapors. The hoods MUST be on when you are obtaining micro melting points or preparing samples for spectroscopy. Safety glasses MUST BE WORN during sample preparation and when obtaining micro melting or boiling points. Disposal and Procedure You are permitted to prepare samples and perform spectroscopy, or boiling- and melting-point determinations, any time there is a science professor in the building. A professor does not need to be present in the laboratory. 1 This really happened to Dr. Berger in graduate school! 2 This may include 1H, 13C, DEPT, COSY, or HETCOR spectra. 88 Micro boiling points should be obtained as detailed in Zubrick. Clean the boiling point tubes with acetone, NOT soap-and-water. When obtaining IR spectra, use the round windows (with the 25-μm spacer) for liquid samples and the thick “mull plates” for solid samples. Solid samples must be ground fine and mixed with a small amount of mineral oil (a “Nujol mull”). IR cell parts should be cleaned with dry isopropyl alcohol. To obtain NMR spectra, you will use NMR tubes. NMR samples must be properly labeled, including the date prepared. Samples without proper labels will be discarded without warning. You are allowed to store prepared samples in the NMR tube, in the rack in the instrument room. However, no more than two NMR samples per student are allowed. We do not have infinite supplies of NMR tubes. Mix your NMR samples with chloroform-d. Solid samples should be mixed in a vial to ensure solubility before putting in the NMR tube! Floating solids will make your spectra unusable! If you are unable to dissolve all your solids, filter your sample into your NMR tube, through a small amount of cotton waste in a Pasteur pipet. See Zubrick, pp. 69-70ff. Samples for 13C NMR must have a concentration of at least 50% by weight 1 in chloroform-d. A good 1H spectrum may be obtained with any concentration over 5% by weight, or even less if you average several (no more than 20!) scans. For best results, NMR samples should have a total volume of ca. 5 mL; see the NMR sample height guide, posted by the hood. 2 Chloroform solutions (i.e. NMR samples) must be placed in the labeled waste bottle provided. This will remain true for the entire term. The waste bottle for NMR samples may be kept in the hood or on the benchtop as long as it is tightly closed. Clean your NMR tubes with isopropyl alcohol or acetone and store them upside down (to prevent dust contamination) in the beaker provided. Rinsings may be flushed down the sink. Check the sink afterward for odors! Answer to Figure 6 The left-hand spectrum in Figure 6 is 3-phenylbutanoic acid; the right-hand spectrum is 2-phenylbutanoic acid. How do the correlation patterns allow you to choose between the two structures? O C OH HO C O 1 Liquid samples should contain about 0.3 mL of the unknown and 0.2-0.3 mL of chloroform-d. For solid samples, weigh about 0.7 mmol of the solid and mix with 0.7 mL of chloroform-d. Samples should be prepared in a small vial and filtered into the NMR tube through cotton waste. 2 If the NMR sample is not tall enough, you will NOT get a good spectrum! 89 Identification of an unknown ester 1 This experiment will use all the spectroscopic knowledge you have, along with a number of tools used in the “real world” such as spectroscopic simulation. You will synthesize an unknown ester from an acid C10H12O2 and an alcohol C4H10O. 2 What is the formula of the resulting ester? You will purify the ester and identify it by NMR spectroscopy. Since the experiment is microscale you will not have enough for 13C NMR; a combination of 1D and 2D 1H NMR should be sufficient. You may use simulated one-dimensional spectra to help solve your structure; however, you must ask for the spectrum of a particular ester, and you are only allowed a total of three simulated spectra. HINTS: Look for patterns in your spectra that allow you to zero in on a particular set of structures. Pay particular attention to the hydrogens on the carbon bonded to oxygen (that is, the O-CH2 or O-CH), if any; where do such hydrogens typically appear? What does the integration in the 1D spectrum tell you? 3 You will have four weeks to solve the structure of your ester. Ask for help if you need it! Safety and disposal guidelines Nothing you will use in this experiment is more than an irritant, except for concentrated sulfuric acid which is quite corrosive. However, the acids and especially the esters are smelly; be thorough when disposing of them and ensure that, for example, pipets are cleaned before discarding so that no odor remains. Procedure 1. You will be given a mixture of an unknown alcohol (approx. 1.5 mmol; C4H10O) and an unknown acid (approx. 2.0 mmol; C10H12O2) in a 3-mL reaction vial. Add two drops of concentrated sulfuric acid. Insert a spin vane and attach a condenser; cap the condenser with a septum pierced by a syringe needle, to allow pressure equalization while minimizing exposure to atmospheric moisture. Reflux the mixture, with stirring, for one hour. Do not overheat as this will result in decomposition of your ester. 2. Dissolve the product in 1 to 1.5 mL of ether. Wash the organic layer twice with 3-mL portions of saturated sodium bicarbonate and once with 3 mL of saturated sodium chloride. Dry the organic layer and remove the ether by evaporation. 3. Obtain the IR spectrum of your product. If you have an excessive amount of alcohol or acid remaining, dissolve in ether and repeat the NaHCO3 and NaCl washings. It is essential that your ester be reasonably pure, or you will not get good NMR results! 1 This experiment is based on S.E. Branz, R.B. Miele, R.K. Okuda and D.A. Straus, J. Chem. Educ. 1995, 72, 659661. 2 The acids and alcohols used are purchased from Aldrich. This should allow you to narrow the range of possibilities. 3 Be careful! Past results have shown that students may get a mixture of the ester and the starting acid, so that integration may not be as useful as splitting patterns. If an acid peak appears in the one-dimensional 1H NMR, its integration may tell you what proportion of your product is ester and what proportion is acid, which should help in peak assignments. 90 4. Use NMR spectroscopy to identify your product. The instructor will be available to help you obtain good spectra. Simulated 1D 1H NMR spectra will be supplied on request; you must ask for the spectrum of a specific C14H20O2 ester. You are limited to three simulated spectra. For the report The report must discuss, in detail, how you identified your ester and show the structures of your alcohol and acid starting materials. Draw the mechanism for the reaction, showing your specific acid-alcohol combination. Pre-laboratory assignment From the Aldrich Catalog, obtain the possible acids (C10H12O2) and alcohols (C4H10O). Draw the structures of each. How would you distinguish the different acids by NMR? The different alcohols? 91 Reaction Studies The following experiments are intended as introductions to the methods used to elucidate how organic chemical reactions happen. Mechanistic organic chemists use these methods, and others like them, to determine the steps and intermediates in chemical reactions and the structural features of molecules that influence reactivity. Not all of these experiments will be used in any given academic year. 92 Factors affecting the reactions of alkyl halides The SN1, SN2, E1 and E2 reactions are discussed in detail in Chapters 8 and 9 of Bruice. In this experiment we will examine the effects of leaving group, alkyl group structure, and solvent polarity on the reaction of alkyl halides. We will use reactions with sodium iodide in acetone and with silver nitrate in ethanol. We will also examine the effect of solvent polarity (40%, 50% and 60% 2-propanol in water) on the reaction of a tertiary alkyl halide with sodium hydroxide. Review Bruice Chapters 8 and 9 before lab, including the effect of nucleophile and solvent on the reactions. Minimum safety standards for this experiment 1. Glass that is wet with alcohol solutions may feel slippery. Be careful handling it. 2. The alkyl halides used in this experiment are inflammable, and some are irritants. Treat them with respect and wash your hands after handling. 3. 2-propanol is inflammable and an irritant; the solutions we will use are similar in composition to commercial rubbing alcohol. Treat them with respect. 4. The sodium hydroxide solutions you will handle will be relatively dilute. Wash your hands carefully and often. 5. Sodium iodide is an irritant. The amounts we will use present essentially no hazard. 6. Silver nitrate is mildly toxic, and will stain your fingers gray. Use care in handling it. Disposal All mixtures from this experiment may be flushed down the sink. Procedure Organic halides that may be used in this experiment: • 1-chlorobutane • 1-bromobutane • 2-chlorobutane • 2-bromobutane • 2-chloro-2-methylpropane • 2-bromo-2-methylpropane • bromocyclopentane • bromocyclohexane • 1-chloro-2-butene (crotyl chloride) • benzyl chloride • bromobenzene 93 1. Reactions with sodium iodide in acetone. Sodium iodide in acetone causes SN2 reactions exclusively. Why is that? a. Preheat a water bath to 50°C (what is the boiling point of acetone?) If your test tubes seem wet, rinse them with acetone. b. Label one test tube for each alkyl halide you will be testing. Place four drops (~0.2 mL) of each alkyl halide in the appropriate test tube. c. Add 2 mL of 15% NaI in acetone to each test tube and shake to mix. Note the time of mixing. Observe the test tubes for 5-10 minutes and note the time necessary for a precipitate to form. d. Take the tubes that have not formed a precipitate and heat them for 5 minutes in the 50°C water bath. Cool them to room temperature and note whether a precipitate has formed or not. e. If there is time, perform a second set of these tests. 2. Reactions with silver nitrate in reagent alcohol. 1 Silver nitrate promotes SN1/E1 solvolysis by ethanol. Why is that? a. Preheat a water bath to 80°C (what is the boiling point of ethanol?) If your test tubes seem wet, rinse them with reagent alcohol. b. Label one test tube for each alkyl halide you will be testing. Place four drops (~0.2 mL) of each alkyl halide in the appropriate test tube. c. Add 2 mL of 1% silver nitrate in reagent alcohol to each test tube and shake to mix. Note the time of mixing. Observe the test tubes for 5-10 minutes and note the time necessary for a precipitate to form. (What is the precipitate?) d. Take the tubes that have not formed a precipitate and heat them for 5 minutes in the 80°C water bath. Cool them to room temperature and note whether a precipitate has formed or not. e. If there is time, perform a second set of these tests. 3. Reactions of a tertiary alkyl bromide with sodium hydroxide in 2-propanol/water solutions. What sort of reaction(s) do you expect from this combination of reagents? a. To prepare 50% 2-propanol (a 50% solution of 2-propanol in water), place 50 mL of 2propanol into a 100-mL graduated cylinder, then add water until the total volume is 100 mL. Mix well. 40% and 60% 2-propanol are prepared in a similar way. In the tests below, if the solution does not become colorless within 15 minutes, stop the reaction, record the time as “greater than 15 minutes” and do not repeat the experiment. b. Place 50 mL of 50% 2-propanol in an Erlenmeyer flask containing a magnetic stirring bar. Add 5 drops of phenolphthalein indicator and exactly 200 μL of 0.5 M NaOH to the flask. Mix well. What color is the solution? c. Add 50 μL of 2-bromo-2-methylpropane (tert-butyl bromide) to the flask, with swirling, and measure the time required for the solution to become colorless. 1 “Reagent alcohol” is a non-potable mixture of methanol, ethanol and isopropyl alcohol with the same dielectric constant as absolute ethanol. 94 d. Repeat b and c using 40% 2-propanol, and using 60% 2-propanol. e. If there is time, perform a second set of these tests. For the report All results will be reported individually, and as appropriate averages with standard deviation. Discuss the observed effects (including primary/secondary/tertiary and other organic group effects; leaving group effects; and solvent effects) in terms of the theory of nucleophilic substitution and elimination reactions. Are your pre-lab predictions borne out by the results you obtained? Pre-laboratory assignment Explain what type(s) of reaction each of the three sets of reaction conditions promotes: SN1, SN2, E1 or E2. Predict which of the organic halides is likely to react under each set of conditions. 