Ketonization of Acetophenone Enol in Aqueous Buffer Solutions. Rate-Equilibrium Relations and Mechanism of the "Uncatalyzed" Reaction

Article abstract of DOI:10.1021/ja00224a039

Rates of ketonization of acetophenone enol, generated by photohydration of phenylacetylene and Norrish type II photoelimination of γ-hydroxybutyrophenone, were measured at 25 deg C in aqueous buffer solutions of six carboxylic acids, six phosphonic acid monoanions, phosphoric acid, and dihydrogen phosphate.Analysis of the carboxylic acid data produced catalytic coefficients from which linear Broensted correlations were constructed, with α = 0.50 for ketonization of the enol and α = 0.32 for the more rapid ketonization of the enolate ion.A more extended, curved Broensted correlation for ketonization of the enolate ion was constructed by combining the carboxylic acid results with catalytic coefficients for the phosphonic acid monoanions, and analysis of this by Marcus theory gave the intrinsic barrier ΔG₀⁺ = 12 kcal mol^-1 and the work terms w^τ = 2 kcal mol^-1 and w^p = 4 kcal mol^-1.These results differ from the much smaller intrinsic barrier and strongly disparate work terms obtained previously in a similar study of isobutyrophenone enol, and an explanation of the difference in terms of a looser transition state (Kreevoy τ = 0.30) for the isobutyrophenone system is offered.Evidence is also supplied which indicates that the so-called "uncatalyzed" ketonization reaction occurs by a stepwise route rather than by a cyclic single-step mechanism.