Aldehydes and Ketones with HCN: Cyanohydrin Formation Overview

Mandelonitrile, a defensive chemical produced by millipedes, is an example of a cyanohydrin featuring a cyano and a hydroxyl group on the same carbon atom.

Cyanohydrins are reversibly formed when HCN adds to the carbonyl group.

Aqueous HCN supplies the strongly basic cyanide nucleophile that catalyzes the addition reaction.

However, using HCN alone, cyanohydrin forms at a negligible rate as the weakly acidic HCN produces very few cyanide nucleophiles.

The reaction rate is enhanced by adding a base or KCN to HCN, to generate more CN^-, and with that, more cyanohydrin.

With simple aldehydes and most aliphatic ketones, equilibrium favors cyanohydrin formation. However, with aryl and hindered ketones, the carbonyl form predominates.

Here, the forward reaction fails since cyanohydrin formation involves rehybridization from sp² to sp³ with reduced bond angles and increased steric hindrance that inhibits the nucleophilic attack on the carbonyl carbon.

Cyanohydrins function as synthetic organic intermediates that can be converted to different useful functional groups.