95 1 H NMR analysis of keto-enol tautomerism 1 All carbonyl compounds with hydrogens alpha to the carbonyl group exist in equilibrium with their enol forms. Normally the equilibrium strongly favors the keto tautomer. However, β-dicarbonyl compounds, in which two carbonyl groups are separated by a methylene (CH2) group, have the possibility of hydrogen bonding between the enol of one carbonyl group and the neighboring carbonyl oxygen: hydrogen bond H O O O Y X H O O Y X HH H O Y X H We can distinguish the keto and enol forms by 1H NMR: while the saturated hydrogens (CH2) between the carbonyl groups in the keto form will appear around 3.5-4.5 ppm, the vinylic hydrogen (=CH) of the enol form appears between about 5.0 and 6.5 ppm. Because the integration of a 1H NMR peak indicates how many hydrogens correspond to that peak, we can determine the ratio of saturated to vinylic hydrogens by integrating the 1H NMR spectrum of each sample. This ratio 2 will give us the equilibrium constant according to K eq = [enol ] = 2 × enol integration [keto] keto integration The equilibrium constant corresponds to the free energy difference between the keto and enol forms: ∆G 0 = − RT ln K eq We will determine the keto-enol equilibrium constants 3 for several β-dicarbonyl compounds in chloroform-d at a concentration of approximately 5% by weight. Samples will be prepared by the instructor. Procedure This experiment will be performed by lab section; each lab section will cooperate in obtaining the necessary spectra. However, data analysis must be performed individually. If different sections use different compounds, all data will be posted to the course website and all students are expected to use all of it. 1. Obtain at least five 1H NMR spectra of each solution, including integration of the relevant peaks. 1 Based on E.J. Drexler and K.W. Field, Journal of Chemical Education 1976, 53, 392-393. See also A. Grushow and T.J. Zielinski, ibid 2002, 79, 707-714. 2 We need to multiply by 2 to allow for the fact that the keto form has two saturated hydrogens between the carbonyl groups, but the enol form has only one vinylic hydrogen! 3 It is STRONGLY suggested that you re-read the section on equilibrium constants from your General Chemistry text. 96 2. Analyze the NMR spectra to obtain the equilibrium constants and free energy differences for the interconversion of each tautomeric pair. You may assume that the probe temperature is 20ºC unless otherwise instructed. For the report Report the equilibrium constants and explain what it is that they are measuring. What do they mean? Explain any observed differences between the β-dicarbonyl compounds studied, in terms of the substituents of each. You may speculate, but be reasonable! For example, consider the effect of alkoxy groups on electrophilic aromatic substitution reactions; are they electron-donating or electronwithdrawing? No experimental section is required in your report. Pre-laboratory assignment In Appendix II of Bruice, look up the pKa values for each of the following compounds: 3-oxobutanal, 2,4pentanedione, ethyl 3-oxobutanoate and diethyl propanedioate. Compare to the pKa values for ethanol, 2propanone and ethyl ethanoate. Explain why the β-carbonyl structures are more acidic. 97 Kinetics of the Esterification of Trifluoroacetic Acid 1 We will use NMR to determine the pseudo-first-order rate constant for esterification of trifluoroacetic acid (Scheme 1). Calculating the rate constant Scheme 1 The rate of esterification of any acid (seen as the rate of appearance of the ester or disappearance of the alcohol) will be dependent on the concentration of the acid and the concentration of the alcohol according to the differential equation O CF3COH + ROH O CF3COR + H2O d [ester ] d [alcohol ] =− = k [acid ][alcohol ] dt dt in which [x] is read “the concentration of x” and k is called the rate constant. 2 This equation tells us that both the acid and the alcohol are involved in the rate-limiting transition state for the reaction; we know this because the rate is dependent on both their concentrations. Since the sum of the exponents of the concentrations is 2 (1 + 1), we call this a “second-order reaction.” It is possible to solve a second-order rate law numerically, but it is much more difficult to solve it analytically. 3 However, if the concentration of one reactant (in our case, the acid) is large compared to that of the alcohol, its concentration does not change significantly during the course of the reaction and we can define a “pseudo-first-order” reaction described by the differential equation d [ester ] d [alcohol ] =− = k ′[alcohol ] dt dt where the “pseudo-first-order rate constant” k ′ = k [acid ] . This allows us to define the rate solely in terms of the concentration of the alcohol. Collecting terms and rearranging, we have − d [alcohol ] = k ′t [alcohol ] which can be integrated thus: d [alcohol ] t = ∫ k ′t dt o [alcohol ]o [alcohol ] − {ln[alcohol ]t − ln[alcohol ]0 } = k ′t −∫ [alcohol ]t so that [alcohol ]t = [alcohol ]0 e − k ′t The integrated rate equation is solved numerically (curve fitting) by the NUTS software, using 1H NMR integration data over a series of scans. The software allows analysis of either the decay of the alcohol 1 This experiment is based on D.E. Minter and J.C. Villarreal, J. Chem. Ed. 1985, 62, 911-912. 2 The “rate constant” is not really a constant, as it varies not only with the specific reaction but with temperature. 3 “To solve analytically” means to be able to write an integrated equation for the concentrations of reactants as functions of time. 98 peak or the growth of the ester; when possible, we will do both. The output from the software is a number (T1 or T2) that is the reciprocal of the pseudo-first-order rate constant k´ for the reaction. 1 The goal of this week’s experiment All experiments will be run at a concentration of 1 molal, that is, 1 mmol of alcohol per gram of trifluoroacetic acid. 2 Solutions will be prepared during the lab period; some data may be collected by the professor prior to lab, and supplied to you. Combined data will be posted to the course web page. We will compare the rates of the esterification of trifluoroacetic acid by a series of alcohols. How do these alcohols differ in their substitution? Data will be obtained under the supervision of the professor, and will be analyzed by you. Account for any differences in the observed rate constants. Are the differences significant? How can you tell? For the report Discuss the mechanism of esterification and try to identify the rate-limiting step. Discuss the effect, if any, of the size (or bulk—what is the difference?) of the organic group on the rate constant for esterification. Where is substitution on the organic group most significant? The report will be due one week after the combined data are posted to the course web page. Please do not parrot back the discussion of mathematical rate laws given above. If you wish to discuss rate laws and reaction kinetics, you should review the material in your general chemistry textbook and cite it properly. No experimental section is required for this report! Pre-laboratory assignment Give at least two plausible reasons for using trifluoroacetic acid, rather than acetic acid, in this NMR experiment. 1 We could also analyze raw data from the experiment by manually integrating each spectrum in the data set. However, because we can’t assume that the integral response for the ester is the same as that for the alcohol, we can only use the alcohol data for this analysis. If we were to use ester integration we would have to obtain an integral after the solution had stood for several hours—we call this “concentration of ester at infinite time” or [ester]∞—and the integration response would probably not be equivalent to that obtained during the experiment. 2 Are these conditions acidic? How strong an acid is trifluoroacetic acid? 99 Synthetic experiments The business of organic chemistry is building new molecular structures. These experiments will introduce some of the reactions in the synthetic organic chemist’s toolkit, with which he builds the most intricate and beautiful structures imaginable. An indispensible reference for these experiments is Zubrick’s Organic Chem Lab Survival Manual. Not all of these experiments will be used in any given academic year. 100 Dehydration of an alcohol Alkenes may be synthesized by treating an alcohol with a strong acid. The acid protonates the hydroxyl group, converting it into the excellent leaving group, water. The elimination reaction proceeds by an E1 mechanism, 1 with the protonated alcohol losing water and then being deprotonated by a base, creating a double bond and regenerating the acid catalyst. This reaction, the reverse of an electrophilic addition reaction, is discussed briefly in Section 10.4 of Bruice. In this experiment, cyclohexene will be synthesized from cyclohexanol. The alcohol is favored if the reaction is allowed to come to equilibrium; to drive the reaction forward, the alkene will be removed as it is formed by the technique of distillation. You will also be removing water from your product by using a drying agent. Techniques used: microscale glassware (Zubrick Chapter 5), microscale distillation (Zubrick Chapter 21), microscale extraction (Zubrick Chapter 16), drying an organic liquid (Zubrick Chapter 10) Minimum Safety Standards for this experiment 16. Hot glass looks the same as cold glass! Before picking up a piece of glassware, be sure to check that it is cool enough to handle. 17. Reagents which have an odor or an appreciable vapor pressure may not be used outside the hood except in closed containers. 18. Look up the MSDS for each reagent used. More specific cautions and procedures are given below. 19. Cyclohexanol is water-soluble enough that residues may be cleaned with soap and water and flushed down the sink. 20. You will be using concentrated phosphoric acid. While this acid does not produce fumes, it is strongly corrosive! Be sure to wash your hands after mixing the initial reaction solution to remove any traces of phosphoric acid. Also wash after handling any other substance produced in this experiment. 21. Cyclohexene has a strong odor. Keep the product in the hood at all times. This includes contaminated glassware and paper towels. Rinse all contaminated glassware with acetone in the hood, into a stoppered waste container, before removing it from the hood. Waste contaminated with cyclohexene may be combined into the waste bottle provided. 22. Bromine is corrosive and toxic and must be used only in the hood. Be sure to familiarize yourself with the MSDS. Wash your hands after handling the bromine bottle. Be sure to ask for gloves before attempting to clean up spillage. 1 See Bruice Chapter 9. 101 Disposal aqueous waste This will smell of cyclohexene and should be flushed down the sink in the hood. The residue from the initial reaction and the aqueous solutions used for washing are both “aqueous waste.” bromine Any waste containing bromine (it will be colored) should be put aside and given to the instructor for neutralization and disposal. This includes paper towels. Exception: the blank solution for your bromine test may be placed in the waste bottle provided. cyclohexanol Residues should be cleaned with acetone and flushed down the sink unless they are contaminated with cyclohexene. cyclohexene Residues should be cleaned with acetone in the hood and placed in the waste bottle provided. drying agent Drying agent should be placed in the container provided and will be disposed of by the instructor. phosphoric acid Residues should be cleaned with water and flushed down the sink. Procedure 1. From the Microscale kit, obtain a 5 mL conical vial, a Hickman-Hinkle still head, and a thermometer adapter. You will also need a thermometer, a heating block and a hot plate. 2. Place about 3.0 g (± 0.5 g) cyclohexanol in the 5 mL vial. Add 1.0 mL concentrated phosphoric acid. Mix well. Add a boiling chip. Attach the vial to the still head. 3. Place the apparatus in the heating block and turn the hot plate power up to full. The reaction solution will begin to boil. 4. Keep the head temperature at 90-95° C as much as possible. Be cautious about heating above 100º for long periods. 1 If the collection chamber of the still head becomes full, remove the product with a Pasteur pipette and place it in a 3 mL conical vial. Continue distilling the solution; your temperature may fluctuate, but try to keep it between 85° and 100°. Eventually, you will have a milliliter or less of dark residue and you will not be able to keep the head temperature above 85°: the reaction is now finished. 