12.21: Aldehydes and Ketones with HCN: Cyanohydrin Formation Mechanism
Cyanohydrins are formed when cyanide nucleophiles and carbonyl compounds like aldehydes and ketones react. A strong base, the cyanide ion, catalyzes cyanohydrin formation. The ions are generated from HCN under aqueous conditions. Once the cyanide ions are generated, the first step involves the nucleophilic attack of the cyanide ions on the electrophilic carbonyl carbon. This attack shifts the π electrons from the C=O to the oxygen atom forming the alkoxide ion intermediate. The alkoxide anion thus formed is highly basic and abstracts a proton from another molecule of HCN to form cyanohydrin. This reaction also regenerates the cyanide catalyst.
The strongly basic cyanide ion also functions as a good leaving group and can easily give back the aldehydes and ketones. In the presence of a strong base, the hydroxyl group on the cyanohydrin gets deprotonated to form the alkoxide anion. The alkoxide anion undergoes internal rearrangement of the delocalized electrons and loses the cyanide ion to form the carbonyl compound.