This reaction occurs very
slowly with pure HCN Therefore, it is catalysed
by a base and  the generated cyanide ion (CN
being a stronger nucleophile readily adds to-)
carbonyl compounds to yield corresponding
cyanohydrin Cyanohydrins 
are 
useful 
synthetic
intermediates (b) Addition of sodium hydrogensulphite: Sodium
hydrogensulphite adds to aldehydes and
ketones to form the addition products