5. Raise the apparatus from the heating block and allow it to cool for about five minutes. While it is cooling, remove the rest of the product from the collection chamber of the still head with a Pasteur pipette and combine it with the product in the 3 mL conical vial. Two phases should be apparent: the alkene and water products of the reaction. 6. Add about 1 mL of 5% sodium carbonate to the vial. Cap the vial and shake. Allow the phases to separate and remove the aqueous layer with a pipette. Repeat the washing with a second portion of 5% sodium carbonate. 1 Normally it will be sufficient to keep the hot plate set at “high.” 102 7. Add about 15-20 granules of drying agent to the product to remove the remaining water. 1 Cover the vial and shake. Let it stand for five or ten minutes. Meanwhile, rinse the distillation apparatus with acetone IN THE HOOD! Let the glassware dry for five or ten minutes. 8. Reassemble the apparatus, placing the dry product in the clean, dry 5 mL vial. (To clean the vessel used for drying your cyclohexene, use first acetone, then soap and water, and then acetone to remove the water.) 9. Distill the product, using the Hickman-Hinkle still head. Do not let the head temperature rise above 85°. Continue boiling until the head temperature begins to fall, or until only a few drops are left in the vial. Never let a boiling flask go completely dry, as it will overheat and could shatter! An explosion is not likely with a thick-walled vial, but you could certainly crack the vial, which is rather expensive. 10. Remove the final product with a Pasteur pipette, and place it in a clean, dry, tared vial. Weigh the vial to determine the amount of product obtained, and report the yield. Show the product to the instructor for an appearance score. 11. Bromine has historically been used to test for the presence of carbon-carbon multiple bonds. Place a sample of your product (about 0.5 mL) and a sample of cyclohexanol in two test tubes, add an equal quantity of acetone to each, and mix. Now add a few drops of bromine solution to each test tube. Describe the bromine and your test solutions before they are mixed. Describe what happens after mixing. For the report… Draw the mechanism of the dehydration reaction, including curved arrows. Discuss the chemistry of the bromine test and draw a mechanism. Report your yield correctly. 1 If CaCl2 is used, add more drying agent if the agent appears to liquefy. 103 Pre-laboratory worksheet for Dehydration of an alcohol 1. Why do we distill the alkene away from the reaction mixture, rather than just allowing the reaction to run at room temperature? 2. Describe how you will be able to determine, by adding bromine (Br2), whether your product is an alkene. 3. What is the purpose of each of the following reagents: phosphoric acid? sodium carbonate? 4. Consider the chemicals used for this experiment. What realistic hazards are present? What safety procedures are necessary beyond wearing glasses and gloves? 5. I, __________________________________, have read and understood the experimental procedure. I am familiar with the hazards and with the required disposal procedures for this experiment. (Sign your name) 104 105 Reduction of a ketone Ketones, which may be synthesized from alcohols, may also be converted into alcohols by reduction with sodium borohydride, as shown: O NaBH4 C OH R R R C R H In this experiment, we will reduce camphor to borneol and/or isoborneol. 1 Camphor, the pungent active ingredient in Vicks Vapo-Rub™, is a chiral molecule, and commercial camphor is typically a racemic mixture of the “left-handed” and “right-handed” isomers: CH3 H3C H3C CH3 H3C H3C O CH3 CH3 O CH3 H3C CH3 O O CH3 S-(-)-camphor R-(+)-camphor “Right-handed” R-(+)-camphor is extracted from the bark of the Chinese camphor tree, Cinnamomum camphora, while the more expensive S-(-)-camphor is obtained from feverfew, Chrysanthemum parthenium. Pure enantiomers of camphor are often used as “chiral auxiliaries” in organic synthesis; the chirality of the camphor influences the chirality of the product. We will use artificially-prepared, racemic camphor in our synthesis, and so expect to obtain racemic product(s), a mixture of R and S enantiomers. Observe, however, that the endo (borneol) and exo (isoborneol) diastereomers of the product are both possible. H3C H3C CH3 CH3 OH H H3C OH borneol H3C H isoborneol Because the endo and exo isomers are not enantiomers, but diastereomers, they have different physical properties. 2 We will determine the ratio of endo to exo isomers by gas chromatography (GC) and a little chemical common sense. 1 A similar procedure has been published by Schoffstall, A.M.; Gaddis, B.A.; Druelinger, M.L. in Microscale and Miniscale Organic Chemistry LaboratoryExperiments, Boston: McGraw-Hill (2000). 2 See Bruice, Chapter 5. 106 Techniques used from Zubrick 6th Ed: microscale addition and reflux (Chapter 24), microscale extraction (Chapter 16), drying an organic liquid (Chapter 10), evaporation under reduced pressure (procedure is not in Zubrick), gas chromatography (Chapters 27, 32) Minimum Safety Standards for this experiment 1. Hot glass looks the same as cold glass! Before picking up a piece of glassware, be sure to check that it is cool enough to handle. 2. Reagents that have an odor or an appreciable vapor pressure may not be used outside the hood except in closed containers. 3. Look up the MSDS for each reagent used. More specific cautions and procedures are given below. 4. You will use a concentrated solution of sodium borohydride in 1% aqueous sodium hydroxide. This material is caustic, and exposure to acid may cause a fire. Handle with care and wash thoroughly with water after use. Do not attempt to clean this with acetone! The reaction that results is violent! 5. You will be using a 3M solution of hydrochloric acid. This is mildly corrosive; take appropriate precautions and wash thoroughly after use. Disposal aqueous layer from extraction Flush down the sink. camphor Residues should be rinsed with acetone and washed with soap and water. Check the sinks after lab to ensure that no camphor odor remains. dichloromethane Most of the dichloromethane will be evaporated. Wipe up spillage with paper towels and allow the dichloromethane to evaporate in the hood. The paper towels may then be disposed of in the wastepaper basket. drying agent Solid spillage may be swept up and placed in the wastebasket. Used drying agent may be placed in the wastebasket if there is no odor; otherwise place in the hood until the odor has vanished. hydrochloric acid solution Flush down the sink. isoborneol and borneol Residues should be rinsed with acetone and washed with soap and water. methanol All methanol solutions may be flushed down the sink. Clean residues with soap and water. product Residues should be treated as camphor or isoborneol. Leftover product may be placed in the wastebasket after dichloromethane residue has been allowed to evaporate. sodium borohydride solution Most will be neutralized in the course of the procedure. Spills should be wiped up with a wet paper towel, which should then be thoroughly rinsed with water. Do not attempt to clean this with acetone unless it has first been thoroughly rinsed with water. sodium chloride Flush down the sink. 107 solution Procedure 1. Weigh about 100 mg of camphor into the 5-mL conical vial. Be sure that you determine the exact mass. 2. Add a bit less than 0.5 mL of methanol to the vial. Place the spin vane in the vial and attach the condenser. 3. Stir your solution vigorously. Measuring with a syringe, carefully add NaBH4 solution via the condenser, about 2 moles of NaBH4 for every mole of camphor. 1 Rinse the condenser with about 0.2 mL of methanol. While continuing to stir, bring the mixture to a boil. Boil for 5-10 minutes, then remove it from the heat and allow it to cool to room temperature. Remove the heating block from the hot plate, being careful not to burn your fingers. During the subsequent step, be careful not to allow your hot plate to heat the vial; this can be accomplished by suspending the vial about ½ inch above the hot plate. At this height, magnetic stirring is still possible. 4. While stirring, add 3M HCl solution, dropwise, through the condenser, until evolution of hydrogen gas ceases (how is it formed?) Add 0.75-1.0 mL of 5% NaCl solution (why don’t we use plain water?) Rinse the condenser with about 1 mL of dichloromethane and stir the mixture vigorously for a few moments. 5. Remove the condenser. Remove the spin vane and the dichloromethane extract. 2 Extract the aqueous layer with one or two more 0.5-mL portions of dichloromethane. Dry the combined extracts, and decant the organic solution into a clean, dry test tube or vial. Rinse the drying agent with 0.5 mL of dichloromethane; combine this with your product mixture (of course). 6. Prepare a GC sample by taking 2-3 drops of your product mixture and adding the same number of drops of methanol. Analyze your product mixture by GC. 3 7. Decant the rest of your product mixture into a clean, dry, labeled and tared beaker and allow the solvent to evaporate overnight in the drying cabinet. Obtain a melting point of your dried product. For the report 1. Have you reduced all the camphor? If not, what is your conversion (the percentage of your material that is product)? 2. Given the structure of camphor, do you expect more borneol (endo alcohol) than isoborneol (exo alcohol), less borneol, or an equal amount? What ratio is observed? Explain your results. 4 1 The solution is 5 to 6 M NaBH4 in 1% aqueous NaOH. The actual concentration will be given to you. 2 If you are not seeing clear separation, add a little water. It may be that your aqueous layer is too dense. 3 GC traces of authentic samples of camphor, borneol and isoborneol will be available for comparison. 4 Online molecular models of camphor, borneol and isoborneol http://www.bluffton.edu/~bergerd/classes/CEM221/classmodels/camphor1.html. 108 are available at Pre-laboratory worksheet for Reduction of a chiral ketone 1. What is the purpose of the methanol? Why don’t we just use water? (HINT: look up camphor in the Merck Index, and see what it says about water vs. alcohol.) 2. Why is sodium borohydride relatively stable in the presence of base, but not in the presence of acid? Explain the chemistry involved. 3. Why do we expect borneol and isoborneol to be separable by GC? What about R-camphor vs. Scamphor? 4. Consider the chemicals used for this experiment. What realistic hazards are present? What safety procedures are necessary beyond wearing glasses and gloves? 5. I, __________________________________, have read and understood the experimental procedure. I am familiar with the hazards and with the required disposal procedures for this experiment. (Sign your name) 109 110 Saponification: Biodiesel Synthesis and Soap-making 1 Introduction As the Earth’s petroleum resources continue to be reduced and as much of the world’s petroleum is found in politically unstable parts of the world, the need for a reasonably priced, more environmentally friendly alternative increases. Biodiesel shows great promise as a readily available replacement for petroleum diesel and can be easily synthesized on scales ranging from laboratory to home workshop to industrial. Biodiesel is also fully and quickly biodegradable. While Otto Diesel’s first engines ran on peanut oil, diesel engines have run on petroleum-based fuel for many decades because of its superior properties (including shelf life, price and viscosity). Because petroleum-based diesel fuel has lower viscosity (is thinner) than pure vegetable oils, modern diesel engines cannot burn unmodified vegetable oil. But vegetable oil can be readily converted into biodiesel, a liquid with similar viscosity to petroleum diesel. Biodiesel can be burned in unmodified modern diesel engines. In recent years there has been significant interest in the production of biodiesel from the waste oils of the food industry. Every year, fast food restaurants in the U.S. produce over 3 billion gallons of used cooking oil. Since many gallons of this used oil inevitably end up in landfills and sewers, the production of biodiesel from waste oil has the potential to significantly reduce environmental impact. Used vegetable oil can only meet a small amount of the diesel fuel demand, and biodiesel is more typically produced from fresh oils. Currently soy is the major oil source, but many other vegetable sources are available. These sources are renewable and the carbon dioxide produced from the burning of the biodiesel is recycled into the next crop through the carbon cycle. Biodiesel is much closer to carbonneutrality than corn-based ethanol, because far less energy is required for processing per energy unit of fuel. (In both cases the carbon released by burning the fuel is captured by the plants that produce new feedstocks for fuel production.) You will be making biodiesel from fresh soybean oil. You will also be making soap. The process we will use is not quite the same as your great-grandparents used, because we will be using alcohol to help things along. But the product will be similar to oldfashioned homemade soap. We will be comparing the soap we produce with samples of commercial bar soap. Theory/Discussion In this experiment you will use similar processes to produce two different products from a a triester of glycerol (a triglyceride). The reaction used to produce diesel fuel is known as transesterification, the process of transforming one type of ester into another type of ester. The reaction uses the strong base sodium methoxide (generated by dissolving sodium hydroxide in methanol) in a base-catalyzed nucleophilic addition-elimination reaction at the carbonyl carbons of the triglyceride. The NaOH, like any other catalyst, is regenerated as a product in the reaction. If the biodiesel is removed from the mixture, glycerol and unreacted NaOH and methanol remain. The glycerol byproduct can be 1 Based on procedures by John E. Thompson, Lane Community College, Eugene, Oregon, and R. Blanski, Littlerock High School, Littlerock, California. 