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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 (alkyl 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: KETONE PHENOL ALDEH'YDE SUBSIDIARY: Halide Nilro Alkene Aromatic Aromatic Aromatic Ether WW7wfifiwgfigfi—V WVfi—eguul 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 Cac 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 L “I. t among the possible choices. Choose only those tests that the solubility tests suggest might L : ' ’ itJ be meaningful. Time will be wasted performing unnecessary tests. There is no substitute 1 r for a firsthand, thorough knowledge of these tests. Study each of the sections carefully ‘I '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. rwns . .. 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. L., 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 Wiberiy, 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., Fatelcy, W. B., and Grasselli, 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 8.: Sons, 1998. Extensive Tables of Compounds and Derivatives Rappoport, 2., ed. Handbook of Tables for Organic Compound Identification. Cleveland: Chemical Rub- ber Co., 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 nemm 0 U H20 C MPO ND . . strong acids cal-box?” alclds NaHCOJ mlufile I some p can 5 insoluble insoluble soluble \—-—> weak acids insoluble solu le ‘ alkencs esters HCl alkynes ethers neutral alcohols amides I compounds ketones Insoluble sum 16 aldehydes sto‘ 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— Experiment 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 crystals using a mortar and pestle. Sometimes gentle heating helps, but strong heating is discouraged, as it often leads to reaction. When colored compounds dissolve, the solution often assumes the color. Using the preceding procedure, determine the solubility of the unknown in each of the fol- lowing solvents: water, 5% HCI, 5% NaHCO3, 5% NaOH, and concentrated H2504. With sulfuric acid, a color change may be observed rather than solution. A color change should be regarded as a positive solubility test. Solid unknowns that do not dissolve in any ofthe test solvents may be inorganic substances. To eliminate this possibility, determine the solubility ofthe unknown in several organic solvents, such as ether. lfthe compound is organic, a solvent that will dissolve it can usually be found. Ifa compound is found to dissolve in water, the pH ofthe aqueous solution should be esti- mated with pH paper or litmus. Compounds soluble in water are usually soluble in all the aque- ous solvents. lfa compound is only slightly soluble in water, it may be more soluble in another aqueous solvent. For instance, a carboxylic acid may be only slightly soluble in water but very sol- uble in dilute base. It often will not be necessary to determine the solubility ofthe unknown in every solvent. Test Compounds Five solubility unknowns can be found on the supply shelf. The five unknowns include a base, a weak acid, a strong acid, a neutral substance with an oxygen-containing functional group, and a neutral substance that is inert. Using solubility tests, distinguish these unknowns by type. Verify your answer with the instructor. Solubility in Water. Compounds that contain four or fewer carbons and also contain oxygen, nitrogen, or sulfur are often soluble in water. Almost any functional group con- taining these elements will lead to water solubility for low-molecular-weight (C4) com- pounds. Compounds having five or six carbons and any of those elements are often insoluble in water or have borderline solubility. Branching of the alkyl chain in a com- pound lowers the intermolecular forces between its molecules. This is usually reflected in a lowered boiling point or melting point and a greater solubility in water for the branched compound than for the corresponding straight-chain compound. This occurs simply be- cause the molecules of the branched compound are more easily separated