Ketonization of enols in aqueous solution: is carbon protonation always rate-determining?
|Year of publication||2012|
|Type||Article in Periodical|
|Magazine / Source||PHOTOCHEMICAL & PHOTOBIOLOGICAL SCIENCES|
|MU Faculty or unit|
|Keywords||KETO-ENOL; DISSOCIATION-CONSTANT; BUFFER SOLUTIONS; ACID; HYDRATION; SYSTEM|
|Description||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 approximate to 2.0 x 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.|