111 used as an additive to soap, among many other uses. The general reaction for forming biodiesel is shown below: O H2C O C R H2C O HC O C OH O NaOH R + 3 CH3OH HC OH H2C +3 H3CO C R OH O H2C O C R On the other hand, when soap is made from a natural oil, sodium hydroxide is consumed in the reaction and not regenerated: O H2C O C R H2C OH O O HC O C R +3 NaOH HC OH H2C +3 + O C R OH Na O H2C O C R Notice the similarities and differences between the reactions. The reaction mechanisms are almost identical. Techniques used: using hot- and cold-water baths; separation of immiscible liquids (Zubrick Chapters 15); vacuum filtration (Zubrick pp. 110-112); taking the infrared spectrum of a liquid (Zubrick Chapter 34); doing more than one thing at a time. You must hand in the pre-lab to be admitted to the laboratory. Minimum Safety Standards for this experiment 1. You will be working with sodium hydroxide, as a pure solid and in concentrated solutions. This is a strong caustic. It will eat holes in your clothes and your skin if not washed off. When cleaning up, be sure to remove all jewelry and anything else covering skin that may have come in contact with sodium hydroxide. 2. Ethanol, vegetable oil and biodiesel are inflammable. Keep away from strong heating, sparks or flames. 3. Methanol is both inflammable and toxic. Keep it away from strong heat sources, and away from your eyes. Wash your hands after handling. 4. The soap you produce may have a high pH. You may use it for washing and bathing, but should not use it near your eyes. 5. You should treat all waste liquids as caustics, as they will contain sodium hydroxide. 112 Disposal All reagents, products and wastes may be disposed of by flushing them down the sink, except for biodiesel, which must be placed in the designated container. Small amounts of biodiesel may be flushed down the sink. Procedure for biodiesel synthesis The following procedure is for synthesizing biodiesel from 100% pure, unused vegetable oil. This method can be modified for recycling used vegetable oil. Used vegetable oil must first be analyzed for free fatty acid and then a pH correction is made before following this procedure. 1. Warm 50 mL of vegetable oil to about 40°C in a 100 mL beaker. You may use your hot water bath from the soap-making procedure, but I recommend that you simply use the hot plate you are heating the water bath on. (Be careful not to overheat the oil!) Warming the oil is not necessary, but increases the reaction rate. 2. Place 10 mL of 0.45 M methanolic NaOH 1 in a 125-mL Erlenmeyer flask equipped with a magnetic stirring bar. Pour the warmed oil into the flask while continuously stirring, using a magnetic stir plate. At first the mixture will become cloudy, but it should soon separate into two layers. Stir for 15-30 minutes on high. 3. Transfer the contents of the flask into a separatory funnel. The mixture will separate into two layers. Which layer is which? 2 Since about 75% of the separation occurs within the first hour, you will be able to see immediate progress. Allow the experiment to stand for at least an hour while you make soap. 4. Drain the glycerol and biodiesel into separate containers. Make sure not to get any biodiesel in the glycerol or glycerol in the biodiesel. If necessary, throw away a small amount of material between the layers. 5. Use the IR spectrometer to identify your products by comparing with known spectra. The biodiesel may be hard to compare, since most oils are comprised of different length carbon chains, but you should be able to find a spectrum of several fatty methyl esters. 3 Comparing to known spectra can easily indentify the glycerol. The presence of glycerol indicates a successful reaction. Procedure for making soap 1. Prepare a hot water bath by filling a 600-mL beaker about half full with tap water. Place this beaker on a hot plate and bring the water to a temperature of about 80-85°C. This will be the warm-water bath. 2. Chill 150 mL of deionized water and 100 mL of 25% NaCl (in separate containers!) in an ice-water bath. Be sure that your salt water container is large enough to hold another 100 mL with vigorous stirring in Step 7 (e.g. a 250-mL flask or 400-mL beaker). 1 That is, NaOH dissolved in methanol. The methanol should be 99% pure or purer; water in particular interferes with this reaction. 2 HINT: what’s the density of glycerol (also known as glycerin)? What’s the density of methyl stearate (from the Aldrich Catalog or the Merck Index)? 3 For example, http://riodb01.ibase.aist.go.jp/sdbs/cgi-bin/cre_index.cgi?lang=eng contains spectral data for methyl stearate, methyl oleate, methyl palmitate, and methyl myristate. 113 3. Add 2.5g of sodium hydroxide pellets to 25mL of 95% denatured ethanol in a 125-mL Erlenmeyer flask. Swirl the flask to get as much of the sodium hydroxide to dissolve as possible, but most of it will not dissolve. 4. Add 5mL of vegetable oil to the 125-mL Erlenmeyer flask and swirl. 5. Submerge the flask in the warm-water bath and heat it for 20 minutes. While heating, stir the solution frequently with a stirring rod. Watch the flask carefully to make sure that it does not boil over. Turn down the temperature of the hot plate if necessary to prevent the contents of the flask from boiling over into water bath. 6. While the flask is heating, set up the vacuum filtration apparatus. 7. After heating the flask for 20 minutes and once a white soapy film has developed, pour the contents of the flask into ice- cold 25% sodium chloride solution. Leave any remaining sodium hydroxide pellets behind in the flask. Stir this mixture vigorously. Place the vessel in the ice-bath and allow it to cool to room temperature. 8. Collect the precipitate by vacuum filtration and wash it three times with ice-cold water. Record the physical characteristics of your soap such as color, texture, and smell. 9. Place the precipitate in a large plastic weighing boat. You can make a cake of this soap by pressing it together (add a little water if necessary) and allowing it to dry. Or you may dispose of the soap after testing it. 10. Test the pH of the soap by dissolving 0.5 g in 10 mL of deionized water. Dip a stirring rod in the solution and touch it to pH test paper. Use the color chart to determine the pH. Compare by testing the pH of commercial bar soap. 11. Test how the soap interacts with hard water by mixing 0.5 g with 10 mL of Bluffton water (taken from the water fountain). Observe and report what happens. Compare by testing commercial bar soap. For the report Discuss the similarities and differences in reactants and products in the two reactions you have completed. Explain why glycerol and soap are water soluble but biodiesel is not. Contrast the test results (steps 10 and 11) of your homemade soap with those of commercial bar soap. Explain the reasons for the results, and for the differences. Include the IR spectra of glycerol and biodiesel, and show how they correspond to published spectra. 114 Pre-laboratory worksheet for Saponification 1. Most oils and fats contain palmitic and stearic acid as building blocks. Give the structures for both these compounds. 2. Describe the role of sodium hydroxide in the reactions we will use. 3. Look up “saponification” in the unabridged dictionary. What is the source of this word? Is it justified to call making biodiesel “saponification”? Why or why not? 4. Consider the chemicals used for this experiment. What realistic hazards are present? What safety procedures are necessary beyond wearing glasses and gloves? 5. I, __________________________________, have read and understood the experimental procedure. I am familiar with the hazards and with the required disposal procedures for this experiment. (Sign your name) 115 116 Multistep Synthesis of Triphenylmethanol It is rare that a useful compound may be synthesized in only one or two steps. Typically several intermediate compounds must be synthesized, purified and characterized along the way to the desired product. Two issues become important in multi-step syntheses: • Yield. A 60% yield may seem acceptable for an organic reaction. However, consider a procedure where five reactions must be carried out in sequence, each reaction using the product of the preceding step as starting material. To obtain the overall yield you must multiply the yield of each step; a 60% yield in each of five steps leads to an overall yield of 7.8%! The procedure is very inefficient. You could compensate by simply running the first step on a very large scale and accepting losses along the way, and in fact if the precursors are cheap this often is done in the “real world” – at least in academia, where getting to the final product is more important than efficiency or waste minimization. However, chemicals are usually not cheap, and it is even more expensive to dispose of large amounts of chemical waste. The better solution is to maximize the yield of each step, and to combine steps where possible. One of the most important parts of industrial research is process optimization, in which the object is to increase the overall yield and reduce the amount of waste generated from chemical manufacturing. • Identity and purity of the intermediates. In a multi-step synthesis, it is essential that the identity of the product of each step be established. You do not want to proceed to the next step if you are not certain of the product from the previous step. Reasonable purity is also essential, as impurities often interfere with subsequent steps either by reacting preferentially with the reagents or simply diluting the reactants to uselessness. Note that small amounts of impurities are seldom a problem, and loss of product by recrystallization or some other purification step may actually do more harm than good by reducing the yield. In this experiment, you will synthesize triphenylmethanol from benzoic acid and bromobenzene in two steps. The synthetic scheme is shown below: O O methanol OCH3 OH acid OH C Br Mg MgBr ether The first reaction is a Fischer esterification, and the second is a nucleophilic acyl substitution using a Grignard reagent. You must suggest a mechanism for each step, for your report. Techniques used: by now, you should be able to figure this out for yourself! Use the index! The most complex new technique is distillation; you should study Zubrick (6th Ed) Chapters 19, 20 and 23 carefully, paying special attention to equipment setup. See also pp. 41-44 (Chapter 4, “The Adapter with Many Names”). You will be using glassware similar to that described in those chapters. You will be using a heating mantle, controlled by a “dimmer switch;” see Zubrick Chapter 18, “Sources of Heat.” 117 Step 1. Esterification of benzoic acid 1 Minimum safety standards for this experiment 1. Hot glass looks the same as cold glass! Before picking up a piece of glassware, be sure to check that it is cool enough to handle. 