from one an- other. Thus, t-butyl alcohol would be expected to be more soluble in water than n-butyl alcohol. When the ratio of the oxygen, nitrogen, or sulfur atoms in a compound to the carbon atoms is increased, the solubility of that compound in water often increases. This is due to the increased number of polar functional groups. Thus, 1,5-pentanediol would be expected to be more soluble in water than 1-pentanol. As the size of the alkyl chain of a compound is increased beyond about four carbons, the influence of a polar functional group is diminished, and the water solubility begins to decrease. A few examples of these generalizations are given here. 436 Part Five Identification of Organic Substances Soluble Borderline Insoluble CH3 0 o 0 CH3—CII—w—(uJ—OH CH3—CH—CH2—(HZ——OH CH3CHZCHZCH2—cli—on (EH, (in, CH3 CH3 CH3—(IZ—CH2—OH CH3—CH—(IJH—CH3 CH3CH2CH2CH2CI-I2—OH (in, 6H CH3 CH3 OOH 0:0H CH3—CH~©:0H (EH, Solubility in 5% BC]. The possibility of an amine should be considered immediately if a compound is soluble in dilute acid (5% HC]). Aliphatic amines (RNHZ, RZNH, R3N) are basic compounds that readily dissolve in acid because they form hydrochloride salts that are soluble in the aqueous medium: R—NHZ + HC] —> Fr-NHg,+ + C1— The substitution of an aromatic (benzene) ring Ar for an alkyl group R reduces the basicity of an amine somewhat, but the amine will still protonate, and it will still generally be soluble in dilute acid. The reduction in basicity in an aromatic amine is due to the resonance delocaliza- tion of the unshared electrons on the amino nitrogen of the free base. The delocalization is lost on protonation, a problem that does not exist for aliphatic amines. The substitution of two or three aromatic rings on an amine nitrogen reduces the basicity of the amine even fur ther. Diary] and triaryl amines do not dissolve in dilute HCl because they do not protonate easily. Thus, AerH and Ar3N are insoluble in dilute acid. Some amines of very high molec- ular weight, such a tribromoaniline (MW = 330), may also be insoluble in dilute acid. NHZ +NH2 +NH2 +NH2 NH3" AROMATIC : = _ H+ Delocalizalion No delocalization ALIPHATIC R—NHz ———“——* R—NH3+ AMINE No delocalization No delocalizalion Solubility in 5% NaHCO3 and 5% NaOH. Compounds that dissolve in sodium bicar- bonate, a weak base, are strong acids. Compounds that dissolve in sodium hydroxide, a strong base, may be either strong or weak acids. Thus, one can distinguish weak and strong acids by determining their solubility in both strong (NaOH) and weak (NaHCO3) basc. The classification of some functional groups as either weak or strong acids is given in the table on page 437. In this experiment, carboxylic acids (pKa ~ 5) are generally indicated when a com- pound is soluble in both bases, while phenols (pKa ~ 10) are indicated when it is soluble in NaOH only. * Experiment 51A Solubifily Tests 437 Compounds dissolve in base because they form sodium salts that are soluble in the aqueous medium. The salts of some high-molecular—weight compounds are not soluble, however, and precipitate. The salts of the long-chain carboxylic acids, such as myristic acid C14, palmitic acid C16, and stearic acid C13, which form soaps, are in this category. Some phenols also produce insoluble sodium salts, and often these are colored due to res- onance in the anion. Weak Acids (soluble in NaOH but not NaHC03) Strong Acids (soluble in both NaOH and NaHC03) Sulfonic acids RSO3H Phenols ArOH Carboxylic acids RCOOH Nitroalkanes RCHZNOZ Ortho- and para-substituted RZCHNOZ di- and trinitrophenols fi-Diketones El) C”) 0“ R—C—CHz— C—R N02 fi-Diesters R ('2') OH I ‘d ROBC—Cfizgc—OR In] as N02 N02 N02 II II R—Cw-NH—C—R Sulfonamides ArSOle-Iz N02 ArSOle-IR Both phenols and carborrylic acids produce resonance-stabilized conjugate bases. Thus, bases of the appropriate strength may easily remove their acidic protons to form the sodium salts. ‘n’ i? ‘1’" R—C—O—H + NaOH -—> R—C—r-O‘ 4-—-D R—C=O Na“ + H20 Delocalizcd anion OH 0— O O O