Base-Promoted α-Halogenation of Aldehydes and Ketones

Aldehydes and ketones undergo α-halogenation in the presence of a full equivalent of base and halogen.

This reaction proceeds via the base-catalyzed deprotonation of α hydrogen to form a nucleophilic enolate intermediate, which then reacts with halogen to give α-halogenated carbonyl compound.

The presence of halogen in the resulting molecule makes the remaining α hydrogens more acidic compared to their parent molecule owing to the electron-withdrawing inductive effect of the halogen.

This leads the monosubstituted compound to undergo further rapid enolization and subsequent halogenation until no acidic α hydrogen remains.