Heres some of the common methods of preparation phenylacetic acid: See especially refferences 3 and 4 for your case i think...
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.
Checked by O. KAMM and A. O. MATTHEWS.