Flash-Photolytic Generation of Acetophenone Enol. The Keto-Enol Equilibrium Constant and pKa of Acetophenone in Aqueous Solution

Article abstract of DOI:10.1021/ja00333a049

Acetophenone enol has been generated by Norrish type II photoelimination of γ-hydroxybutyrophenone, and its rate of ketonization has been measured in dilute aqueous HCl solutions.These data, in combination with the specific rate of acid-catalyzed enolization of acetophenone, give a value of the keto-enol equilibrium constant which is in good agreement with another result based upon enolization and ketonization rate constants measured in NaOH solutions.The average of these determinations gives pK_E = 7.90 +/- 0.02.This, when combined with the known acid-dissociation constant of acetophenone enol, leads to pK_a^K = 18.24 +/- 0.03 for the acidity constant of acetophenone ionizing as a carbon acid.The present results allow an estimate of the specific rate of proton transfer from H3O⁺ to the β-carbon atom of acetophenone enolate ion, k = (4.2 +/- 1.2) x 10¹⁰ M^-1 s^-1, which is so great as to suggest that this reaction occurs through proton transfer down hydrogen-bonded solvent bridges between acid and substrate.