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Org. Synth. 1923, 3, 45
DOI: 10.15227/orgsyn.003.0045
DIPHENYLACETIC ACID
[Acetic acid, diphenyl-]
Submitted by C. S. Marvel, F. D. Hager, and E. C. Caudle.
Checked by J. B. Conant and A. W. Sloan.
1. Procedure
In a 1-l. round-bottomed flask are placed 250 cc. of glacial acetic acid, 15 g. of red phosphorus, and 5 g. of iodine. The mixture is allowed to stand for fifteen to twenty minutes until the iodine has reacted, and then 5 cc. of water (Note 1) and 100 g. (0.44 mole) of benzilic acid (p. 89) are added. A reflux condenser is attached, and the mixture is boiled continuously for at least two and one-half hours. After the reaction is complete, the hot mixture is filtered with suction to remove the excess red phosphorus (Note 2). The hot filtrate is slowly poured into a cold, well-stirred, filtered solution of 20–25 g. of sodium bisulfite in 1 l. of water (Note 3). This procedure removes the excess iodine and precipitates the diphenylacetic acid as a fine white or slightly yellow powder (Note 4). The product is filtered with suction, washed with cold water, and dried thoroughly on filter paper. The yield is 88–90 g. (94–97 per cent of the theoretical amount) of a solid melting at 141–144° (Note 5). If a crystalline product is desired, the acid is dissolved in about 500 cc. of hot 50 per cent alcohol and then cooled. The melting point after recrystallization is 144–145°.
2. Notes
1. In place of phosphorus, iodine, and water, a dilute solution of hydriodic acid and phosphorus may be used.
2. If difficulties are encountered in filtering the hot acetic acid solution through filter paper, an asbestos filter may be prepared and used to advantage.
3. Some samples of sodium bisulfite caused part of the diphenyl-acetic acid to dissolve. This can always be avoided if, after the solution of sodium bisulfite is prepared, a current of sulfur dioxide is passed in, until the solution is acid to litmus.
4. Sometimes, if the acetic acid solution is poured into the water too rapidly, the product will be slightly pink and a reprecipitation from acetic acid solution will be necessary.
5. The melting points of benzilic acid and diphenylacetic acid lie very close together. However, it is very easy to test for complete reduction by treating a little of the product with cold concentrated sulfuric acid. If even a trace of benzilic acid remains the sulfuric acid will turn red.
3. Discussion
Diphenylacetic acid can be prepared by the reduction of benzilic acid with hydriodic acid and red phosphorus,1 by carbonation of the reaction product of phenylsodium and diphenylmethane,2 and by hydrolysis of 1,1-dichloro-2,2-diphenylethylene.3

References and Notes
  1. Jena, Ann. 155, 84 (1870); Klingemann, Ann. 275, 84 (1893); Zinssen, Ber. 24, 3556 (1891); Chichibabin, Ber. 44, 442 (1911).
  2. I. G. Farbenind. A.-G., Ger. pat. 671, 098 [C. A. 33, 3391 (1939)].
  3. Sheibley and Prutton, J. Am. Chem. Soc. 62, 840 (1940).

Appendix
Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)

red phosphorus

alcohol (64-17-5)

sulfuric acid (7664-93-9)

acetic acid (64-19-7)

sulfur dioxide (7446-09-5)

PHOSPHORUS (7723-14-0)

sodium bisulfite (7631-90-5)

iodine (7553-56-2)

Benzilic acid (76-93-7)

Diphenylmethane (101-81-5)

hydriodic acid (10034-85-2)

Diphenylacetic acid,
Acetic acid, diphenyl-,
diphenyl-acetic acid (117-34-0)

phenylsodium

1,1-dichloro-2,2-diphenylethylene