2. Reagents which have an odor or an appreciable vapor pressure may not be used outside the hood except in closed containers. 3. Look up the MSDS for each reagent used. More specific cautions and procedures are given below. 4. Concentrated sulfuric acid causes severe burns. Be careful not to spill it on yourself! You may not notice the burn until you wash your hands! Disposal aqueous layers from extractions Flush down the hood sink. benzoic acid Dry spillage may be swept up and thrown in the trash; aqueous or methanolic solutions may be flushed down the sink. drying agent Place on a paper towel in the hood until it no longer smells, then place in the wastebasket. ether Place ether residues in the waste bottle provided; small amounts may be poured onto the benchtop in the hood and allowed to evaporate. Rinse containers with acetone or isopropyl alcohol and flush the rinsings down the hood sinks. methanol Flush down the sink. methyl benzoate Store, properly labeled, in the drying cabinet for next week. Procedure 1. In a 50-mL round-bottomed flask, combine 6 g benzoic acid and 20 mL methanol. Carefully add 2 mL of concentrated sulfuric acid. Swirl to mix. 2. Add a boiling chip, attach a reflux condenser, and heat to boiling. Boil the mixture gently for 30 minutes. 3. Cool the solution and decant into a separatory funnel containing 50 mL of water. Rinse the flask with 20-25 mL of ether and add the ether to the funnel. Separate. Wash the organic layer with 25 mL portions of water, of 5% sodium bicarbonate, 2 and of saturated aqueous sodium chloride. 4. Dry the organic layer. Decant the dry ether into a round-bottomed flask and wash the drying agent with another 5-10 mL of ether. Distill the mixture at atmospheric pressure; your product is the material boiling above 190º. 1 1 This procedure is taken from Williamson, K.L. Macroscale and Microscale Organic Experiments, 3rd ed.; Houghton Mifflin: Boston, 1999; pp. 480-1. 2 Shake gently (but not too gently) until the funnel no longer “burps.” 118 5. Analyze your product by IR spectroscopy. How do you know it’s an ester? How do you know it’s dry? Record the yield for this step in your lab notebook. Step 2. Addition of phenylmagnesium bromide to methyl benzoate. 2 All quantities in this experiment must be calculated, by you, according to the following criteria: • You will use no more than 3 grams of methyl benzoate, or as much as you have if you don’t have 3 grams. 3 Measure methyl benzoate by volume, not by weight! • You should have a ca. 2-5% excess of Grignard over methyl benzoate (that is, for every mole of methyl benzoate you have, you should make 2.04-2.10 moles of phenylmagnesium bromide since the stoichiometry is 1:2). • You should have a ca. 1-2% excess of bromobenzene over magnesium when you make the Grignard reagent. Measure the bromobenzene by volume, not by weight! Never fear, the pre-lab walks you through all these calculations. Minimum safety standards for this experiment 1. Hot glass looks the same as cold glass! Before picking up a piece of glassware, be sure to check that it is cool enough to handle. 2. Reagents which have an odor or an appreciable vapor pressure may not be used outside the hood except in closed containers. 3. Look up the MSDS for each reagent used. More specific cautions and procedures are given below. 4. Your ether solution of Grignard reagent is pyrophoric and must be treated with great respect. 5. 10% sulfuric acid is not as bad as the concentrated acid you used last week. Nevertheless, treat it with respect! Disposal aqueous layers from extractions Flush down the hood sink. bromobenzene Small quantities may be flushed down the sink, rinsing with acetone or isopropyl alcohol. drying agent Place on a paper towel in the hood until it no longer smells, then place in the wastebasket. ether Place ether residues in the waste bottle provided; small amounts may be poured onto the benchtop in the hood and allowed to evaporate. Rinse containers with 1 Do not run water through the condenser for the latter part of the distillation; running water through a condenser while still temperatures are above about 120º can crack the glass. 2 This procedure is taken from Williamson, K.L. Macroscale and Microscale Organic Experiments, 3rd ed.; Houghton Mifflin: Boston, 1999; pp. 457-61. 3 If you need more methyl benzoate, it will be available for you. 119 acetone or isopropyl alcohol and flush the rinsings down the hood sinks. magnesium Place spillage in the trash. methyl benzoate Turn in leftover methyl benzoate to the instructor. mother liquor from crystallization Place in the waste bottle provided. triphenylmethanol Final product, once all analysis is complete, should be turned in to the instructor. Procedure 1. Your equipment is in the drying oven! This includes a 100-mL round-bottomed flask, a separatory funnel with the Teflon stopcock removed, a condenser, and a Claisen adapter. 2. Fill a drying tube with Drierite and fit it to your thermometer adapter. Remove your glassware from the oven and assemble it; the Claisen adapter goes into the top of the RB flask, and the condenser and separatory funnel into the Claisen adapter. The drying tube goes into the top of the condenser. 1 You should put a stopper into the top of the separatory funnel, or another drying tube if materials are available. Glassware should be assembled before it cools to room temperature. 3. Place your magnesium into the RB flask. Mix your bromobenzene with about 10 mL of dry ether 2 in the separatory funnel and run the mixture into the flask. The instructor will start your reaction by breaking several pieces of magnesium in the flask. 3 The reaction should start within a few minutes (you will see bubbles, and the ether will turn metallic grey or brown). 4. If the reaction does not begin to boil on its own within five or ten minutes, bring the reaction mixture to a gentle boil (only a few bubbles forming!) Once the reaction begins, be ready to remove heat if it gets too vigorous! 5. Once the reaction starts, add 10-15 mL of dry ether to the separatory funnel. After the reaction has gotten underway, you will need to add ether from the funnel from time to time to keep it going. 4 When all ether has been added and the reaction slows down (as it will do near the end) you should apply gentle heat so that the mixture continues to boil. You should also swirl the reaction periodically to mix it. The reaction is complete when only a few small pieces of metal or metal contaminants remain in the flask. 6. Mix your methyl benzoate with about 10 mL of dry ether in the separatory funnel. Cool the reaction flask briefly in an ice-water bath, then remove. Slowly add the methyl benzoate solution, using cooling as needed to control the reaction. After addition is complete, reflux the mixture for half an hour (gently! When this reaction starts, it’s exothermic at first!) 1 See Zubrick, Chapter 23. 2 You may want to rinse your graduated cylinder(s) with dry ether. 3 This is to expose a fresh magnesium surface; the reaction takes place at the surface of the magnesium. 4 Ether is necessary to keep the Grignard reagent in solution. If there is not enough ether, the Grignard reagent accumulates on the magnesium surface and prevents the reaction between bromobenzene and magnesium from happening. 120 7. Pour the reaction mixture into a 250-mL Erlenmeyer flask containing about 50 mL 10% sulfuric acid and about 25 g ice. Rinse the reaction flask with 1-2 mL of 10% sulfuric acid, and 10-20 mL of ordinary ether. Swirl the mixture in the Erlenmeyer until the solids have dissolved; 1 you may need to add either more ether or more 10% sulfuric acid. 8. Separate the layers and wash the organic layer with another portion of 10% sulfuric acid, then with saturated sodium chloride solution. Dry the ether layer, and decant it into a clean, dry 125-mL Erlenmeyer flask. Wash the drying agent with a small amount of ether and add the ether to the flask. At this point the Erlenmeyer flask should be tightly stoppered and set aside until the following week. The flask must, of course, be properly labeled. 2 Be sure to clean your glassware and put it in the drying oven for tomorrow’s lab section. If you took it out of the oven, it goes back in the oven. Of course, if there is no “tomorrow’s lab section” this does not apply! One week later… 9. Add about 25 mL of heptane to the flask, then heat gently on a hot plate until you begin to see crystals of triphenylmethanol. 3 Remove the flask from the heat and allow crystals to form, first at room temperature and then at 0º. Filter the crystals, washing with heptane or petroleum ether, and determine the yield from your first crop. Discard the mother liquor in the waste bottle. At this point you should show the crystals to the instructor for your appearance score. 10. Analyze your product by melting point. How pure do you think it is? For the report Draw the mechanisms of all reactions you have performed in this synthesis (that is, esterification; formation of the Grignard; and addition of the Grignard to your ester). Determine the yield for each step, and the overall yield (from benzoic acid). 1 You will see residual magnesium “fizzing” as it reacts with the acid to generate hydrogen gas. 2 If crystals form while the ether solution is standing, you may not want to add heptane in step 9! 3 The purpose of this is to evaporate enough ether that the triphenylmethanol will crystallize. 121 122 Pre-laboratory assignment for week 1: Esterification 1. What is the function of the sulfuric acid? 2. What hazards are associated with ether? Could you substitute another solvent for the ether during the extraction? Suggest a possible substitution. 3. How will you confirm the identity of your product? 4. Consider the chemicals used for this experiment. What realistic hazards are present? What safety procedures are necessary beyond wearing glasses and gloves? 5. I, __________________________________, have read and understood the experimental procedure. I am familiar with the hazards and with the required disposal procedures for this experiment. (Sign your name) 123 124 Pre-laboratory assignment for week 2: Grignard reaction 1. What is the most important hazard associated with this experiment? How can you minimize it? 2. How much methyl benzoate did you obtain last week? (Do not enter more than 3 grams.) This is the amount you will use this week. _________ grams methyl benzoate How many moles of methyl benzoate are in that many grams? _________ moles methyl benzoate What is the volume of methyl benzoate you will use? _________ mL methyl benzoate How many moles of Grignard will you need to react with your methyl benzoate? (Consider reaction stoichiometry!) Add 5% to this number and enter it in the blank; this is the number of moles of Grignard you will be making. How many grams of magnesium will you need to make that many moles of Grignard? How many grams of bromobenzene will you need? Add 2% to this number and enter it in the blank; this is the number of grams of bromobenzene you will use. How many milliliters of bromobenzene are there in that many grams? _________ moles Grignard _________ grams magnesium _________ grams bromobenzene _________ mL bromobenzene 3. How will you confirm the identity of your final (triphenylmethanol) product? 4. Consider the chemicals used for this experiment. What realistic hazards are present? What safety procedures are necessary beyond wearing glasses and gloves? 5. I, __________________________________, have read and understood the experimental procedure. I am familiar with the hazards and with the required disposal procedures for this experiment. (Sign your name) 125 126 Synthesis of an Ether Using Phase-Transfer Catalysis Ethers may be synthesized by a nucleophilic substitution reaction, in which an organic oxide anion reacts with an alkyl halide to produce a new carbon-oxygen bond. This reaction is called the Williamson Ether Synthesis. In this experiment, you will synthesize 1-ethyl-4-n-propoxy-benzene, 1 an alkyl aryl ether. The main problem in this synthesis is to bring the phenoxide nucleophile, which is polar, into contact with the 1-iodopropane electrophile, which is non-polar. This difficulty is solved by the use of tetramethylammonium bromide as a phase-transfer catalyst. 2 Minimum Safety Standards for this experiment 1. Hot glass looks the same as cold glass! Before picking up a piece of glassware, be sure to check that it is cool enough to handle. 2. 