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°.