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.