Side-Chain Reactions of Benzene Derivatives

Abstract

WE have recently examined a number of side-chain reactions in the light of the postulate that the differences in the rates of reaction of a series of similarly constituted compounds under identical conditions are to be ascribed solely to different energies of activation, substituents contributing additively to the total energy¹. Our results² for the reaction of hydrogen ion with various p-substituted aceto-phenones X.C₆H₄.CO.CH₂R (acid-catalysed proto-tropy) indicate that the energies of activation are given by the expression E=E₀ C (a²), where C and a are constants for the series, E₀ is the value of E for the unsubstituted compound, and is the dipole moment of C₆H₆X. The substituents dealt with included three halogens, to which the equation applies accurately, but did not include inclined groups such as OAlk and NAlk². We further suggested³ that the expression E=E₀±C (a²) might be applicable to side-chain reactions in general, the negative and positive signs referring respectively to those of Classes A and B⁴. A review of fourteen reactions led us to the conclusion that, for m-substituted compounds, the equation is valid except when the substituent is a halogen, while, with the substituent in the p-position, complications arise, as anticipated, from the operation of electromeric effects, and in a number of Class B reactions the term in µ² changes sign, an observation for which there was no obvious explanation.