Kinetic Studies on the Keto-Enol Tautomeric Reactions of Benzoylacetones

The rate of keto-enol tautomerization of p-substituted benzoylacetones in the absence of catalyst in 60% methanol at 20-30°C were followed spectrophotometrically. Absorption curves of keto and enol form were obtained by personal computer-treatment of the data taken (absorbance and wavelength), followed by preparation of calibration curves. Conversion percentage was obtained graphically. The rate for the reactions satisfied the reversible first-order kinetics : Rate=k1 (a-c-x)-k1 (c+x). Here, k1 and k-1 are the first-order rate constants of the ketonization and the enolization ; a, c and x are the concentration of the whole tautomer, of the keto form at the initial time and of consumed enol form at time t, respectively. In the activation parameters, enthalpies are nearly equal to the entropy terms in the ketonization, whereas each entropy term becomes a dominant factor in the enolization. A plot of log k1 (and/or log k-1) vs. log CH2O gave a straight line with a slope of ca. 3 in the ketonization (and/or enolization). From these findings, a probable mechanism which involves the hydrogen bonding annular complex by three molecules of water between enol hydrogen and methine carbon in the ketonization and/or methylene hydrogen and benzoyl oxygen in the enolization is postulated and discussed.