Kinetic Studies on the Keto-Enol Tautomeric Reactions of Benzoylacetophenones

The rates of keto-enol tautomerization of 2- (p-ubstituted benzoyl) acetophenones in the absence of catalyst in methanol containing 20 vol% water at 2030°C were spectrophotometrically measured. Absorption curves of pure keto and enol forms were obtained by analysis of obtained absorbance and wavelength with a personal computer. Conversion percentage was obtained from calibration curves.The reactions obeyed the reversible first-order rate equation : Rate=k1 (a-c-x) -k-1 (c+x), where, k1 and k-1 are first-order rate constants for ketonization and enolization : a, c and x are concentrations of the whole tautomers, keto form at the initial time and consumed enol form at time t, respectively.Each entropy in the ketonization or enolization seems to indicate the presence of transition state with aggregated water molecule.A plot of log k1 (or log k-1) vs. log CH2O gave a straight line with a slope of ca. 3 for the ketonization and enolization as well. These findings support a mechanism in which a hydrogen bonding annular complex is produced by three molecules of water between enol hydrogen and the methine carbon in ketonization and/or methylen hydrogen and benzoyl oxygen in enolization.