Hydroxide ion catalyzed reverse aldol-type condensation reactions of para-substituted benzoylacetaldehydes

The apparent hydroxide ion catalyzed decomposition of para-substituted benzoylacetaldehydes to give para-substituted acetophenones and formate ions is characterized by rate saturation at high hydroxide ion concentrations (0.1--1 M); thus the order in hydroxide ion varies from 1 at low hydroxide ion concentrations to 0 at high hydroxide ion concentrations. These kinetics suggest that the critical transition state contains aldehyde hydrate monoanion and hydroxide ion or the kinetically equivalent aldehyde hydrate dianion in analogy to earlier work. A number of rate constants, including those for product-forming decomposition of aldehyde hydrate monoanion apparently catalyzed by hydroxide ion, were calculated from the experimental data and estimates of various equilibrium constants. For decomposition of aldehyde hydrate monoanions catalyzed by hydroxide ions, the calculated rho = 1 and the deuterium solvent kinetic isotope effects k/sup D//k/sup H/ = 1.1--1.5 suggest that heavy atom reorganization plays a major role in product formation. (U.S.)