19.7: Nucleophilic Addition of HCN- Cyanohydrin Formation

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Aldehydes and unhindered ketones undergo a nucleophilic addition reaction with HCN to yield cyanohydrins, $RCH(OH)C≡ N$ . 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 small amount of base is added to generate the nucleophilic cyanide ion, CN^–. Addition of CN^– takes place by a typical nucleophilic addition pathway, yielding a tetrahedral intermediate that is protonated by HCN to give cyanohydrin product plus regenerated CN^–.

Benzaldehyde and cyanide form a tetrahedral intermediate, which in presence of hydrogen cyanide gives mandelonitrile (88 percent) where benzene is single-bonded to C single-bonded to hydrogen, nitrile, and hydroxyl groups.

Cyanohydrin formation is somewhat unusual because it is one of the few examples of the addition of a protic acid (H–Y) to a carbonyl group. As noted in the previous section, protic acids such as H₂O, HBr, HCl, and H₂SO₄ don’t normally yield carbonyl addition products because the equilibrium constants are unfavorable. With HCN, however, equilibrium favors the cyanohydrin adduct.

Cyanohydrin formation is useful because of the further chemistry that can be carried out on the product. For example, a nitrile ( $R–C≡N$ ) can be reduced with LiAlH₄ to yield a primary amine (RCH₂NH₂) and can be hydrolyzed by hot aqueous acid to yield a carboxylic acid. Thus, cyanohydrin formation provides a method for transforming an aldehyde or ketone into a different functional group.

Hydrogen cyanide converts benzaldehyde to mandelonitrile. Mandelonitrile is reduced to 2-amino-1-phenylethanol using lithium aluminum hydride in tetrahydrofuran, then hydronium. It’s hydrolyzed to mandelic acid (90 percent) in hydronium with heat.

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