4-Ethylphenol is a toxic, smelly irritant (it may make you sneeze). Spills should be cleaned with soap and water in the hood. This reagent must be kept inside the hood at all times! Weighing must be performed in closed containers. Nothing contaminated with this reagent may be removed from the hood until the odor is gone. The smell will probably permeate your clothes and skin by the end of the experiment. 3. 1-Iodopropane is toxic and carcinogenic. The quantities we will use are relatively safe. Keep this reagent in the hood. 4. Tetramethylammonium bromide is toxic and should be treated with respect. Because it will absorb water from the air, the container should be kept tightly closed when not in use. This reagent may be used outside the hood. 5. You will be working with concentrated solution of sodium hydroxide. This solution is quite caustic and should be handled with great care! Wash thoroughly after handling. 6. Diethyl ether has no major hazards associated with it except extreme flammability. Keep away from flames and use only in the hood; excessive exposure can cause narcosis. 7. Magnesium sulfate is sold as “Epsom Salts.” Alumina is non-toxic (inhalation of large amounts leads to mechanical lung damage, or silicosis). There are no significant hazards associated with these substances in the quantities used for this experiment. Disposal Alumina Spillage may be swept up and placed in the wastebasket. Diethyl ether Most of the ether will be evaporated. Wipe up spillage with paper towels and allow the ether to evaporate in the hood. The paper towels may then be disposed of in the wastepaper basket. 1 “n-Propoxy” does not mean there is nitrogen present! Was there an “amino” or “amine” anywhere in the name? The “n” means that the propoxy group is “normal,” that is, unbranched, as opposed to isopropoxy. This old-style naming convention is still used in industry. 2 Look up “phase-transfer catalysis” in your textbook. 127 Drying column May be disposed of in the container for used Pasteur pipettes. 4-Ethylphenol Residues on glassware should be cleaned with acetone and flushed down the sink in the hood. Spills may be cleaned with soap and water, with the wash water kept in the hood. 1-Iodopropane Residues should be cleaned with acetone and flushed down the sink. Magnesium sulfate Spillage may be swept up and placed in the wastebasket. 4-propoxyethylbenzene Product residues should be cleaned with acetone and flushed down the sink. Sodium hydroxide solutions Clean with water and flush down the sink. Tetramethylammonium bromide Residues should be cleaned with soap and water and flushed down the sink. Procedure 1. Weigh 150 mg 4-ethylphenol into a 3-mL conical vial with a spin vane. Add about 15 mg tetramethylammonium bromide, 250 µL 25% aqueous NaOH and 150 µL 1-iodopropane. 1 Attach a condenser. 2. Heat the reaction mixture on medium to medium-high heat (90-100° C) for 60 minutes with stirring. Measure the temperature occasionally by carefully inserting a thermometer bulb into the small hole in the heating block. 3. Allow the reaction to cool and remove the spin vane. Two phases should be apparent: an upper organic layer and a lower aqueous layer. Add 0.5 mL diethyl ether, shake, and allow the layers to separate again. 4. Separate the layers. Wash the aqueous layer with another 0.5 mL diethyl ether. Discard the aqueous layer, and combine the ether layers. 1 Adjust the amount of iodopropane according to the amount of 4-ethylphenol. 128 5. Wash the product with about 0.25 mL 5% aqueous NaOH followed by about 0.25 mL water. Add 0.25 mL ether to the product and pull the solution through a drying column containing magnesium sulfate (1 cm) over alumina (2 cm) in a Pasteur pipette. Wash the column with 3 mL of ether, collecting the material in a tared conical vial. 6. Evaporate the solvent using an empty Pasteur pipet, vacuum adapter and aspirator. Determine the mass of your product and report the yield. 7. Analyze your product by 1H NMR and COSY. 1 Prove the structure of your product from its spectra. Pasteur pipette O-ring goes here Water aspirator For the report Prove the structure of your product from the NMR spectra. Draw a detailed reaction mechanism, and discuss the role of the phase-transfer catalyst in this reaction. Pre-laboratory assignment 1. Draw the structures of your starting materials and the expected product. 2. What features do you expect to see in the hydrogen NMR spectrum of the product? 1 See the section, “Introduction to two-dimensional NMR spectroscopy.” 129 Alcohol Synthesis by Hydroboration/Oxidation Alkenes may be converted to alcohols by the hydroboration/oxidation sequence shown below. Is this reaction an electrophilic addition? What regiochemistry do you expect? Why? BH3 C C H BH2 C C H2 O2 OH - H OH C C In this experiment we will convert 1-octene into an alcohol. Read about techniques for doing reactions with exclusion of air and/or moisture in Zubrick. Hydroboration is commonly used in organic synthesis and usually works well. However, the procedure takes some care due to the nature of borane reagents. Borane (BH3) is highly reactive and will spontaneously burn on exposure to air; its reaction with water is similarly exothermic. For this reason, boranes were once considered as possible rocket fuels, but have been abandoned because, in large quantities, they are more dangerous to handle than liquid hydrogen and have lower energy density. (Boron oxides also tend to encrust the combustion chamber.) The reagent we will use is a complex of borane and tetrahydrofuran (THF). (What is the Lewis structure of this complex? Show formal charges.) In this form, borane is somewhat stabilized, but the reagent must still be handled with caution. Follow the instructions carefully, and always think ahead. Work in the hood; make all transfers carefully and quickly using a syringe; and rinse the syringe and needle immediately after use. (Why?) Although you will be using only a small amount of borane in this experiment, it is still potentially hazardous. Minimum Safety Standards for this experiment 1. Hot glass looks the same as cold glass! Before picking up a piece of glassware, be sure to check that it is cool enough to handle. 2. Use utmost care in handling the borane-THF reagent. Remember that it must not come in contact with air or water! Rinse the syringe and needle with ether, then with acetone, then with water, immediately after use! The reagent is corrosive, but after it has reacted with water borane is biologically innocuous. Take the same precautions with THF as you would with any volatile organic liquid. 3. Diethyl ether has no major hazards associated with it except extreme flammability. Keep away from flames and use only in the hood; excessive exposure can cause narcosis. 4. 30% hydrogen peroxide is a strong oxidizing agent. As such it is corrosive, and may cause a fire if it comes in contact with highly flammable materials. 5. Sodium hydroxide solution is caustic. Use it with care and wash your hands afterwards, paying special attention to areas under jewelry and clothing which may not be easily rinsed. 6. 0.1 M hydrochloric acid is not especially dangerous. Treat it with respect but don’t worry about it unduly. 7. 1-Octene is not seriously poisonous but is smelly and quite flammable (flash point 8º) and should be used only in the hood. Octyl alcohols are not hazardous in the quantities we will be using. 8. Magnesium sulfate powder is a respiratory irritant if inhaled, but has no other associated hazards. 130 Disposal Borane-THF complex, solution in THF Residues on your syringe should be cleaned with ether, then with acetone, then with water, immediately after use, and flushed down the sink. The reagent in your reaction mixture will be destroyed during workup. Diethyl ether All ether used will be evaporated. Spills in the hood should be allowed to evaporate; spills outside the hood should be wiped up immediately and the paper towels placed in a hood to dry. They may then be disposed of in the trash. Hydrochloric acid solution Residues may be flushed down the sink. Hydrogen peroxide, 30% Residues may be flushed down the sink. Small spills may be cleaned up with wet (not dry!) paper towels; if there is a spill of more than one or two mL, call the professor immediately! Magnesium sulfate Spills may be swept up and placed in the trash. The area of the spill should then be wiped with wet paper towels, which may also be placed in the trash. Octanol residues Small spills should be washed with acetone, then wiped up. Residues on glassware may be cleaned with soap and water. Contaminated disposable pipets may be thrown directly in the trash. Amounts larger than 0.5 mL must be placed in a waste bottle. Octene residues Small spills should be washed with acetone, then wiped up. Residues on glassware may be cleaned with soap and water. Contaminated paper towels or disposable pipets should be allowed to dry in the hood before throwing in the trash. Sodium hydroxide solution Residues may be flushed down the sink. Procedure 1. Dry the glassware you will need in the oven for at least one hour, disassembled: 5-mL vial, spin vane, and Claisen head. You will also need a drying tube and a septum cap, but they need not be oven-dried. Drying tube (fill with Drierite) 2. Remove the glassware from the oven and assemble it as soon as you are able to touch it comfortably. Place the apparatus in an ice bath after it is assembled. When it is assembled, it should look like the picture shown at right. 3. Rinse the syringe with dry ether, then add 0.3 mL 1-octene through the septum with the syringe. Do not rinse the syringe. Claisen head 131 4. While stirring the reaction mixture, add 0.6 mL of 1M borane/THF solution through the septum dropwise, over 5 minutes. 5. Remove the apparatus from the ice bath and stir for 45 minutes to allow the reaction to take place. 6. Add 3-5 drops of water to destroy any unreacted borane. Remove the Claisen head and, while stirring, add 0.3 mL of 3M NaOH solution to the reaction mixture, followed by 0.3 mL of 30% hydrogen peroxide added dropwise over 5-10 minutes. 7. Attach the condenser to the vial and heat at 50-90° for an hour. Monitor the temperature by periodically inserting a thermometer into the heating block. 8. Allow the reaction mixture to cool. Extract the aqueous layer twice with 0.5-mL portions of diethyl ether and combine the ether extracts with the organic layer. Wash the organic layer once with 0.75 mL of 0.1M HCl, then three times with water. 9. Add enough ether to the organic layer so that its volume is at least 2 mL. Dry the organic layer over anhydrous magnesium sulfate. 10. Insert a cotton plug into a Pasteur pipet. Filter your organic layer through the cotton, and wash the drying agent with two 0.5-1 mL portions of ether, filtering the wash ether through the cotton. Evaporate the ether and THF from the combined, filtered organic layers by blowing air on them through a clean Pasteur pipet. 11. Analyze your product by IR and NMR. You must determine the following: is your product an alcohol? Is there any remaining 1-octene? What is the regiochemistry 1 of your product? For the report Show how your NMR spectrum indicates the regiochemistry1 of the product. Draw a detailed mechanism of this reaction in your report, and explain the regiochemistry you found. Pre-laboratory assignment 1. Explain why it is important to rinse your syringe needle immediately after using it to measure boraneTHF. 2. What is the Lewis structure of the borane-THF complex? 3. What product is expected from this reaction? 1 That is, have you made 1-octanol or 2-octanol? 132 The Diels-Alder Reaction The Diels-Alder reaction is the archetype of a class of reactions called pericyclic reactions. In these reactions, several bonds form or break simultaneously through a system of interacting bond orbitals. The Diels-Alder reaction won its discoverers the 1950 Nobel Prize in Chemistry 1 because of its utility in making 6-membered rings, which are important structural features in natural products—not to mention its significance in stimulating the theory of chemical bonding. A Diels-Alder reaction requires a diene and a dienophile. 