15.7 : アルデヒドおよびケトンの酸触媒によるα-ハロゲン化

Aldehydes or ketones undergo α-halogenation in an acid-catalyzed condition to yield a monohalogenated product.

Here, the C=O oxygen is first protonated by the acid to form a resonance-stabilized cation. The cation intermediate then undergoes deprotonation at the α carbon to yield the enol tautomer.

As the electron-releasing –OH group renders the C=C bond highly nucleophilic, the double bond rapidly attacks an electrophilic halogen molecule to form a monohalogenated carbocation.

Finally, deprotonation of the carbocation yields the α-halo aldehydes or ketones.

The presence of an electron-withdrawing halogen atom reduces the reactivity of the C=O oxygen atom, thus preventing multiple halogenation.

Further, the acid formed as a byproduct can catalyze the first step of enolization, thus making the reaction autocatalytic.

The acid-catalyzed α-halogenation reaction is useful to convert ketones into α,β-unsaturated ketones via E2 elimination reactions, generating a new π bond.