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.