2 One of the best dienes, and the one we will use in this experiment, is 1,3-butadiene. However, 1,3-butadiene is unstable and must be used immediately upon preparation to prevent polymerization. We will prepare 1,3-butadiene in situ from butadiene sulfone, and the diene will then react with the dienophile, maleic anhydride, to produce the product. Discuss the thermal decomposition of butadiene sulfone to butadiene and sulfur dioxide in your report. Is this a pericyclic reaction? O O O S O + O O butadiene sulfone O O maleic anhydride Safety considerations You will be using very high heat (150-200ºC) for this experiment. Be careful not to touch hot objects with your bare hands! Look up safety and toxicity data for the chemicals used. Are the amounts we will use safe? Disposal The mother liquor from filtering the product, must be disposed of in a designated waste container. The dry final product may be disposed of in the trash. NMR samples dissolved in acetone may be flushed down the sink. 1 See http://www.nobel.se/chemistry/laureates/1950/ 2 Look up these terms in your textbook. 133 Procedure Are the amounts called for stoichiometric? How much leeway can you allow yourself in the amounts of materials used? 1. Place 340 mg butadiene sulfone, 180 mg maleic anhydride, and 300 µL xylene in the 3 mL vial with the spin vane. Attach the condenser and heat the mixture to reflux, with stirring. 1 Reflux the mixture for 15 minutes. (Monitor the temperature, which should be 150-200°C, by occasionally inserting a thermometer into the heating block.) Allow the vial to cool to room temperature. 2. Add petroleum ether to the cooled reaction mixture dropwise until the solution becomes slightly cloudy. Since the product is only slightly soluble in petroleum ether, this step causes the solution to be near saturation with product, which is needed for crystallization. 3. Heat the solution until it becomes clear. Cool the solution, first in air then on ice, to allow the crystals to form. Recover the product by filtration through the Hirsch funnel with a vacuum adapter. 4. Determine the yield and melting point, and obtain the IR and NMR spectra of the product. 2 Justify the assigned structure on the basis of the spectroscopic results. You will be graded on the yield and appearance of your product. For the report Draw the mechanism for this reaction. Do you expect exo product or endo product? Is there a structural difference between the endo and exo product? Pre-laboratory assignment Explain how you will show, by NMR, that you have the expected product. 1 What is the boiling point of xylene? (We are using a mixture of isomers.) How strongly should you heat the mixture for xylene to reflux? 2 The product is insoluble in chloroform, so we will use acetone-d6 as the NMR solvent. Perdeuteroacetone is quite expensive, and NMR solutions must be prepared under the direct supervision of the laboratory instructor. 134 Diels-Alder reaction, alternate version See the previous procedure for background. For this reaction, we will use maleic anhydride as the dienophile and trans,trans-2,4-hexadiene-1-ol as the diene. The reactants and the expected product are shown below: OH O O O + O O OH O Safety considerations Look up safety and toxicity data for the chemicals used. Are the amounts we will use safe? You can clean the chemicals used with soap and water, using isopropyl alcohol for rinsing. Procedure Are the amounts called for stoichiometric? How much leeway can you allow yourself in the amounts of materials used? 5. Place exactly 44 mg of powdered maleic anhydride (it will have been powdered for you) in a 10-mL beaker. Add exactly 50 μL of trans,trans-2,4-hexadiene-1-ol, and take the mixture back to your hood. Stir the mixture with a heavy spatula, until the two components blend. Continue stirring for 10-15 minutes until crystals form. Allow the beaker to cool to room temperature. 6. Obtain the IR (in Nujol) and NMR spectra of the product. We will obtain both normal 1H NMR and COSY spectra. While you can dissolve a small amount of your product in chloroform-d, acetone-d6 is a better solvent for your product. Acetone-d6 is very expensive, and is strictly controlled by the instructor. You will need to have direct supervision to make acetone-d6 solutions. 7. Test for the presence of a carboxylic acid group. Carboxylic acids will react with bases to form watersoluble salts. If the base is a carbonate base, carbon dioxide will also be liberated – but only in the presence of water. For example, Alka-Seltzer® is a dry combination of citric acid and sodium bicarbonate. These do not react in the solid state, but when water is added the reaction is quite vigorous. a. Take a very small portion of your product (about the size of a flea) and put it on a watch glass. Add a similar-sized portion of dry sodium bicarbonate (NaHCO3). Now add a few drops of water. Observe what happens. b. Repeat this test with salicylic acid, something we know to have carboxylic acid groups present. 135 For the report Justify the assigned structure on the basis of the spectroscopic results. Things to consider when interpreting the NMR: • Compare your hydrogen spectrum to the simulated spectra of the expected product, and to the measured spectra of the diene and dienophile we used. This may help you determine the conversion you obtained in this reaction. • Notice that in the simulated spectrum of the product, the carboxylic acid OH hydrogen appears at 11 ppm, but in your actual spectrum it may not appear because of a downfield shift associated with hydrogen-bonding. Where is hydrogen bonding taking place in the expected product? Include a mechanism for the formation of the expected product in your report. Why is the product an ester? Pre-laboratory assignment Describe the NMR spectrum you expect to see if the product is as advertised. 136 Wittig synthesis of 1,4-diphenylbutadiene 1 The Wittig reaction is a simple, highly specific way of synthesizing alkenes from aldehydes or ketones, which is discussed in Chapter 17 of Bruice. We will use the Wittig reaction to make 1,4-diphenylbutadiene from cinnamaldehyde and benzyltriphenylphosphonium chloride. The thrust of this experiment is determination of the stereochemistry of the product: is it cis or trans? How stereospecific is the reaction? Minimum Safety Standards for this experiment 1. BENZYLTRIPHENYLPHOSPHONIUM CHLORIDE is highly toxic. 2 It may be handled only in a single, designated hood. Stringent contamination control will be practiced. You will be briefed on the expected procedures. The hood will be decontaminated after use. Latex examination gloves should provide sufficient protection. Do not breathe the dust. 2. 50% SODIUM HYDROXIDE is a strong caustic and will eat through both skin and clothing. Handle with care and wash thoroughly after handling, paying particular attention to covered skin (under clothing and jewelry). Because this substance will dissolve glass, do not leave it in glass for more than 24 hours. 3. CINNAMALDEHYDE is a smelly irritant. Handle with care, only in the hood, and avoid breathing the vapors. 4. 1,4-DIPHENYLBUTADIENE is an irritant. Handle with care. 5. DICHLOROMETHANE has no major hazards associated with it. Use only in the hood; excessive exposure can cause narcosis. Disposal Alcohol solutions Flush down the sink. Benzyltriphenylphosphonium chloride The area in which this reagent has been used must be washed thoroughly with soap and water. Cinnamaldehyde Small spills and residues should be cleaned with soap and water and flushed down the sink. Dichloromethane Spills should be wiped up and left to evaporate in the hood. Residues should be placed in the appropriate waste bottle. Diphenylbutadiene Remaining product must be turned in to the instructor. Sodium hydroxide, 50% Residues may be flushed down the sink with plenty of water. Spills should be wiped up with paper towels, which should then be flushed out with water and discarded. 1 Based on J.W. Lehman, Operational Organic Chemistry, 3d Ed. Upper Saddle River, NJ: Prentice-Hall (1999), pp. 315-322. 2 LD50 is about 48 mg/kg in mice, intravenous. Estimated from data for the iodide salt, NIOSH report NX#00885. 137 Procedure 1. Combine 1 mmol of benzyltriphenylphosphonium chloride with 1 mmol of cinnamaldehyde in about 1 mL of dichloromethane. Add 0.5 mL of 50% aqueous NaOH and a spin vane, and stir vigorously at room temperature for 30 minutes. PROCEDURAL NOTE: While wearing latex gloves, roughly weigh the benzyltriphenylphosphonium chloride into a reaction vial in a hood, then close the vial and move it to the analytical scale for a true weight. Be sure to tare your capped vial! Afterwards, add the cinnamaldehyde BY VOLUME in the hood, and return to your own hood with the CAPPED vial; there you may add the dichloromethane and the strong base. 2. Remove the reaction mixture from the reaction vessel. Wash the reaction vessel with 2 mL of dichloromethane followed by 1.5 mL of water. Combine the washings with the reaction mixture, and shake to extract the product into the organic layer. Separate the organic layer and remove the solvent under a stream of air. 3. Add 3 mL of 60% aqueous ethanol to the solidified residue and break up the solid as finely as possible with a spatula or stirring rod. This removes triphenylphosphine oxide and other impurities. Collect the solid by vacuum filtration, washing it on the filter with several small portions of ice-cold 60% ethanol. Dry the solid, and obtain the yield and melting point. If time permits, recrystallize your product from 95% ethanol, using the Craig tube. 1 4. Characterize your product and prove its detailed structure including cis/trans stereochemistry. 2 For the report... Discuss the Wittig reaction. What product do you expect, based on the structure of the intermediates? Prove the structure of your product. Pre-laboratory assignment 1. Normally we use a much stronger base (lithium diisopropylamide or butyllithium) to make a phosphonium ylide from a phosphonium salt. Explain why sodium hydroxide is strong enough for this particular experiment. 2. Draw both stereoisomers of the expected product, and explain how you can use hydrogen NMR to tell them apart. 1 See Zubrick for instructions for microscale recrystallization. 2 HINT: use the coupling constants of the vinylic hydrogens. 138 Oxidation of a bifunctional alcohol Traditionally, chromium(VI) (Cr6+) compounds have been used for the oxidation of alcohols. However, chromium(VI) is toxic and both use and disposal of its compounds are hazardous. Therefore, procedures have been recently developed which use more environmentally benign oxidizing agents. In this experiment we will use sodium hypochlorite (NaOCl), the active ingredient in household bleach, as the oxidizing agent. Hypochlorite (or in this experiment, hypochlorous acid) functions as a source of “Cl+,” converting alcohols into organic hypochlorites that can then undergo elimination to give carbonyl groups: OCl OH RC RC H + HOCl R R OCl RC H + H2O O H + H 2O R C + H3O+ + Cl- R R One purpose of this experiment is to determine the specificity of hypochlorite/hypochlorous acid as an oxidizing agent for alcohols. In order to do this we will oxidize the bifunctional compound 2,2,4trimethylpentane-1,3-diol. 1 There are three possibilities: both alcohols could be oxidized; only the secondary alcohol could be oxidized; only the primary alcohol could be oxidized. The primary alcohol, if oxidized, could be oxidized to either an aldehyde or to a carboxylic acid. OH NaOCl OH ? If the primary alcohol were oxidized to an aldehyde, it is possible that the product would form a cyclic hemiacetal, as sugars do. 2 Examine, for example, the IR spectrum of 5hydroxypentanal in the Aldrich Library of FT-IR Spectra. If the primary alcohol were oxidized to an acid, would you expect to form a lactone (cyclic ester) under the reaction conditions? Why or why not? This experiment is time-consuming because of the extended reaction time. Be sure to work efficiently and plan thoroughly, or you may have to leave the laboratory without finishing. This will result in a grade of zero! 1 This experiment is taken from Pelter, M.W.; Macudzinski, R.M.; Passarelli, M.E. J. Chem. Educ. 2000, 77, 1481. 2 See McMurry Chapter 19, as well as the chapter on carbohydrates. 139 Minimum Safety Standards for this experiment 1. Hot glass looks the same as cold glass! Before picking up a piece of glassware, be sure to check that it is cool enough to handle. 2. Reagents which have an odor or an appreciable vapor pressure may not be used outside the hood except in closed containers. 3. Look up the MSDS for each reagent used. More specific cautions and procedures are given below. 