Mechanism of acid-catalysed alcoholysis of epoxides. Part III. Alcoholysis of 1,2-epoxypropane, 1,2-epoxy-3-phenoxypropane, 1,2-epoxy-3-phenylpropane, and 2,3-epoxypropan-1-ol

Abstract

Enthalpies and entropies of activation have been determined for the acid-catalysed methanolysis of 1,2-epoxypropane, and of XCH₂·[graphic omitted] (X = Ph, PhO, or OH), and for the acid-catalysed ethanolysis and propan-2-olysis of 1,2-epoxy-3-phenylpropane. Products have been analysed by g.l.c. and the results have been used to dissect the measured rate coefficients into rate coefficients for normal and abnormal attack. Entropies of activation for the methanolysis in the normal position (ca.–17 cal mol^–1 K^–1) have been interpreted in terms of an A2 mechanism, while those for the abnormal position (ca.–14 cal mol^–1 K^–1) have been interpreted in terms of a borderline A2 mechanism. The more positive values for the methanolysis of 1,2-epoxy-3-phenoxypropane apparently arise from restricted bond rotations in the initial state.

The Hughes–Ingold solvation theory has been used to explain the variation of ΔS‡ and ΔH‡ with change in solvent in the acid-catalysed reactions of 1,2-epoxy-3-phenylpropane with the three alcohols.

The effect of substituents has been analysed in terms of the Taft linear free-energy relationship. The negative values of the polar reaction constants and the large postive values of the steric reaction constants are in accord with the mechanisms already proposed.

The effect of the solvent on the reactions is examined and certain quantitative correlations are briefly considered.