Na+ fl HQC—b +H20 Delocalized anion In phenols, substitution of nitro groups in the ortho and para positions of the ring increases the acidity. Nitro groups in these positions provide additional delocalization in the conju- gate anion. Phenols that have two or three nitro groups in the ortho and para positions often dissolve in both sodium hydroxide and sodium bicarbonate solutions. Solubility in Concentrated Sulfuric Acid. Many compounds are soluble in cold con- centrated sulfuric acid. Of the compounds included in this experiment, alcohols, ketones, aldehydes, and esters are in this category. Other compounds that also dissolve include 438 Part Five Identification of Organic Substances alkenes, alkynes, ethers, nitroaromatics, and amides. Because several different kinds of compounds are soluble in sulfuric acid, further chemical tests and spectroscopy will be needed to differentiate among them. Compounds that are soluble in concentrated sulfuric acid but not in dilute acid are ex- tremely weak bases. Almost any compound containing a nitrogen, an oxygen, or a sulfur atom can be protonated in concentrated sulfuric acid. The ions produced are soluble in the medium. 4. R—o—H + H2804 —> R—(IJ—H + HSO4‘ m4 R+ + H20 + Hso4- H (n) TIHI R—C—R + H2804 —5 R—C—R + H504— ‘n’ +cll)—H R—C—OR + H2504 ——+ R—C—OR + H804“ \ / l | _ /c=c\ + st04 m+ R—p—g—R + Hso4 R R H Inert Compounds. Compounds not soluble in concentrated sulfuric acid or any of the other solvents are said to be inert. Compounds not soluble in concentrated sulfuric acid in- clude the alkanes, most simple aromatics, and the alkyl halides. Some examples of inert compounds are hexane, benzene, chlorobenzene, chlorohexane, and toluene. Experiment 51 B Tests for the Elements (N, S, X) —Br —1\ll— —CEN —NO2 —-c1 ---1 /8\ Except for amines (Experiment 51G), which are easily detected by their solubility behav— ior, all compounds issued in this experiment will contain heteroelements (N, S, Cl, Br, or I) only as secondary functional group. These will be subsidiary to some other important functional group. Thus, no alkyl or aryl halides, nitro compounds, thiols, or thioethers will be issued. However, some of the unknOWns may contain a halogen or a nitro group. Less frequently, they may contain a sulfur atom or a cyano group. , :- Experiment751 B Tests for the Elements (N, S, X) 439 mugs-LE -,;-_ Consider as an example p—bromobenzaldehyde, an aldehyde that contains bromine as a ring substituent. The identification of this compound would hinge on whether the investi- gator could identify it as an aldehyde. It could probably be identified without proving the existence of bromine in the molecule. That information, however, could make the identifi- , cation easier. In this experiment, methods are given for identifying the prescnce of a halo- , gen or a nitro group in an unknown compound. Also given is a general method (sodium % fusion) for detecting the principal heteroelements that may exist in organic molecules. Classification Tests Nitro Groups Ferrous hydroxide N, S, X (Cl, Br, I) Beilstein test Sodium fusion Silver nitrate Sodium iodide/acetone WASTE DISPOSAL Dispose of all solutions containing silver into a waste container designated for this purpose. Any other . l'li aqueous solutions should be disposed of in the container designated for aqueous waste. Any remaining " organic compounds must be disposed of in the appropriate organic waste container under the hood. This is particularly true of any solution containing benzyl bromide, which is a lachrymator. TESTS FOR A HALIDE Beilstein Test :11} Procedure. Bend a small loop in the end ot'a short length ofcopper wire. Heat the loop end of the wire in a Bunsen burner flame. AFter cooling, dip the wire directly into a small sample of ‘ the unknown. IF the unknown is a solid and won’t adhere to the copper wire, place a small . - amount oFthe substance on a watch glass, wet the copper wire in distilled water, and place the I wire into the sample on the watch glass. The solid