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B - Unknwn1 - 428 Part Five Identification of Organic...

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Unformatted text preview: 428 Part Five Identification of Organic Substances Experiment 51 Identification of Unknowns Qualitative organic analysis, the identification and characterization of unknown com- pounds, is an important part of organic chemistry. Every chemist must learn the appropri- ate methods for establishing the identity of a compound. In this experiment, you will be issued an unknown compound, and will be asked to identify it through chemical and spec- troscopic methods. Your instructor may give you a general unknown or a specific un- known. With a general unknown, you must first determine the class of compound to which the unknown belongs, that is, identify its main functional group; then you must de- termine the specific compound in that class that corresponds to the unknown. With a spe- cific unknown, you will know the class of compound (ketone, alcohol, amine, and so on) in advance, and it will be necessary to determine only whatever specific member of that class was issued to you as an unknown. This experiment is designed so that the instructor can issue several general unknowns or as many as six successive Specific unknowns, each having a different main functional group. Although there are well over a million organic compounds that an orga'nic chemiSt might be called on to identify, the scope of this experiment is necessarily limited. In this textbook, just over 300 compounds are included in the tables of possible un- knowns given for the experiment (see Appendix 1). Your instructor may wish to ex- pand the list of possible unknowns, however. In such a case, you will have to consult more extensive tables, such as those found in the work compiled by Rappoport (see References). In addition, the experiment is restricted to include only seven important functional groups: Aldehydes Amines Ketones Alcohols Carboxylic acids Esters Phenols Even though this list of functional groups omits some of the important types of compounds (alkyi halides, alkenes, alkynes, aromatics, ethers, amides, mercaptans, ni- triles, acid chlorides, acid anhydrides, nitro compounds, and so on), the methods intro- duced here can be applied equally well to other classes of compounds. The list is sufficiently broad to illustrate all the principles involved in identifying an unknown compound. in addition, although many of the functional groups listed as being excluded will not appear as the major functional group in a compound, several of them will frequently _ appear as secondary, or subsidiary, functional groups. Three examples of this are pre- sented here. 0 . N Br C— CH3 OZN OH CH30 CH=CH— CHO MAJOR: KEl‘ONE PHENOL ALDEH'YDE SUBSIDIARY: Halide Nilro Alkene Aromatic Aromatic Aromatic Ether WW7wfifiwgfigfi—V fives—e444“. V “.4. Experiment 51 Identification of Unknowns 429 The groups included that have subsidiary status are -— CI Chloro —N02 Nitro C=C Double Bond —Br Bromo —CEN Cyano CEC Triple Bond —I Iodo —OR Alkoxy Q Aromatic The experiment presents all the chief chemical and Spectroscopic methods of deter- mining the main functional groups, and it includes methods for verifying the presence of the subsidiary functional groups as well. It will usually not be necessary to determine the presence of the subsidiary functional groups to identify the unknown compound correctly. Every piece of information helps the identification, however, and if these groups can be de- tected easily, you should not hesitate to determine them. Finally, complex bifunctional compounds are generally avoided in this experiment; only a few are included. HOW TO PROCEED Fortunately, we can detail a fairly straightforward procedure for determining all the neces— sary pieces of information. This procedure consists of the following steps: Part One: Chemical Classification 1. Preliminary classification by physical state, color, and odor Melting-point or boiling-point determination; other physical data Purification, if necessary Determination of solubility behavior in water and in acids and bases 5:199:24 Simple preliminary tests: Beilstein, ignition (combustion) 6. Application of relevant chemical classification tests Part Two: Spectroscopy 7. Determination of infrared and NMR spectra Part Three: Optional Procedures 8. Elemental analysis, if necessary 9. Preparation of derivatives 10. Confirmation of identity Each of these steps is discussed briefly in the following sections. PRELIMINARY CLASSIFICATION Note the physical characteristics of the unknown, including its color, its odor, and its phys- ical state (liquid, solid, crystalline form). Many compounds have characteristic colors or odors, or they crystallize with a specific crystal structure. This information can often be found in a handbook and can be checked later. Compounds with a high degree of conjuga- tion are frequently yellow to red. Amines often have a fishlike odor. Esters have a pleasant fruity or