Anna Paola Pelliccioli, Peter Sebej and Jakob Wirz, "Ketonization of enols in aqueous solution: Is carbon protonation always rate-determining?", Photochem. Photobiol. Sci. 2012, 11, 967-971.


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 10E7 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, pKa,cE = 11.4, which is too high by about two pK units.