15.31 : 通过烯醇离子对酮进行 α-烷基化

A ketone, in the presence of a strong base like LDA, loses an acidic α hydrogen to give an enolate ion, which is stabilized by resonance.

Its hybrid structure reveals negative charges on the carbonyl oxygen and the α carbon.

The ambident enolate nucleophile can attack an electrophile via two potential reactive sites: the anionic oxygen or the anionic α carbon.

Reactions through O-attack are less preferred than C-attack. This is because the oxygen site is less accessible, as the positive counterpart of the base stays more strongly attached to the oxygen than to the α carbon.

Moreover, in the O-attack product, the strong carbonyl bond is lost, which is otherwise retained in the C-attack product. 

In summary, to alkylate a ketone, the ketone is first reacted with a strong base to give an enolate, which undergoes a C-attack with an alkyl halide in an S_N2 reaction to produce an α-alkylated ketone.