floral odor. Acids have a sharp and pungent odor. A part of the training of every good chemist includes cultivating the ability to recognize familiar or typical odors. As a note of caution, many compounds have distinctly unpleasant or nauseating odors. Some 430 Part Fwe Identification of Organic Substances have corrosive vapors. Sniff any unknown substance with the greatest caution. As a first step, open the container, hold it away from you, and using your hand, carefully waft the va- pors toward your nose. If you get past this stage, a closer in5pection will be possible. MELTING-POINT OR BOILING—POINT DETERMINATION The single most useful piece of information to have for an unknown compound is its melt— ing point or boiling point. Either piece of data will drastically limit the compounds that are possible. The electric melting-point apparatus gives a rapid and accurate measurement (see Technique 6, Section 6.7, and 6.8). To five time, you can often determine two separate melting points. The first determination can be made rapidly to get an approximate value. Then you can determine the second melting point more carefully. The boiling point is easily obtained by a simple distillation of the unknown (Technique 8, Section 8.4), by reflux (Technique 6, Section 6.11), or by a micro boiling-point determina- tion (Technique 6, Section 6.10). The simple distillation has the advantage in that it also purifies the compound. A Hickman head should be used if a simple distillation is per- formed, and you should be sure the thermometer bulb is fully immersed in the vapor of the distilling liquid. For an accurate boiling-point value, distill the liquid rapidly. You must distill more than 0.75 mL of liquid. 1f the solid is high-melting (>200°C), or the liquid high-boiling (>200°C), a ther- mometer correction may be needed (Technique 6, Sections 6.12 and 6.13). In any event, al- lowance should be made for errors of as large as i5°C in these values. PURIFICATION If the melting point of a solid has a wide range (ca 5°C), it should be recrystallized and the melting point redetermined. If a liquid was highly colored before distillation, if it yielded a wide boiling-point range, or if the temperature did not hold constant during the distillation, it should be redis- tilled to determine a new temperature range. A reduced-pressure distillation is in order for high-boiling liquids or for those that show any sign of decomposition on heating. Occasionally column chromatography may be necessary to purify solids that have large amounts of impurities and do not yield satisfactory results on crystallization. Acidic or basic impurities that contaminate a neutral compound may often be removed by dissolving the compound in a low-boiling solvent, such as CHZCIZ or ether, and extract- ing with 5% NaHCO3 or 5% HCl, reSpectively. Conversely, acidic or basic compounds can be purified by dissolving them in 5% NaHCO3 or 5% HCl, respectively, and extracting them with a low-boiling organic solvent to remove impurities. After neutralization of the aqueous solution, the desired compound can be recovered by extraction. SOLUBILI'IY BEHAVIOR Tests on solubility are described fully in Experiment 51A. They are extremely important. Determine the solubility of small amounts of the unknown in water, 5% HO, 5% NaHC03, 5% NaOH, concentrated H2804, and organic solvents. This information reveals whether a compound is an acid, a base, or a neutral substance. The sulfuric acid test reveals whether a neutrai compound has a functional group that contains an oxygen, a nitrogen, or a sulfur atom that can be protonated. This information allows you to eliminate or to choose various functional-group possibilities. The solubility tests must be made on all unknowns. Experiment 51 Identification of Unknowns 431 The two combustion tests, the Beilstein test (Experiment 51B) and the ignition test (Exper- iment 51C) can be performed easily and quickly, and they often give valuable information. It is recommended that they be performed on all unknowns. ! l PRELIMINARY TESTS CHEMICAL CLASSIFICATION TESTS The solubility tests usually suggest or eliminate several possible functional groups. The chemical classification tests listed in Experiments 51D to 511 allow you to distinguish ;‘ .4. t among the possible choices. Choose only those tests that the solubility tests suggest might L : ' ’ itJ I be meaningful. Time will be wasted performing unnecessary tests. There is no substitute 1 r 5;.le for a firsthand, thorough knowledge of these tests. Study each of the sections carefully . 