12.15: Aldehydes and Ketones with HCN: Cyanohydrin Formation Overview
Cyanohydrins are compounds that contain –CN and –OH groups on the same carbon atom. They are formed by the nucleophilic addition of the cyanide ions to the carbonyl group. Cyanide ions are highly basic and nucleophilic and can be generated from HCN under aqueous conditions. However, since HCN is a weak acid, the number of cyanide ions generated is very small. Hence, a small amount of base or KCN/NaCN is added to HCN to increase the concentration of the cyanide ions in the reaction mixture.
Cyanohydrin formation is favored in the case of aldehydes and simple ketones. However, when the bulky groups are present in ketones, the ketone gets sterically hindered, and the equilibrium generally shifts to the left, favoring the reactant.
Cyanohydrins are the key intermediates in the synthesis of various organic compounds. Under acidic hydrolysis conditions, cyanohydrins can be converted into α-hydroxy acids or α,β-unsaturated acids. The conversion to α-hydroxy acids constitutes a key step in the Kiliani–Fischer synthesis of sugars. On the other hand, the reduction of cyanohydrin in the presence of lithium aluminum hydride gives a β-amino alcohol.
Cyanohydrin also occurs in nature in the form of mandelonitrile in millipedes. Millipedes use mandelonitrile as protection against predators and other organisms. Upon release, the mandelonitrile gets converted into benzaldehyde and HCN, preventing other animals from approaching the millipede.