should adhere to the wire. Now heat the wire in the Bunsen burner flame again. The compound will first burn. After the burning, a green Flame will be produced ifa halogen is present. Test Compounds. Try this test on bromobenzene and benzoic acid. Halogens can be detected easily and reliably by the Beilstein test. It is the simplest method for determining the presence of a halogen, but it does not differentiate among chlorine, bromine, and iodine, any one of which will give a positive test. However, when the identity of the unknown has been narrowed to two choices, of which one has a halogen and one does not, the Beilstein test will often be enough to distinguish between the two. A positive Beilstein test results from the production of a volatile copper halide when an organic halide is heated with copper oxide. The c0pper halide imparts a blue-green color to the flame. This test can be very sensitive to small amounts of halide impurities in some com- pounds. Therefore, use caution in interpreting the results of the test if you obtain only a weak color. 440 Part Five Identification of Organic Substances W Silver Nitrate Test Procedure. Add one drop oFa liquid or five drops oFa concentrated ethanoiic solution ofa solid unknown to 2 mL ofa 2% ethanolic silver nitrate solution. IF no reaction is observed aFter 5 minutes at room temperature, heat the solution in a hot water bath at about 100°C and note whether a precipitate Forms. lfa precipitate Forms, add two drops of 5% nitric acid and note whether the precipitate dissolves. Carboxylic acids give a false test by precipitating in silver nitrate, but they dissolve when nitric acid is added. Silver halides, in contrast, do not dissolve in nitric acid. Test Compounds. Appiy this test to benzyl bromide (a—bromotoluene) and bromobenzene. Discard all waste reagents in a suitable waste container in the hood, because benzyl bromide is a Iachrymator. This test depends on the formation of a white or off—white precipitate of silver halide when silver nitrate is allowed to react with a sufficiently reactive halide. CH3CH20H RX +Ag+NO3‘ —* AgX + R+NO3'—H—> R-O— CHZCH3 Precipitate The test does not distinguish among chlorides, bromides, and iodides but does distinguish labile (reactive) halides from halides that are unreactive. Halides substituted on an aro- matic ring will not usually give a positive silver nitrate test; however, alkyl halides of many types will give a positive test. The most reactive compounds are those able to form stable carbocations in solution and those equipped with good leaving groups (X = 1, Br, Cl), Benzyl, ally], and tertiary halides react immediately with silver nitrate. Secondary and primary halides do not react at room temperature but react readily when heated. Aryi and vinyl halides do not react at all, even at elevated temperatures. This pattern of reactivity fits the stability order for various carbocations quite well. Compounds that produce stable carbocations react at higher rates than those that do not. CH2+ i i + = R—$+ > R—gH > R‘HCHz-l' > CH3+ > R Benzyl ' 3° 2“ 1” Methyl Aryl and and RCH=CH—CH2+ RCH=CH+ Ally] Vinyl The fast reaction of benzylic and allylic halides is a result of the resonance stabilization that is available to the intermediate carbocations formed. Tertiary halides are more reactive than secondary halides, which are in turn more reactive than primary or methyl halides be- cause alkyl substituents are able to stabilize the intermediate carbocations by an electron— releasing effect. The methyl carbocations have no alkyl groups and are the least stable of all carbocations mentioned thus far. Vinyl and aryl carbocations are extremely unstable be- cause the charge is localized on an spZ-hybridized carbon (double-bond carbon) rather than on one that is sp3-hybridized. 