'l ‘ until you understand the significance of each test. Also, it will be helpful to actually try the tests on known substances. In this way, it will be easier to recognize a positive test. Appro- priate test compounds are listed for many of the tests. When you are performing a test that is new to you, it is always good practice to run the test separately on both a known sub— stance and the unknown at the some time. This practice lets you compare results directly. Once the melting or boiling point, the solubilities, and the main chemical tests have been made, it will be possible to identify the class of compound. At this stage, with the melting point or boiling point as a guide, it will be possible to compile a list of possible compounds. Inspection of this list will suggest additional tests that must be performed to distinguish among the possibilities. For instance, one compound may be a methyl ketone and the other may not. The iodoform test is called for to distinguish the two possibilities. The tests for the subsidiary functional groups may also be required. These tests are de- scribed in Experiments 51B and 51C. These tests should also be studied carefully; there is no substitute for firsthand knowledge about these either. Do not perform the chemical tests either haphazardly or in a methodical, comprehen- sive sequence. Instead, use the tests selectively. Solubility tests automatically eliminate the need for some of the chemical tests. Each successive test will either eliminate the need for another test or dictate its use. You should also examine the tables of unknowns carefully. The boiling point or the melting point of the unknown may eliminate the need for many of the tests. For instance, the possible compounds may simply not include one with a double bond. Efficiency is the key word here. Do not waste time performing nonsensical or unnec- essary tests. Many possibilities can be eliminated on the basis of logic alone. How you proceed with the following steps may be limited by your instructor’s wishes. Many instructors may restrict your access to infrared and NMR spectra until you have nar- rowed your choices to a few compounds all within the same class. Others may have you determine these data routinely. Some instructors may want students to perform elemental analysis on all unknowns; others may restrict it to only the most essential situations. Most unknowns can be identified without either spectroscopy or elemental analysis. Again, some instructors may require derivatives as a final confirmation of the compound’s iden- tity; others may not wish to use them at all. SPECTROSCOPY Spectroscopy is probably the most powerful and modern tool available to the chemist for determining the structure of an unknown compound. It is often possible to determine struc- ture through spectroscopy alone. On the other hand, there are also situations for which 432 Part FWE Identification of Organic Substances spectroscopy is not of much help and the traditional methods must be relied on. For this reason, you should not use spectroscopy to the exclusion of the more traditional tests but rather as a confirmation of those results. Nevertheless, the main functional groups and their immediate environmental features can be determined quickly and accurately with spec- troscopy. ELEMENTAL ANALYSIS Elemental analysis—which allows you to determine the presence of nitrogen, sulfur, or a specific halogen atom (Cl, Br, I) in a compound—is often useful; however, other informa- tion often renders these tests unnecessary. A compound identified as an amine by solubility tests obviously contains nitrogen. Many nitrogen-containing groups (for instance, nitro groups) can be identified by infrared spectroscopy. Finally, it is not usually necessary to identify a specific halogen. The simple information that the compound contains a halogen (any halogen) may be enough information to distinguish between two compounds. A sim- ple Beilstein test provides this information. DERIVATIVES One of the principal tests for the correct identification of an unknown compound comes in trying to convert the compound by a chemical reaction to another known compound. This second compound is called a derivative. The best derivatives are solid compounds, be- cause the melting point of a solid provides an accurate and reliable identification of most compounds. Solids are also easily purified through crystallization. The derivative provides a way of distinguishing two otherwise very similar compounds. Usually they will have de- rivatives (both prepared by the same reaction) that have different melting points. Tables of unknowns and derivatives are listed in Appendix 1. Procedures for preparing derivatives are given in Appendix 2. CONFIRMATION OF IDENTITY A rigid and final test for identifying an unknown can be made if an “authentic” sample of the compound is available for comparison. One can compare infrared and NMR Spectra of the unknown compound with the spectra of the knowu compound. If the spectra match, peak for peak, then the identity is probably certain. Other physical and chemical properties can also be compared. If the compound is a solid, a convenient test is the mixed melting point (Technique 6, Section 6.4). Thin-layer or gas-chromatographic comparisons may also be useful. For thin-layer analysis, however, it may be necessary to experiment with several different development solvents to reach a satisfactory conclusion about the identity of the substance in question. Although we cannot be complete in