Reaction Condensation of an Aldehyde by Potassium Cyanide to a Benzoin

REACTION CONDENSATION OF AN ALDEHYDE BY POTASSIUM CYANIDE TO A BENZOIN 

Mix 10 grammes of benzaldehyde with 20 grammes of alcohol and treat the mixture with a solution of 2 grammes of potassium cyanide and 5 c.c. of water. Boil on the water-bath for one hour (reflux condenser). The hot solution is poured into a beaker and allowed to cool slowly the crystals separating out are filtered off, washed with alcohol, and dried on the water-bath. For conversion into benzil (see next preparation), they need not be recrystallised. In order to obtain perfectly pure benzoin, a small portion of the crude product is recrystallised from a little alcohol in a test-tube. Melting-point, 1340- Yield, about 90% of the theory. 

If an aromatic aldehyde of the type of benzaldehyde is warmed in alcohol solution with potassium cyanide, substances are obtained which possess the same composition, but with double the molecular weight of the aldehyde  

Since a small quantity of potassium cyanide has the power to condense large quantities of the aldehyde, the reaction may possibly take place as indicated by the equations below. In the first phase, one molecule of aldehyde reacts with one molecule of potassium cyanide. - 

The hydrocyanic add then reacts with a second molecule of aide- 

---