IN a 5-l. round-bottom flask, fitted with a stopper holding
a reflux condenser and separatory funnel, are placed 500 g.
of powdered sodium cyanide (96-98 per cent pure) and 450 cc.
of water. The mixture is warmed on a water bath in order
to dissolve most of the sodium cyanide, and then 1 kg.
of benzyl chloride (b. p. 170-180'0) mixed with 1 kg.
of alcohol is run in through the separatory funnel in the course
of one-half to three-quarters of an hour. The mixture is then
heated with a reflux condenser on the steam bath for four hours,
cooled and filtered with suction to remove most of the sodium chloride.
It is well to wash the filtered salt with a small portion of
alcohol in order to remove any benzyl cyanide which may have been
mechanically held. The flask is now fitted with a condenser,
and as much alcohol as possible is distilled off on the steam bath.
The residual liquid is cooled, filtered if necessary, and the layer
of benzyl cyanide separated. This crude benzyl cyanide is now
placed in a Claisen distilling flask and distilled _in vacuo_,
the water and alcohol coming over first, and finally the cyanide.
It is advantageous to use a fractionating column or, better still,
a Claisen flask with a modified side-arm[1] (Vol. I, p.
40, Fig. 3) which gives the same effect as a fractionating column.
The material is collected from 135-140'0/38 mm. (115-120'0/10 mm.).
The yield is 740-830 g. (80-90 per cent of the theoretical amount).
[1] J. Am. Chem. Soc. 39, 2718 (1917). 2. Notes
The quality of the benzyl chloride markedly affects the yield
of pure benzyl cyanide. If a poor technical grade is used,
the yields will not be more than 60-75 per cent of the theoretical,
whereas consistent results of about 85 per cent or more were always
obtained when a product was used that boiled over 10'0. The technical
benzyl chloride at hand yielded on distillation about 8 per cent
of high-boiling material; a technical grade from another source was
of unusual purity and boiled over a 2'0 range for the most part.
It is advisable to distil off the last portion of alcohol and water
_in vacuo_ and also to distil the benzyl cyanide _in vacuo_,
since under ordinary pressures a white solid invariably separates
during the distillation.
One method of purifying the benzyl cyanide is to steam distil it
after the alcohol has been first distilled from the reaction mixture.
At ordinary pressures, this steam distillation is very slow and,
with an ordinary condenser, requires eighteen to twenty hours
in order to remove all of the volatile product from a run of 500 g.
of benzyl chloride. The distillate separates into two layers; the benzyl
cyanide layer is removed and distilled. The product obtained in this
way is very pure and contains no tarry material, and, after the excess
of benzyl chloride has been removed, boils practically constant.
This steam distillation is hardly advisable in the laboratory.
The benzyl cyanide, prepared according to the procedure as outlined,
is collected over a 5'0 range. It varies in appearance
from a colorless to a straw-colored liquid and often develops
appreciable color upon standing. For a product of special purity,
it should be redistilled under diminished pressure and collected
over a 1-2'0 range. For most purposes, such as the preparation
of phenylacetic acid or ester, the fraction boiling 135-140'0/38 mm.
is perfectly satisfactory. 3. Other Methods of Preparation
Benzyl cyanide occurs naturally in certain oils.[1] The only feasible
method of preparing it that has been described in the literature is
the one in which alcoholic potassium cyanide and benzyl chloride[2]
are employed. The cheaper sodium cyanide is just as satisfactory
as the potassium cyanide and therefore is the best material to use.
Gomberg has recently prepared benzyl cyanide from benzyl chloride
and an aqueous solution of sodium cyanide.[3]
[1] Ber. 7, 519, 1293 (1874); 32, 2337 (1899)
[2] Ann. 96, 247 (1855); Ber. 3, 198 (1870); 14, 1645
(1881); 19, 1950 (1886).
[3] J. Am. Chem. Soc. 42, 2059 (1920).