this experiment in terms of the functional groups covered, or the tests described, the experiment should provide a good introduc- tion to the methods and the techniques chemists use to identify unknown compounds. Textbooks that cover the subject more thoroughly are listed in the References. You are encouraged to consult these for more information, including specific methods and clas- sification tests. rwnc . .. Experiment 51A Solubility Tests 433 REFERENCES Comprehensive Textbooks Cheronis, N. D., and Entrikin, J. B. Identification of Organic Compounds. New York: Wiley-interscience, 1963. Pasto, D. J., and Johnson, C. R. Laboratory Test for Organic Chemistry. Englewood Cliffs, NJ: Prentice- Hall, 1979. Shriner, R. I... Fuson, R. C., Curtin, D. Y., and Morrill, T. C. The Systematic Identification of Organic Compounds, 6th ed. New York: John Wiley & Sons, 1980. Spectroscopy , Bellamy, L. J. The Infra-red Spectra of Complex Molecules, 3rd ed. New York: Methuen, 1975. Colthup, N. B., Daly, L. H., and Wiberly, S. E. Introduction to Infrared and Raman Spectroscopy, 3rd ed. San Diego, CA: Academic Press, 1990. Dyer, J. R. Application of Absorption Spectroscopy of Organic Compounds. Englewood Cliffs, NJ: Prentice-Hall, 1965. Lin-Vien, D., Collhup, N. B., Fateley, W. B., and Gtasselli, J. G. The Handbook oflnfrared and Roman Characteristic Frequencies of Organic Molecules. San Diego, CA: Academic Press, 1991. Nakanishi, K infrared Absorption Spectroscopy. San Francisco: Holden-Day, 1962. Pavia, D. L, Lampman, G. M., and Kriz, G. 5., Jr. Introduction to Spectroscopy: A Guide for Students of Organic Chemistry, 2nd ed. Philadelphia: W. B. Saunders, 1996. Silverstein, R. M., and Webster, F. X. Spectrometric identification of Organic Compounds, 6th ed. New York: John Wiley & Sons, 1998. Extensive Tables of Compounds and Derivatives Rappoport, 2., ed. Handbook of Tables for Organic Compound identification. Cleveland: Chemical Rub- ber (20., 1967. Experiment 51A Solubility Tests Solubility tests should be performed on every unknown. They are extremely important in determining the nature of the main functional group of the unknown compound. The tests are very simple and require only small amounts of the unknown. In addition, solubility tests reveal whether the compound is a strong base (amine), a weak acid (phenol), a strong acid (carboxylic acid), or a neutral substance (aldehyde, ketone, alcohol, ester). The com— mon solvents used to determine solubility types are 5% HCl Concentrated H2804 5% NaHC03 Water 5% NaOH Organic solvents The solubility chart on page 434 indicates solvents in which compounds containing the various functional groups are likely to dissolve. The summary charts in Experiments 51D through 51] repeat this information for each functional group included in this experi- ment. In this section, the correct procedure for determining whether a compound is soluble in a test solvent is given. Also given is a series of explanations detailing the reasons that 434 Part Five Identification of Organic Substances compounds having specific functional groups are soluble in only specific solvents. This is accomplished by indicating the type of chemistry or the type of chemical interaction that is possible in each solvent. WASTE DISPOSAL Dispose of all aqueous solutions in the container designated for aqueous waste. Any remaining organic compounds must be disposed of in the appropriate organic waste container. _ low MW turns red litmus blue—bases turns blue litmus red—acids Soluble low MW litmus is unchanged—neutral neutral 0 U H20 C MPO ND . . strong acids carboxylrjlic alclds NaHC03 soluble I some p can 5 insoluble insoluble soluble \—-—> weak acids insoluble solu le ‘ alkencs esters HCl alkynes ethers neutral alcohols amides . compounds ketones Insoluble lu le aldehydes so H2304 nitro compounds insoluble alkanes \ inert alkyl halides compounds aromatic compounds Solubility chart for compounds containing various functional groups. SOLUBI LlTY TESTS Procedure Place about 1 mL oFthe solvent in a small test tube. Add one drop oFan unknown liquid from a Pasteur pipe: or a Few crystals oF an unknown solid From the end ofa spatula, directly into the solvent. Gently tap the test tube with your finger to ensure mixing, and then observe whether any mixing lines appear in the solution. The disappearance oFthe liquid or solid or the appearance of P— Experirnent 51A Solubility Tests 435 the mixing lines indicates that solution is taking place. Add several more drops ofthe liquid or a few more crystals ofthe solid to determine the extent ofthe compound’s solubility. A common mistake in determining the solubility ofa compound is testing with a quantity of the unknown too large to dissolve in the chosen solvent. Use small amounts. It may take several minutes to dissolve solids. Compounds in the form of large crystals need more time to dissolve than pow— ders or very small crystals. In some cases, it is helpful to pulverize a compound with large crysta...
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