Mechanism of acid-catalysed alcoholysis of epoxides. Part I. Reactions of substituted (1,2-epoxyethyl)benzenes

Abstract

Rate coefficients have been determined by a spectrophotometric method for the acid-catalysed methanolysis at 30° of (1,2-epoxyethyl)benzene and of XC₆H₄·[graphic omitted] (X =p-Br, m- or p-Cl, m-Me, m-MeO, or m- or p-NO₂), and for the acid-catalysed ethanolysis, propan-1-olysis, and butan-1-olysis at 30° of similar epoxides with X = H, m-Cl, m-Me, or p-NO₂. Alcoholysis is unidirectional giving the abnormal products (primary alcohols) only.

The values of the entropy of activation (ΔS‡) for methanolysis are in the range –12·3 to –13·5 cal mol^–1 K^–1, and the Hammett ρ-values for abnormal attack are between –3·8 and –4·2, which suggests a consistency of mechanism. The reaction is discussed in terms of a borderline A2 mechanism.

A slow acid-catalysed reaction has also been discovered for the epoxides XC₆H₄·[graphic omitted] (X = H, p-Br, p-Cl, or m- or p-Me), which is regarded as a rearrangement of the epoxide to the corresponding arylacetaldehyde. With four other epoxides (X =m-Cl, m-MeO, or m- or p-NO₂) this reaction was not detected.