15.8 : Base-Promoted α-Halogenation of Aldehydes and Ketones

α-Halogenation of aldehydes and ketones is a reaction involving the substitution of α hydrogens with halogens in the presence of a base. The reaction begins with the abstraction of α hydrogen by the base to produce a nucleophilic enolate ion. This intermediate undergoes a subsequent nucleophilic substitution with the halogen to produce a monohalogenated carbonyl compound. If the starting substrate has more than one α hydrogen, it is difficult to stop the reaction at the stage of monosubstitution. This is due to the electron withdrawing inductive effect of the halogen that makes the remaining hydrogens highly acidic. As a consequence, the monohalogentaed compound undergoes further rapid enolization and halogenation until all α hydrogens are replaced by halogens.