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Synthesis of Benzyl Cyanide


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Benzyl Cyanide [1]

In a 5000ml round-bottomed flask, fitted with a stopper holding a reflux condenser and separatory funnel, are placed 500 g. (10 moles) of powdered sodium cyanide (96-98 percent pure) and 450ml of water. The mixture is warmed on a water bath in order to dissolve most of the sodium cyanide, and then 1000g (8 moles) of benzyl chloride (bp 170-180C)[3] mixed with 1000g of 95 percent alcohol is run in through the separatory funnel in the course of 30-45 minutes. The mixture is then heated under 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 under reduced pressure, the water and alcohol coming over first, and finally the cyanide (it is preferable to distill the last part of the solvents and the benzyl cyanide under vacuum). It is advantageous to use a fractionating column. The material is collected at 135-140C at 38 mmHg. (115-120C at 10 mmHg.). The yield is 740-830g. (80-90 per cent of the theoretical amount). It is preferable to wash the final product in order to remove foul-smelling benzyl isocyaide, and to significantly lengthen the shelf-life of the product. The once-distilled benzyl cyanide is shaken vigorously for five minutes with an equal volume of warm (60C) 50 percent sulfuric acid, prepared by adding 275ml of concentrated sulfuric acid to 500 ml of water. The benzyl cyanide is separated and washed with an equal volume of saturated sodium bicarbonate solution followed by an equal volume of half-saturated sodium chloride solution. It is then dried and distilled under reduced pressure. The loss in the washings is negligible.

Benzyl Cyanide [2]

6 kilos of potassium cyanide are dissolved in water in an enameled stirring vessel fitted with reflux condenser. There is then added slowly a solution of 10 kilos of benzyl chloride[3] in an equal weight of alcohol. When all has been added, the mixture is heated for three to four hours at the boil. The liquid separates into two layers, the upper one reddish brown, contains the benzyl cyanide, the lower aqueous layer the alcohol and potassium chloride (which is precipitated as a crust on the walls of the vessel on cooling). The brown oily layer is washed several times with water and then distilled at ordinary pressure. Traces of alcohol first come over and the temperature then rises quickly to 195C. The residual liquor is now cooled somewhat and the benzyl cyanide distilled off in vacuo without fractionation. The crude benzyl cyanide so obtained is used for the next operation.

References

[1] Org Syn Coll Vol 1, p 107-109
[2] Perfumery Essent. Oil Record 14, 336 (1924)
[3] Preparation of Benzyl Chloride