Ketonization of enols in aqueous solution: is carbon protonation always rate-determining?

Abstract

The pH–rate profiles for the ketonization of the (E)- and (Z)-photoenols of o-methylacetophenone (MA) in aqueous solution were determined by nanosecond laser flash photolysis. Carbon protonation of the enol anions of MA by solvent water is exceptionally fast, k₀′^K ≈ 2.0 × 10⁷ s⁻¹, too fast to permit establishment of the acid–base equilibrium on the enol oxygen prior to ketonization. Analysis of the pH–rate profile of the (E)-enol using the common assumption of rate-determining carbon protonation would lead to an erroneous value for the acidity constant of that enol, pK_a,c^E = 11.3, which is too high by about two pK units.