Aldehydes and Ketones with HCN: Cyanohydrin Formation Mechanism

Cyanohydrin formation requires the generation of a strong base and a strong nucleophile, the cyanide anion, from HCN by using a catalytic amount of a base, or KCN.

The base-catalyzed mechanism of cyanohydrin formation involves two steps: Nucleophilic attack and proton transfer.

In the first step, the cyanide anion attacks the carbonyl electrophile to give an addition intermediate—the alkoxide ion.

Following this, the negatively charged oxygen atom on the intermediate abstracts a proton from HCN to give the addition product—the cyanohydrin—along with simultaneous regeneration of the cyanide catalyst.

Since the cyanide anion is also a good leaving group, the reaction can be reversed to give back the carbonyl compound.

Treating the cyanohydrin with a strong base deprotonates the hydroxyl group to give an alkoxide anion, and subsequent loss of the cyanide ion forms the carbonyl compound.