Recall that α-alkylation of ketones occurs via the generation of an enolate ion followed by a substitution reaction.

The enolate formation from ketones takes place in the presence of a base that plays a crucial role in deciding the reaction outcome.

A base whose conjugate acid is stronger than the ketone favors the reverse reaction, resulting in an equilibrium mixture with a higher ketone and lower enolate concentration.

As a result, self-condensation of ketones and nucleophilic attack of the base on alkyl halide competes with α-alkylation.

Conversely, reactions involving a base whose conjugate acid is weaker than the ketone ensures complete and irreversible conversion of the ketone to its corresponding enolate.

This eliminates the chances of other undesirable side reactions, and the resulting enolate acts as a nucleophile that can undergo substitution reactions with the alkyl halide to give substituted ketones.