Aldehydes and unhindered ketones react with HCN to yield cyanohydrins,
RCH(OH)C=N. For example, benzaldehyde gives the cyanohydrin com-monly called mandelonitrile in 88% yield on treatment with HCN:
Studies carried out in the early 1900s by Arthur Lapworth showed that cyanohydrin formation is reversible and base-catalyzed. Reaction occurs
Slowly when pure HCN is used but rapidly when a smll amount of base
is added to generate the nucleophilic cyanide ion, CN_ . Alternatively, a smll
amount of KCN can be added to HCN to catalyze the reaction.
Addition of CN_to a ketone or aldehyde occurs by a typical nucleophilic
addition pathway, yielding a tetrahedral intermediate that is protonated by HCN to give cyanohydrin product plus regenerated CN_.
Cyanohydrin formation is unusual because it is one of the few exam-
ples of the addition of a protic acid (H-Y) to a carbonyl group. As noted in
the previous section, reagents such as H2O, HBr, HCl, and H2SO4 don’t nor-mally form isolable carbonyl adducts because their equilibrium constant for reaction is unfavorable. With HCN, however, the equilibrium favors the cyanohydrin adduct.