On the analysis of substituent effects. Part 3. The geometry of the ring in substituted benzenes

Abstract

In Part 1 it was proposed that in any substituted benzene molecule the ring exists in one of two discrete states, neither of which possesses regular hexagonal geometry at zero substituent field. Here further physical and chemical evidence for the existence of such ring states is presented and discussed within the context of a polarisation and charge-transfer analysis of electronic effects. A quantitative expression for the ring bond angles in terms of electronic characteristics of the substituents is given.

The two states are labelled H (for higher) and L (for lower) according to their relative abilities to transmit substituent electronic effects. The difference is explained in terms of the balance between charge transfer and overlap repulsion within the ring. Deviations from the quantitative expression in certain para-disubstituted benzenes are interpreted in terms of twc kinds of enhanced intra-ring charge-transfer effect, one of which has no analogue in resonance theory.