15.33: Factors Affecting α-Alkylation of Ketones: Choice of Base
α-Alkylation of ketones is achieved in the presence of alkyl halides and a base. The reaction proceeds via the formation of an enolate ion followed by nucleophilic substitution. The choice of base employed is essential as it is the key factor in determining the reaction outcome.
The reaction involving bases like EtO− whose conjugate acid EtOH (pKa = 15.9) is stronger than the ketone (pKa = 19.2) results in an equilibrium mixture with higher ketone concentration. As a consequence, side reactions become predominant over α-alkylation. Using bases like LDA, whose conjugate acid NH(CHMe2)2 is weaker (pKa = 36) than the ketones, leads to an irreversible enolate ion formation, excluding undesirable side reactions. Hence, the nucleophilic enolate further undergoes substitution with alkyl halides to produce the desired α-alkylated ketone.