Ketonization of enols in aqueous solution: is carbon protonation always rate-determining?
Summary of "Ketonization of enols in aqueous solution: is carbon protonation always rate-determining?"
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, k(0)'(K)≈ 2.0 × 10(7) 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.
Department of Chemistry, University of Basel, CH-4056, Basel, Switzerland. J.Wirz@unibas.ch.
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