cyanohydrins

Meaning: A cyanohydrin is formed by nucleophilic addition of cyanide to a carbonyl. CN⁻ attacks the electrophilic C=O carbon, the π bond breaks, and a tetrahedral alkoxide forms. Protonation (by HCN or water) gives the –OH. Overall: RCHO + HCN ⇌ RCH(OH)C≡N. The reaction is reversible. It is more favorable for aldehydes and unhindered ketones than for bulky ketones, because aldehydes are less crowded and more electrophilic.

On the MCAT: Expect a carbonyl compound treated with NaCN or KCN plus acid (or HCN with a trace of base). You may be asked to name the product or to predict the effect of steric bulk on reactivity. The nitrile is the key to follow-up steps. Aqueous acid with heat converts –C≡N to –COOH, giving an α-hydroxy acid. LiAlH₄ reduces the nitrile to a primary amine, giving a β-amino alcohol. If the carbonyl carbon becomes a stereocenter, as in an aldehyde other than formaldehyde or an unsymmetrical ketone, cyanide attacks either face of the planar C=O equally. The product is a racemic mixture.

Common confusion: The nucleophile is CN⁻, not HCN. HCN is a weak acid and a poor nucleophile, so a little cyanide ion must be present for the reaction to go. Also, adding cyanide to a carbonyl is different from SN2 displacement of a halide by cyanide. Both add one carbon, but only the carbonyl addition makes an –OH on the same carbon.