Evidence for a cyclic AA11 mechanism in the hydrolysis of benzylidene diacetates

Abstract

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 k_obs+H_o against log a_w 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 A_A11. 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 A_Ac2 mechanism at lower acidities.

Only p-methoxybenzylidene diacetate showed an appreciably uncatalysed reaction at 25°, and this reaction occurs by an S_N1 type of process which also appears to involve a cyclic mechanism.