A Qualitative Experiment for Organic Chemistry Lab Christine K. F. Hermann Box 6949, Radford University, Radford, VA24142 We recently incorporated a qualitative organic experiment into the regular organic chemistry lab. This experiment 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 a n alcohol, a carboxylic acid, a n amine, an aldehyde, or a ketone. Two laboratory periods were allowed for each student 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 spectrum by dissolving in carbon tetrachloride or deuterated dimethyl sulfoxide. The students ran the NMR under the supervision of a n instructor. To obtain the IR spectra, the students made a KBr pellet or placed the neat unknown between salt plates. Again, the students obtained their own spectra by using the FTIR spectrometer under supervision. The students were not allowed to use the computer spectral library. The four classification tests were each used with a compound 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-dinitrophenylhydrazineto test for aldehydes and ketones, chromic acid to test for alcohols, 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 result for the functional group(~) tested. Optional procedures included the test for unsatnration using bromine or potassium dichromate, the determination for the Dresence of a halogen bv the Beilstein test or w ~ t hd v e r nitrate, and the n&.aiization equwalent for the detennin:uion of the cau~valcntwe~ehtot'a rarboxvlic acid. The instructor was available for checking the boiling points, the melting points, the interpretation of the spectra, 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 incorrectly identified the unknown. Experimental 2+Dinitrophenylhydrazine Test Dissolve 20 mg or two drops of the unknown in 1mL of 95% ethanol. Add 1mL of the 2,4-dinitrophenylhydrazine reagent (see Table 2). Shake the solution vigorously. A yellow to red precipitate is a positive test, indicating the presence of aldehydes or ketones.
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 dioxide 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 indicated by the red color dissipating immediately, without the evolution of hydrogen bromide gas. Most compounds containing double bonds or triple bonds give positive tests. Aromatic compounds react with bromine and produce hydrogen bromide gas. Potassium Permanganate Test Dissolve 20 mg or two drops of the unknown in 1mL of water or 95% ethanol. Add one drop of a 1% potassium permanganate solution. Apositive test is the loss of the purple color and the formation of a brown solid. Compounds containing double or triple bonds give positive results. Aromatic rings give negative results. Positive results also are produced by aldehydes, some alcohols, phenols, and aromatic amines. Table 1. Known compounds Classification Test
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 sulfuricacid. Dilute with 15 mLof water.
Chromic Acid Test--Jones Reagent Dissolve 20 mg or two drops of the unknown in 1mL of pure acetone. Add one or two drops of the chromic acid reagent (see Table 2). The formation of a green precipitate is a positive test. Primary alcohols, secondary alcohols, and
Known Positive
Volume 71
Number 11 November 1994
991
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.
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 125-mL Erlenmeyer flask. The solution is titrated with a sodium hydroxide solution of known molarity (about 0.1 MI. Phenolphthalein is used a s a n indicator. The neutralization equivalent (NE)is calculated by the following equation:
Silver Nitrate Test
Dissolve 20 m e or two d r o ~ of s the unknown in 1 mL of 95% ethanol. ~ d this i mixtire to 2 mL of a 2% silver nitrate solution. If no ~ r e c i ~ i t a t i oisn 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. Secondary and tertiary halides give a positive result after heat is applied to the solution. Aryl and vinyl halides do not react a t all.
992
Journal of Chemical Education
a
NE =
g of acid M of NaOH x L of NaOH used
The neutralization equivalent or a multiple thereof is equal to the molecular weight, depending upon the number 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 WAY:New Ymk, 1980 r, 49. . . Cornoounds,6th d.: 3. Pavia,D. L.:Lampman,G. M.;Knz,G.S.OrgonieLobomfory~hniq~s,3rded.; W. B. Saunders Company: New Ymk. 1988; pp 682690.