444 Part Five Identification of Organic Substances Experiment 51C Tests for Unsaturation F"\C—C/R / _ \ R R R—CEC—R The unknowns to be issued for this experiment have neither a double bond nor a triple bond as their only functional group. Hence, simple alkenes and alkynes can be ruled out as possi- ble compounds. Some of the unknowns may have a double or a triple bond, however in ad- dition to another more important functional group. The tests described allow you to determine the presence of a double bond or a triple bond (unsaturation) in such compounds. Classification Tests Unsaturation Aromaticity Bromine-carbon tetrachloride Potassium permanganate ignition test WASTE DISPOSAL Test reagents that contain bromine should be discarded into a special waste container designated for this purpose. Methylene chloride and carbon tetrachloride must be placed in the organic waste container des- ignated for the disposal of halogenated organic wastes. Dispose of all other aqueous solutions in the con- tainer designated for aqueous waste. Any remaining organic compounds must be disposed of in the apprOpriate organic waste container. TESTS FOR SIMPLE MULTIPLE BONDS Bromine in Carbon Tetrachloride or Methylene Chloride Procedure. Dissolve 50 mg oFa solid unknown or two drops oFa liquid unknown in 1 ml. of carbon tetrachloride (or 1,2-dimethoxyethane). Add a 2% (by volume) solution of bromine in carbon tetrachloride, dropwise, and shake it until the bromine color persists. The test is positive if more than five drops of the bromine solution are needed so that the color remains for 1 minute. Usually, many drops oFthe bromine solution will be needed iFunsaturation is present. Hydrogen bromide should not be evolved. If hydrogen bromide gas is evolved, you will note a “fog” while you blow across the mouth oFthe test tube. The HBr can also be detected by a moist- ened piece oflitmus or pH paper. hhydrogen bromide is evolved, the reaction is a substitution re- action (see discussion below) and not an addition reaction, and a double or tripie bond is prob- ably not present. Experiment 51C Tests for Unsaturatian 445 Methylene Chloride. Even though carbon tetrachloride is used in very small quantities in this test, it poses certain health hazards (see p. 17), and another solvent may be preferable. Methy— lene chloride (dichloromethane) can be substituted for carbon tetrachloride. Certain problems 7 arise, however, because methylene chloride slowly reacts with bromine, presumably by a light- induced free-radical process, to produce HBr. After about 1 week, the color ofa 2% solution of bromine in methylene chloride fades noticeably, and the odor ofthe HBr can be detected in the reagent. Although the decolorization tests still work satisfactorily, the presence of HBr makes it difficult to distinguish between addition and substitution reactions. A freshly prepared solution of bromine in methylene chloride must be used to make this distinction. Deterioration of the reagent can be forestalled by storing it in a brown glass bottle. Most other substitute solvents also present problems. Ethers, for insrance, react slowly in the same way as methylene chloride, and hydrocarbons, such as hexane, are not general enough solvents to be able to dissolve all the possible test compounds. Test Compounds. Try this test with cyclohexene, cyclohexane, toluene, and acetone. A successful test depends on the addition of bromine, a red liquid, to a double or a triple bond to give a colorless dibromide: Br \ / \ l / + Bl'2 -"—’ /(|:-C\ Br Red Colorless Not all double bonds react with bromine—carbon tetrachloride solution. Only those that are electron—rich are sufficiently reactive nucleophiles to initiate the reaction. A double bond that is substituted by electron-withdrawing groups often fails to react or reacts slowly. Fu- maric acid is an example of a compound that fails to give the reaction. H\C_C/COOH HOOC/ \H Fumaric acid Aromatic compounds either do not react with bromine—carbon tetrachloride reagent or they react by substitution. Only the aromatic rings that have activating groups as sub— stituents (-— OH, -—OR, or —NR2) give the substitution reaction. OH OH H H H H + Brz —> + ortho isomers + HBr H H H H e“?