Scheme 6.

Illustration of the stabilizing resonance interaction between a substituent -X and the reactant carbonyl group, and of the stabilizing inductive interaction between electron-withdrawing–X and the enolate oxygen anion. The resonance and inductive substituent effects are assumed to dominate in determining the changes in pK_E for enolization and in (pK_a)_XH for ionization at oxygen, respectively; and, to be negligible for acetaldehyde (X = H). Negligible polar effects on (pK_E) and resonance effects on (pK_a)_XH are assumed for this analysis.