XVII
PHENYLACETIC ACID
C6H5CH2CN + 2H2O + H2SO4—> C6H5CH2CO2H + NH4HSO4
Prepared by ROGER ADAMS and A. F. THAL. Checked by O. KAMM and A. O. MATTHEWS.
1. Procedure
IN a 5-l. round-bottom flask, fitted with a mechanical stirrer and reflux condenser, are mixed 1150 cc. of water, 840 cc. of commercial sulfuric acid and 700 g. of benzyl cyanide (preparation III, p. 9). The mixture is heated under a reflux condenser and stirred for three hours, cooled slightly and then poured into 2 l. of cold water. The mixture should be stirred so that a solid cake is not formed; the phenylacetic acid is then filtered off. This crude material should be melted under water and washed by decantation several times with hot water. These washings, on cooling, deposit a small amount of phenylacetic acid which is filtered off and added to the main portion of material. The last of the hot water is poured off from the material while it is still molten and it is then transferred to a 2-l. Claisen distilling flask and distilled in vacuo. A small amount of water comes over first and is rejected; about 20 cc., containing an appreciable amount of benzyl cyanide, then distils. This fraction is used in the next run. The distillate boiling 176-189'0/50 mm. is collected separately and solidifies on standing. It is practically pure phenylacetic acid, m. p. 76-76.5'0; it amounts to 630 g. (77.5 per cent of the theoretical amount). As the fraction which is returned to the second run of material contains a considerable portion of phenylacetic acid, the yield actually amounts to at least 80 per cent.
For the preparation of small quantities of phenylacetic acid, it is convenient to use the modified method given in the Notes.
2. Notes
The standard directions for the preparation of phenylacetic acid specify that the benzyl cyanide is to be treated with dilute sulfuric acid prepared by adding three volumes of sulfuric acid to two volumes of water. There action, however, goes so vigorously that it is always necessary to have a trap for collecting the benzyl cyanide which is blown out of the apparatus. The use of the more dilute acid, as described in the above directions, is more satisfactory.
The phenylacetic acid may also be made by boiling under a reflux condenser for eight to fifteen hours, without a stirrer, but this method is not nearly so satisfactory as that described in the procedure.
When only small quantities of the acid are required, the following modified procedure is of value. One hundred grams of benzyl cyanide are added to a mixture containing 100 cc. of water, 100 cc. of concentrated sulfuric acid, and 100 cc. of glacial acetic acid. After this has been heated for forty-five minutes under a reflux condenser, the hydrolysis is practically complete. The reaction mixture is then poured into water, and the phenylacetic acid isolated in the usual manner.
The odor of phenylacetic acid is disagreeable and persistent.
3. Other Methods of Preparation
The standard method of preparation of phenylacetic acid is by the hydrolysis of benzyl cyanide with either alkali[1a] or acid.[2a] The acid hydrolysis runs by far the more smoothly and so was the only one studied. There are numerous other ways in which phenylacetic acid has been formed, but none of them is of practical importance for its preparation. These methods include the following: the action of water on phenyl ketene;[3a] the hydrolysis and subsequent oxidation of the product between benzaldehyde and hippuric acid;[1] the reduction of mandelic acid;[2] the reduction of benzoylformic acid with hydriodic acid and phosphorus;[3] the hydrolysis of benzyl glyoxalidone;[4] the fusion of atropic acid with potassium hydroxide;[5] the action of alcoholic potash upon chlorophenylacetylene;[6] the action of benzoyl peroxide upon phenylacetylene;[7] the alkaline hydrolysis of triphenylphloroglucinol;[8] the action of ammonium sulfide upon acetophenone;[9] the heating of phenylmalonic acid;[10] the hydrolysis of phenylacetoacetic ester;[11] the action of hydriodic acid upon mandelonitrile.[12]
[1a] Ann. 96, 247 (1855); Ber. 14, 1645 (1881); Compt. rend. 151, 236 (1910).
[2a] Ber. 19, 1950 (1886).
[3a] Ber. 44, 537 (1911).
[1] Ann. 370, 371 (1909)a
[2] Chem. (2) 1, 443 (1865); Ber. 14, 239 (1881).
[3] Ber. 10, 847 (1877)
[4] J. prakt. Chem. (2) 82, 52, 58 (1910).
[5] Ann. 148, 242 (1868).
[6] Ann. 308, 318 (1899).
[7] J. Russ. Phys. Chem. Soc. 42, 1387 (1910); Chem. Zentr. 1911 (I) 1279.
[8] Ann. 378, 263 (1911).
[9 Ber. 21, 534 (1888); J. prakt. Chem. (2) 81, 384 (1910).
[10] Ber. 27, (1894).
[11] Ber. 31, 3163 (1898)
[12] Inaugural Dissertation of A. Kohler (1909), Univ. of Bern.