- H Br Some ketones and aldehydes react with bromine to give a substitution product, but this reaction is slow except for ketones that have a high enol content. When substitution occurs, not only is the bromine color discharged, but hydrogen bromide gas is also evolved. 446 Part Five Identification of Organic Substances Potassium Permanganate (Baeyer Test) Procedure. Dissolve 25 mg oFa solid unknown or two drops of'a liquid unknown in 2 mL of water or 95% ethanol (1,2-dimethoxyethane may also be used). Slowly add a 1% aqueous so- lution (weight/volume) oF potassium permanganate, drop by drop while shaking, to the un- known. in a positive test, the purple color of the reagent is discharged, and a brown precipitate of manganese dioxide Forms, usually within 1 minute. IF alcohol was the solvent, the solution should not be allowed to stand For more than 5 minutes, because oxidation ofthe alcohol will begin slowly. Because permanganate solutions undergo some decomposition to manganese dioxide on standing, any small amount of amount 0F precipitate should be inter- preted with caution. Test: Compounds. Try this test on cyclohexene and toluene. This test is positive for double and triple bonds but not for aromatic rings. It depends on the conversion of the purple ion Mn04‘ to a brown precipitate of MnOZ following the oxi- dation of an unsaturated compound. \ / _ \ / /c=c\+ MnO4 ——> /c—c\+ Mno2 OH OH Purple Brown Other easily oxidized compounds also give a positive test with potassium permanganate solution. These substances include aldehydes, some alcohols, phenols, and aromatic amines. If you suspect that any of these functional groups are present, you should interpret the test with caution. Spectroscopy Infrared Double Bonds (C = C) Triple Bonds ( C = C) C=C stretch usually occurs near 1680— C=C stretch usually occurs near 2250— 1620 cm“. Symmetrical alkenes 2100 cm'l. The peak is usually may have no absorption. sharp. Symmetrical alkynes show C—H stretch of vinyl hydrogens occurs no absorption. >3000 em", but usually not higher C-—H stretch of terminal acetylenes than 3150 cm“. occurs near 3310—3200 cm". C—H out-of—plane bending occurs near 1000—700 cm‘1 (see Appendix 3). Nuclear Magnetic Resonance Vinyl hydrogens have resonance near 5—7 ppm and have coupling vaIUCs as follows: IN", = 11-18 Hz, J“, = 6—15 Hz, J gem”, = 0—5 Hz. Allylic hydrogens have resonance near 2 ppm. Acetylenic hydrogens have resonance near 2.8—3.0 ppm. Experiment 51H Alcohols 461 Nuclear Magnetic Resonance The resonance position of amino hydrogens is extremely variable. The resonance may also be very broad (quadrupole broadening). Aromatic amines give resonances near 7 ppm due to the aromatic ring hydrogens. Derivatives The derivatives of amines that are most easily prepared are the acetamides and the beam- mides. These derivatives work well for both primary and secondary amines but not for ter- tiary amines. 0 0 CH3—(“3—c1 + RNHZ —. CH3—(”:— NH—R + HCI Acetyl chloride An acetamide 0 o Qii—CI + RNH2 —» Qg—NH—R + HCl Benzoyl chloride A benzamide The most general derivative that can be prepared is the picric acid salt, or picrate, of an amine. This derivative can be used for primary, secondary, and tertiary amines. OH 0— N02 N02 N02 N02 + R3Ni —> R3NH+ N02 NO2 Picric acid A picrate For tertiary amines, the methiodide salt is often useful. CH3I+ R3N: "—- CH3—NR3+I‘ Amethiodide Procedures for preparing derivatives from amines can be found in Appendix 2. Experiment 51 H Alcohols RCHZOH R R—i—OH R :CH—OH R 462 Part Five Identification of Organic Substances Alcohols are neutral compounds. The only other classes of neutral compounds used in this experiment are the aldehydes and ketones and the esters. Alcohols and esters usually do not give a positive 2,4-dinitrophenylhydrazine test; aldehydes and ketones do. Esters do not react with acetyl chloride or with Lucas reagent, as alcohols do, and they are easily dis- tinguished from alcohols on this basis. Primary and secondary alcohols