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THE LABORATORY PREPARATION OF A SIMPLE VITAMIN: p-AMINOBENZOIC ACID CHESTER B. KREMER The City College, New York, N. Y.
INTEREST in an organic laboratory preparation is frequently heightened when the preparation is known to possess physiological activity. Thus the preparation of the simple antibiotic sulfanilamide has always been a favorite in the organic laboratory. In a similar sense, a synthesis of the simple vitamin p-aminobenzoic acid has been found to stimulate student interest. Although the role p-aminobenzoic acid plays as a vitamin is not too clearly defined, there appears to be agreement that it is one of three vitamins concerned with skin pigmentation. The other two vitamins are pantothenic acid and biotin. One cannot rightly term p-aminobenzoic acid the "anti-gray hair" vitamin, capable of converting gray hair into black hair, even though its absence in the diet of certain animals results in the production of gray hair. Aging in the human is a far more complicated process. The vitamin also has practical medical application in the treatment of typhus and Rocky Mountain spotted fever. Another interesting property is its ability to inhibit the actiou of sulfanilamide on streptococci. From the laboratory point of view, the preparation of p-aminobenzoic acid possesses several interesting features. First, it is a "sequence" type of experiment, in which the student starts with a simple organic compound and, utilizing the product of each step, progresses to the desired molecule. The chemistry involved is simple and easily within the scope of the first-year organic chemistry student. Further, those procedures and techniques involved in acylation, oxidation, and hydrolysis are covered in the one experiment. Finally, readily crystallizable compounds are obtained in good yield a t each stage. While it is feasible to start the sequence with a simpler molecule (for example, toluene), it is suggested that p-toluidine be employed as the starting compound. The chemistry involved is outlined below:
The reduction of p-nitrobenzoic acid with ammonium sulfide was first used to prepare p-aminobenzoic acid.' Better results were obtained when tin and hydrochloric acid were substituted as reducing agents2 Among the first to employ oxidation of aceto-ptoluidide, followed by hydrolysis of the acetamino group, were Hoffmann3and Kaiser4who used potassium permanganate as the oxidant, and Ullmann5 who used calcium permanganate. The necessary apparatus for the experiment is simple and standard. A three-neck, round-bottomed flask, fitted with a reflux condenser and stirrer, is suitable for the permanganate oxidation. If a three-neck flask is not available, an ordinary round-bottomed flask, fitted with an addition tube equipped with side arm attachment for a reflux condenser, serves equally as well. Good stirring is essential during the course of the oxidation. PROCEDURE
Preparation of Aceto-p-toluidide. Place 32 g. (0.3 mole) of powdered p-toluidine in a mixture of 750 ml. of water and 27 ml. of concentrated hydrochloric acid. If necessary, warm on a steam bath with stirring, t o aid solution. If the solution is dark colored, add one or two grams of decolorizing charcoal, stir for several minutes, and filter by gravity. Prepare a solution of 48 g. (0.34 mole) of sodium acetate trihydrate in 80 ml. of water. Warm the decolorized solution of p-toluidine hydrochloride to 50°C. Add 33.4 ml. (0.35 mole) of acetic anhydride, stir quickly, and immediately add the previously prepared sodium acetate solution. Mix thoroughly and cool in an ice-bath. Filter the crystals with suction, wash three times with small amounts of cold water, and air dry. Yield, 80-90 per cent; m. p., 146-7°C. Preparation of p-Acelaminobenzoic Acid. Equip a
' FISCHER, G., Ann., 127, 142 (1862).
' W~LBRAND, J., AND F. BEILSTEIN, ibid., 128, 264 (1863). a HOFFMANN, A. W., Ber., 9,1303 (1876). ' KAISER,A., ibid., 18, 2943 (1885). ULLMANN, F., ihid., 36, 1801 (1903).
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2-liter, three-neck, round-bottomed flask with a reflux condenser and mechanical stirrer. Place 20 g. (0.13 mole) of aceto-p-toluidide and 50 g. of magnesium sulfate hydrate in the flask and add 1500 ml. of water. Heat the mixture on a steam bath, start the stirrer, and add a sludge of 60 g. of powdered potassium permanganate in a small amount of water through the third neck of the flask. Stopper the flask and heat a t 85'C. (approximately the temperature maintained by the steam bath) for two hours with constant stirring. Filter the hot solution through a large fluted filter paper. Wash the precipitated manganese dioxide with a small amount of hot water. If the filtrate shows the presence of excess permanganate (purple color), add not more than one ml. of ethanol, heat on the steam bath for another 30-60 minutes (or allow to stand overnight), and once more filter the hot solution through a fluted filter. Cool the colorless filtrate and acidify with
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excess dilute sulfuric acid. Filter with suction and air dry. Yield, 60-70 per cent; m. p., 250-2°C. Preparation of p-Aminobenzoic Acid. Place the p-acetaminobenzoic acid obtained above in a 300-ml., round-bottomed flask attached to a reflux condenser. Add hydrochloric acid (made by mixing equal volumes of concentrated hydrochloric acid and water) in the ratio of 6 ml. of the diluted acid to each gram of the p-acetaminobenzoic acid. Reflux gently (strong heatiug results in the loss of hydrogen chloride through the condenser) for 30 minutes. Cool, add an equal volume of water, and make just alkaline t o litmus paper with ammonia water (use dilute ammonia for the final stages). For each 30 ml. of the final solution, add one ml. of glacial acetic acid, chill in an ice bath, and initiate crystallization by scratching with a glass rod. Filter with suction and dry. The purified product melts a t 186-7°C.