4. Household bleach is caustic and a strong oxidizer; glacial acetic acid is corrosive. Wash thoroughly after handling. Disposal drying agent Solid spillage may be swept up and placed in the wastebasket. Used drying agent must be kept in the hood until the odor is gone, then placed in the wastebasket. ether Most of the ether will be evaporated. Wipe up spillage with paper towels and allow the ether to evaporate in the hood. The paper towels may then be disposed of in the wastepaper basket. glacial acetic acid Residues should be cleaned with water and flushed down the sink in the hood. household bleach Clean with soap and water and flush down the sink. Reaction product Residues should be cleaned with acetone and flushed down the sink. sodium carbonate solution Flush down the sink. sodium chloride solution Flush down the sink. sodium hydroxide solution Flush down the sink. 2,2,4-trimethylpentane-1,3-diol Residues on glassware should be cleaned with acetone and flushed down the sink. Spills should be swept into the trash, then cleaned with soap and water. 140 Procedure 1. Place about 300 mg of the alcohol into the 5-mL conical vial. Determine the mass of the alcohol carefully. Add 300-350 µL glacial acetic acid. Insert the spin vane. Glacial acetic acid may be used ONLY in the hood! 2. Attach the condenser. Stir the solution using a magnetic stirrer. Add about 2.5 mL of 5% sodium hypochlorite (NaOCl) solution (household laundry bleach!) dropwise via the condenser, over a 2minute period. 3. Stir vigorously at room temperature for 60 minutes. Periodically check your solution with starchiodine paper 1 to ensure that it contains excess hypochlorite; if the starch-iodine test is negative (no color change), add more bleach, about 100 μL at a time, until a positive test is obtained. 4. After the hour is up, pour your mixture into a large, capped vial with about 3 mL of brine; 2 you may want to remove the spin vane first! Extract three times with 1-mL portions of diethyl ether. 5. Wash the combined extracts in your 5-mL vial, with three 0.75-mL portions of 5% sodium carbonate followed by two 0.75-mL portions of 5% aqueous NaOH. Wash the ether layer with about ½ mL of water; if the water layer is acidic to litmus paper, you will need to perform more washings with 5% NaOH. You should add more ether if your organic layer goes below about 2 mL in volume. 6. Dry the combined ether extracts over one of the available drying agents. Transfer the ether layer to a clean vial, and remove the ether by evaporation, using a Pasteur pipet, the vacuum adapter and a water aspirator. 7. Determine the yield of your product. Analyze your product by IR and NMR. What are the possible products? Which of them have you made? For the report Calculate the yield of this and every synthesis you perform. Discuss the spectroscopic results, and show which of the possible products they indicate. How might it be possible to determine the structure of the product by IR alone? Why did you wash the reaction mixture with strong base? Consider both reaction mixture and possible products! Pre-laboratory assignment Draw all of the possible oxidation products. Consider: (a) primary alcohols may be oxidized to aldehydes or to carboxylic acids; (b) both alcohols may be oxidized, or only one. Explain how you would distinguish between the different possible oxidation products by hydrogen NMR. 1 Dip a stirring rod or spatula, NOT the test paper! 2 What is “brine”? 141 The Aldol Condensation In the Aldol condensation, strong base is used to convert an aldehyde or ketone (the Aldol donor) into an enolate, which then attacks the carbonyl group of an aldehyde (the Aldol acceptor). Finally, the adduct is dehydrated to produce a double bond. The reaction is often performed with the same compound acting as donor and acceptor, but not always. In this experiment, we will use acetone as the donor and benzaldehyde as the acceptor. Notice that acetone has two identical α-carbons, each of which can lose a proton to create an enolate nucleophile. Therefore, it is possible for each acetone molecule to react with two benzaldehyde molecules. The actual product of this reaction, either 4-phenyl-3-buten-2-one or 1,5-diphenyl-1,4-pentadien-3-one, depends on the reaction conditions. You will be expected to determine the identity of the product obtained. O O vs. Write a mechanism for the formation of each of the two possible products in this reaction. Minimum Safety Standards for this experiment 1. 1.5 M NaOH is caustic. Use care in handling and wash thoroughly afterward. 2. Benzaldehyde is a smelly irritant. Use only in the hood, and wash thoroughly after handling. 3. Acetone and ethanol are not toxic externally. Disposal All materials used in this experiment may be flushed down the sink. Large spills of benzaldehyde must be wiped up with a paper towel, and the paper towel allowed to dry in the hood before disposal. Leftover product may be discarded in the wastebasket. Procedure 1. Assemble the 3 mL reaction vial with spin vane and condenser (no water circulation) and secure on a magnetic stirrer. Add 160 µL benzaldehyde and 60 µL acetone to the vial. 1 Quickly add 1 mL sodium hydroxide solution. 2 2. Stir this mixture with a magnetic stirrer for 30 minutes 3 at room temperature. Record your observations. 3. Collect the product by vacuum filtration using the Hirsch funnel. Wash the product by returning it to the vial and stirring with 1 mL water, followed by vacuum filtration. 1 Repeat this for a total of four washings, being careful not to lose the product! 1 What is the stoichiometry? 2 The solution is 1.5 M NaOH in 40/60 ethanol/water. 3 Use your judgement, depending on what you observe! 142 4. After the final washing, pull air through the product for 10 minutes to dry it. Determine the mass of your product. If you have sufficient product, recrystallize from 95% ethanol. 5. Obtain the mass, melting point, and IR and NMR spectra of your product. Identify it and determine the crude and final yields. How might you change the reaction conditions in order to increase the yield of the other possible product? For the report Explain the mechanism of the reaction and how you distinguished the two possible products from each other. Pre-laboratory assignment 1. Explain how you will distinguish the two possible products by NMR. 2. Draw a complete reaction mechanism for the formation of either one of the two possible products. 1 During the vacuum filtration, wash with small amounts of 60/40 ethanol/water. 143 The Perkin Reaction The original Perkin reaction is between a carboxylic acid anhydride and an aromatic aldehyde to produce an α,β-unsaturated carboxylic acid. This reaction requires a base, usually the carboxylate ion similar to the anhydride, so that mixed anhydrides are not produced in side reactions. After the main reaction (a nucleophilic addition to the aldehyde), hydrolysis of the anhydride gives the final product. What reaction have we studied which is similar to the Perkin reaction? Propose a mechanism for the Perkin reaction in a general case (acetic anhydride/sodium acetate reacting with benzaldehyde). Use Newman projections 1 to explain whether the major product is expected to be cis or trans. In the variation of the Perkin reaction that will be carried out in this experiment, the enolate will be produced from rhodanine, a heterocycle analogous to an anhydride. There is an α-carbon in rhodanine that serves as the nucleophile. The carbonyl compound we will use will be o-chlorobenzaldehyde. The product is o-chlorobenzylidene rhodanine, a compound with antibacterial and antifungal activity. S S S S NH H + C O Cl H NH C Cl O O rhodanine o-chlorobenzaldehyde o-chlorobenzylidene rhodanine NOTE: The entire procedure must be performed in a fume hood! Safety and disposal guidelines Glacial acetic acid is corrosive and smelly; it may be used only in the hood. All liquid residues from this experiment (apart from NMR solutions) may be flushed down the sink, but this must be done in the hood! Any leftover product may be disposed of in the trash. 1 See McMurry Chapter 4, and the conformation section of the molecular models website www.bluffton.edu/~bergerd/Models/newman.html 144 Procedure 1. Mix 30 mg rhodanine and 50 mg sodium acetate with 1.0 mL glacial acetic acid and 47 µL o-chlorobenzaldehyde 1 in a 3-mL reaction vial. Add a boiling chip to the vial, attach the condenser, and heat in a heating block at 140-150° for 30 minutes. 2. Cool the reaction on ice and recover the product by vacuum filtration using the Hirsch funnel. Wash the product in the filter with two portions of cold glacial acetic acid. 3. The product is difficult to recrystallize, so only a small amount will be further purified. Recrystallize 10 mg of the crude product in the Craig tube using 1.0 mL glacial acetic acid. 4. Let both the crude and purified products dry overnight in the drying cabinet. Weigh the product to determine the yield. Determine the melting point of the purified product (lit. 191°) and obtain the NMR 2 and IR spectra of the crude. (No spectrum for this compound appears in the Aldrich Library of NMR Spectra.) Assign the structure based on the spectra. For the report Include a detailed mechanism of this reaction in your report. Consider the stereochemistry of the reaction. Would you expect to have produced the E or Z isomer? Pre-laboratory assignment 1. Draw a detailed mechanism for the reaction we expect to perform in this experiment. 2. Which isomer of the product is more stable: E or Z? Explain. 1 The instructor will measure this for you. Be sure to adjust the exact amount depending on the mass of your rhodanine. 2 Due to the small amounts used, you will not be able to obtain 13C NMR. Consider whether a COSY spectrum will be helpful! (Really think about it: why waste 45 minutes if the spectrum won’t help? On the other hand, if it will help it’s time well spent.) 145 A Synthesis Using Meldrum’s Acid 1 In this experiment, you will identify the product of a reaction between Meldrum’s Acid and formaldehyde. O O H2C=O DMF O ? O Minimum Safety Standards for this experiment 1. Meldrum’s Acid is an irritant. Treat it with respect. 2. Dimethylformamide (DMF) is smelly, an irritant and a teratogen. It will penetrate latex gloves and go through human skin, along with anything dissolved in it. Avoid direct contact and wash your hands thoroughly after using. 3. 37% formaldehyde3 is smelly, a lachrymator and possible carcinogen; however, the amounts used in this experiment are small and you will not handle the concentrated reagent directly. Disposal 1. Spilled Meldrum’s Acid may be discarded in the trash. 2. The aqueous waste produced in this reaction, as well as DMF and formaldehyde rinsings, must be flushed down the sink in the hood. 3. Large DMF spills must be cleaned up with paper towels. These towels must remain in the hood until their odor is gone. Wash thoroughly after cleaning up such a spill. 4. Chloroform-d solutions must be placed in the appropriate waste bottle. 5. Leftover product may be discarded in the trash. 1 Based on Crouch, R.D.; Holden, M.S. J. Chem. Educ. 2002, 79, 477-478. 146 Procedure 1. Place 1.00 mmol 1 of Meldrum’s acid (2,2-dimethyl-1,3-dioxane-4,6-dione) in a 3-mL conical vial and add about 200 μL of DMF. Add a spin vane and stir gently to dissolve the solid. 2. Add 38 μL of 37% aqueous formaldehyde. 2 Cap the vial and stir for 90 min at room temperature. 3. Add 500 μL of tap water and cool in an ice bath for 10-15 min. 4. Collect the product by filtration, using a Hirsch funnel. Rinse the flask with a minimum amount of cold water. Solid will form in the filter flask and a second crop of product can be obtained from the filtrate to increase your yield. 5. Dry your product in the drying cabinet. Record the mass and melting point of your dried product and obtain appropriate NMR spectra. Be sure that a product appearance grade is recorded by the instructor before you make up your NMR solutions. For the report Identify your product; it forms according to the stoichiometry in the reaction conditions. Discuss its spectral data (especially NMR). Propose (and draw) a reasonable mechanism for the formation of your product. Pre-laboratory assignment You know, of course, the starting materials and conditions. What products are possible? Likely? Consider the stoichiometry of the reaction! 3 For guidance, see the sections of your text on carbonyl alpha-substitution reactions. 1 It is not necessary to get precisely 1.00 mmol, but you should (a) get as close as possible and (b) record the exact amount of Meldrum’s acid you use. 2 This will be dispensed by the instructor. Adjust the amount of formaldehyde based on the actual amount of Meldrum’s acid used. 3 37% formaldehyde is a solution in which each milliliter contains 0.37 grams of formaldehyde (37 grams per 100 mL). 147 ...
View Full Document

Ask a homework question - tutors are online