are easily oxi- dized; esters and tertiary alcohols are not. A combination of the Lucas test and the chromic acid test will differentiate among primary, secondary, and tertiary alcohols. Solubility Characteristics Classification Tests HCl NaHCO3 NaOH H2804 Ether (-) (-) H (+) (+) Water: <C5(+) >Coi‘) Acetyl chloride Lucas test Chromic acid test Iodoform test WASTE DISPOSAL Any solution containing chromium must be disposed of by placing it in a waste container specifically identified for the diSposal of chromium wastes. Dispose of all other aqueous solutions in the container designated for aqueous waste. Any remaining organic compounds must be disposed of in the appropriate organic waste container. CLASSIFICATION TESTS Acetyl Chloride Procedure. Cautioust add about 10—1 5 drops ofacetyl chloride, drop by drop, to about 0.5 mL ofthe liquid alcohol contained in a small test tube. Evolution ofheat and hydrogen chloride gas indicates a positive reaction. Adding water will sometimes precipitate the acetate. Acid chlorides react with alcohols to form esters. Acetyl chloride forms acetate esters. 0 0 ll || CH3—C—Cl + ROI-l -—-+ CHg—C—O—R + HCl Usually the reaction is exothermic, and the heat evolved is easily detected. Phenols react with acid chlorides somewhat as alcohols do. Hence, phenols should be eliminated as pos— sibilities before this test is attempted. Amines also react with acetyl chloride to evolve heat (see Experiment 51G). This test does not work well with solid alcohols. Lucas Test Procedure. Place 2 mL of Lucas reagent in a small test tube, and add three to Four drops of the alcohol. Stopper the test tube, and shake it vigorously. Tertiary (3°), benzylic, and allylic al- cohols give an immediate cloudiness in the solution as the insoluble alkyi halide separates From the aqueous solution. After a short time, the immiscible alkyl halide will Form a separate layer. Experiment 51H Alcohols 463 Secondary (2°) alcohols produce a cloudiness after 2—5 minutes. Primary (1 °) alcohols dissolve in the reagent to give a clear solution. Some secondary alcohols may have to be heated slightly to encourage reaction with the reagent. Note: This test works only for alcohols that are soluble in the reagent. This often means that alcohols with more than six carbon atoms cannot be tested. Test Compounds. Try this test with a 1-butanol (n-butyl alcohol), 2-butanol (sec-bury! alco- hol), and 2-methyI—2-proponal (t-butyl alcohol). Reagent. Cool 10 mL ofconcentrated hydrochloric acid in a beaker, using an ice bath. While Still cooling and while stirring, dissolve 16 g of'anhydrous zinc chloride in the acid. This test depends on the appearance of an alkyl chloride as an insoluble second layer when an alcohol is treated with a mixture of hydrochloric acid and zinc chloride (Lucas reagent): ZnCI2 R-—OH + HCl R— C] + H20 Primary alcohols do not react at room temperature; therefore, the alcohol is seen simply to dissolve. Secondary alcohols react slowly, whereas tertiary, benzylic, and allylic alcohols react instantly. These relative reactivities are explained on the same basis as the silver ni- trate reaction, which is discussed in Experiment 51B. Primary carbocations are unstable and do not form under the conditions of this test, hence no results are observed for primary alcohols. ’l R . l l 51' ' _ R—(lj—om znc12 —> R—(IJ—CIJ---ZnC12 —> R— (13* g+R—(lj—c1 R R H R R The Lucas test does not work well with solid alcohols. Ch romic Acid Test Procedure. Dissolve one drop ofa liquid or about 10 mg oFa solid alcohol in 1 mL oFreagent- grade acetone. Add one drop ofthe chromic acid reagent, and note the result that occurs within 2 seconds. A positive test For a primary or a secondary alcohol is the appearance of'a blue-green color. Tertiary alcohols do not produce the test result within 2 seconds, and the solution remains orange. To make sure that the acetone solvent is pure and does not give a positive test result, add one drop of'chromic acid to 1 mL ofacetone that does not have