Evidence for a cyclic A A11 mechanism in the hydrolysis of benzylidene diacetates

The acid-catalysed hydrolyses of a series of benzylidene diacetates have been studied. The rates of hydrolysis were found to depend on the Hammett acidity function H0, and plots of log kobs+Ho against log aw had negative slopes. The hydrolyses therefore occur by an A1 mechanism, and the correlation of the rates of reaction with σ+ establishes that the mechanism is AA11. The entropies of activation obtained are consistent with the formation of a cyclic transition state, in which a protonated acetoxy-group acts as a Lewis acid in assisting the loss of the second-acetoxy-group. p-Nitrobenzylidene diacetate appears to hydrolyse by the above mechanism at high acid concentrations, and by an AAc2 mechanism at lower acidities.Only p-methoxybenzylidene diacetate showed an appreciably uncatalysed reaction at 25°, and this reaction occurs by an SN1 type of process which also appears to involve a cyclic mechanism.