A Qualitative Experiment for Organic Chemistry Lab Christine K. F. Hermann Box 6949, Radford University, Radford, VA24142

We recently incorporated a qualitative organic experi- ment into the regular organic chemistry lab. This experi- ment was introduced near the end of the second semester. The use of spectroscopy and the chemistry of the functional groups had been discussed in the lecture by this time (1 ) .

The students were told that the unknowns were either an alcohol, a carboxylic acid, an amine, an aldehyde, or a ketone. Two laboratory periods were allowed for each stu- dent to identify one unknown. The identifications were based on melting points or boiling points, the IR and NMR spectra, and a set of four classification tests. The boiling points were corrected for standard pressure (2).

The students prepared their unknowns for NMR spec- trum by dissolving in carbon tetrachloride or deuterated dimethyl sulfoxide. The students ran the NMR under the supervision of an instructor.

To obtain the IR spectra, the students made a KBr pel- let or placed the neat unknown between salt plates. Again, the students obtained their own spectra by using the FT- IR spectrometer under supervision. The students were not allowed to use the computer spectral library.

The four classification tests were each used with a com- pound that gave a known positive test, a compound that gave a known negative test, and the unknown. The known compounds are listed in Table 1. The classification tests used were reaction with 2,4-dinitrophenylhydrazine to test for aldehydes and ketones, chromic acid to test for alco- hols, sodium bicarbonate to test for carboxylic acids, and acetyl chloride to test for amines. Each student reported the observations with each test, followed by a discussion of the whether the unknown gave a positive or negative re- sult for the functional group(~) tested.

Optional procedures included the test for unsatnration using bromine or potassium dichromate, the determina- tion for the Dresence of a halogen bv the Beilstein test or w ~ t h d v e r nitrate, and the n&.aiization equwalent for the detennin:uion of the cau~valcnt we~eht ot'a rarboxvlic acid.

The instructor was available for checking the boiling points, the melting points, the interpretation of the spec- tra, and the results of the classification tests.

The unknowns were chosen from a list of compounds that aoneared in the derivative tables of their laboratorv textbobk (3) .

From a class of 43 students, only four students incor- rectly identified the unknown.

Experimental 2+Dinitrophenylhydrazine Test

Dissolve 20 mg or two drops of the unknown in 1 mL of 95% ethanol. Add 1 mL of the 2,4-dinitrophenylhydrazine reagent (see Table 2). Shake the solution vigorously. A yel- low to red precipitate is a positive test, indicating the pres- ence of aldehydes or ketones.

Chromic Acid Test--Jones Reagent

Dissolve 20 mg or two drops of the unknown in 1 mL of pure acetone. Add one or two drops of the chromic acid re- agent (see Table 2). The formation of a green precipitate is a positive test. Primary alcohols, secondary alcohols, and

aldehydes give a positive test. Tertiary alcohols give a negative test.

Sodium Bicarbonate Tesf

Place 20 mg or two drops of the unknown compound in 5'; sodium bicarbonate solution Evolution 01' carbon diox- ide gas is a positive test, indicating that a carboxylic acid is present.

Acetyl Chloride Test

Place 20 mg or two drops of the unknown in a test tube. Slowly add 10 drops of acetyl chloride. Evolution of heat and gas is a positive test. Alcohols, primary amines, and secondary amines give a positive test. Tertiary amines give a negative test.

Bromine Test

Dissolve 20 mg or two drops of the unknown in 0.5 mL of carbon tetrachloride or methylene chloride. Add one drop of a 2% solution of bromine in carbon tetrachloride. Shake the mixture. Apositive test for unsaturation is indi- cated by the red color dissipating immediately, without the evolution of hydrogen bromide gas. Most compounds con- taining double bonds or triple bonds give positive tests. Aromatic compounds react with bromine and produce hy- drogen bromide gas.

Potassium Permanganate Test

Dissolve 20 mg or two drops of the unknown in 1 mL of water or 95% ethanol. Add one drop of a 1% potassium per- manganate solution. Apositive test is the loss of the purple color and the formation of a brown solid. Compounds con- taining double or triple bonds give positive results. Aro- matic rings give negative results. Positive results also are produced by aldehydes, some alcohols, phenols, and aro- matic amines.

Table 1. Known compounds

Classification Test Known Positive Known Negative

2,4-Dinitrophenylhydrazine Acetophenone Toluene

Chromic Acid Ethanol Toluene

Sodium Bicarbonate Butanoic acid Toluene

Acetyi chloride Aniline Toluene

Table 2. Preparation of Reagents

2.4-dinitrophenylhydrazine Dissolve 1.5 g of 2,4- reaaent dinitroohenvlhvdrazine in 7.5 mL of

concentrated sulfuric acid. Add the mixture to 10 mL of water and 35 mL of 95% ethanol, with stirring.

chromic acid reagent Dissolve 5.0 g of chromium trioxide in 5 mL of concentrated sulfuric acid. Dilute with 15 mLof water.

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Weilstein Test Neutralization Equivalent

A copper wire is bent around to form a small loop. I t is heated in a flame for a few minutes. Afier the wire has cooled, dip it into a solution of the unknown dissolved in ethanol. Place the wire hack into the flame. Agreen flame is a positive test for the presence of a halogen.

Silver Nitrate Test

Dissolve 20 me or two d r o ~ s of the unknown in 1 mL of 95% ethanol. ~ d i this mixtire to 2 mL of a 2% silver ni- trate solution. If no ~ r ec i~ i t a t i on is observed within five minutes, heat the sol;tiongently, then cool. Apositive test will he the formation of a precipitate. If the precipitate is formed, add two drops of a 5% nitric acid solution to the mixture. Carhoxylic acids produce a solid that dissolves in nitric acid. A precipitate that does not dissolve in nitric acid indicates the presence of a halogen. Benzyl, ally], and tertiary halides give a positive test immediately. Secon- dary and tertiary halides give a positive result after heat is applied to the solution. Aryl and vinyl halides do not re- act a t all.

Approximately 0.2 g (measured to three decimal places) of the carboxvlic acid is dissolved in 50 mL of water or 95!1 ethanol in a 125-mL Erlenmeyer flask. The solution is titrated with a sodium hydroxide solution of known molar- ity (about 0.1 MI. Phenolphthalein is used as an indicator. The neutralization equivalent (NE) is calculated by the fol- lowing equation:

g of acid NE = M of NaOH x L of NaOH used

The neutralization equivalent or a multiple thereof is equal to the molecular weight, depending upon the num- ber of -COOH groups present in the structure.

Literature Cited 1. Modson, R. T.; Boyd, R. N. Olgonlc Ckrnislry. 6UI ed.; Rentice-Hdl: Englewood

Cliffs, NJ, 1992. 2. SWner,R. L.;Fuaon,R. C.;Curtin,D.Y; Mon3l.T.C. TheSysfornoflcIdenfifimtion

oforeonic Cornoounds, 6th d.: WAY: New Ymk, 1980 r, 49. . . 3. Pavia,D. L.:Lampman,G. M.;Knz,G.S.OrgonieLobomfory~hniq~s,3rded.; W.

B. Saunders Company: New Ymk. 1988; pp 682690.

992 Journal of Chemical Education