an unknown dissolved in it. The orange color oFthe reagent should persist Forat least 3 seconds. IFit does not, a new bottle of acetone should be used. Test Compounds. Try this test with 1-butanol (n-butyl alcohol), 2-butanol (sec-butyl alco- hol), and 2-methyl-2-pr0panol (t-butyl alcohol). CAUTION: Many compounds of chromium(Vl) are suspected carcinogens (see p. 19) and should be handled with care. Avoid contact. i ll i _i 464 Part: Five Identification of Organic Substances Reagent. Dissolve 1 g of chromic oxide Cr03 in 1 mL of concentrated sulfuric acid. Carefully add the mixture to 3 mL ofwater. This test is based on the reduction of chromium(VI), which is orange, to chromiumflll), Which is green, when an alcohol is oxidized by the reagent. A change in color of the reagent from orange to green represents a positive test. Primary alcohols are oxidized by the reagent to carboxylic acids; secondary alcohols are oxidized to ketones. 2 Cr03 + 2 H20 3—» 2 H20 0431+ H2Cr207 + H20 H R—tlz—H R—C—H fl R—CmOH (l)H (l ("3 Primary alcohols H R—(lZ—R R—C—R a ll Secondary alcohol Although primary alcohols are first oxidized to aldehydes, the aldehydes are further Oxidized to carboxylic acids. The ability of chromic acid to oxidize aldehydes but not ketones is taken advantage of in a test that uses chromic acid to distinguish between alde-hydes and ketones (Experiment 51D). Secondary alcohols are oxidized to ketones, but no further. Tertiary alcohols are not oxidized at all by the reagent, hence this test can be used to distinguish primary and secondary alcohols from tertiary alcohols. Un- like the Lucas test, this test can be used with all alcohols regardless of molecular WCight and solubility. m lodoForm Test Alcohols with the hydroxyl group at the 2-position of the chain give a positive iodoform test. See the discussion in Experiment 51D, Aldehydes and Ketones. ——.__—~—H—'_—__—Hv_ Spectroscopy Infrared 0“H stretch. A medium to strong, and usually broad, absorption comes in the region 3600—3200 cm‘l. In dilute solutions or with little hydrogen bonding, there is a sharp absorption near 3600 cm“. In more concentrated solutions, or with considerable hydrogen bonding, there is a broad absorption near 3400 cm‘l. Sometimes both bands appear. C—‘O stretch. There is a strong absorption in the region 1200—1500 cm“. Primary alcohols absorb nearer 1050 cm‘]; tertiary alcohols and phenols absorb nearer 1200 cm'l. Secondary alcohols absorb in the middle of this range. Experiment 51I Esters 465 Nuclear Magnetic Resonance The hydroxyl resonance is extremely concentration-dependent, but it is usually found be- tween 1 and 5 ppm. Under normal conditions, the hydroxy] proton does not couple with protons on adjacent carbon atoms. Derivatives The most common derivatives for alcohols are the 3,5-dinitrobenzoate esters and the phenylurethanes. Occasionally the a-naphthylurethanes (Experiment 51F) are also pre- pared, but these latter derivatives are more often used for phenols. 0 N ozN 2 r it C—Cl + ROH ——> C—O—R + HCl OzN ozn 3.5-Dinitrobenzoyl A 3,5-dinitrobenzoate chloride ‘u’ ON: C=O + ROH -—> QNH—C—O—R Phenyl isocyanate A phenylurethane Procedures for preparing these derivatives are given in Appendix 2. Experiment 511 Esters 0 il C \ R/ O—R' Esters are formally considered “derivatives” of the corresponding carboxylic acid. They are frequently synthesized from the carboxylic acid and the appropriate alcohol: R—COOH + R'—OH 35—1» R—COOR' + H20 Thus esters are sometimes referred to as though they were composed of an acid part and an alcohol part. Although esters, like aldehydes and ketones, are neutral compounds that have a car- bonyl group, they do not usually give a positive 2,4-dinitrophenylhydrazine test result. The two most common tests for identifying esters are the basic hydrolysis and ferric hydroxa- mate tests. ...
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B - Unknwn1 - 428 Part Five Identification of Organic...

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