Note ↑: Through induction, oxygen pulls some of the electron density out of these C–H bonds, weakening them. This makes it relatively easy to deprotonate the α-carbon of an aldehyde or ketone, as shown in Figure 7.1.

Note ↑: The pKa values of aldehyde and ketone -hydrogens range from 16 to 21, much lower than the pKa values of ethene (44) or ethyne (25), but comparable with those of alcohols (15–18). Strong bases can therefore remove an -hydrogen. The anions that result are known as enolate ions or simply enolates.

We know that acid strength is enhanced by stabilization of the conjugate base. In the enolate ion, the inductive effect of the positively polarized carbonyl carbon strongly stabilizes the negative charge at the -position. Aldehydes are stronger acids than ketones because their carbonyl carbon bears a larger partial positive charge. Further strong stabilization is provided by delocalization of charge onto the electronegative oxygen, as described by the resonance forms just pictured. The effect of delocalization is also reflected in the electrostatic potential map of the acetone enolate shown down below (on an attenuated scale), which exhibits negative charge (red) on the -carbon as well as on the oxygen. An example of quantitative enolate formation is the deprotonation of cyclohexanone (pK_a = 19) by lithium diisopropylamide (LDA, pK_a of amine = 36).