On the analysis of substituent effects. Part 1. Hammett and related plots

Abstract

The overall pattern of significant deviations from straight lines through the origin in Hammett-like plots is shown to be remarkably simple when the choice of standard substituent constants is based on ¹³C substituents chemical shifts at the β site in ring-substituted styrenes. This simplicity is explained in terms of a novel theoretical analysis of substituent effects. It could also be explained in terms of field and resonance substituent effects if the concept that the resonance effect supplements the field effect was replaced with the concept that the resonance effect modifies the field effect by an amount that can vary but only up to a maximum. The concept of through-resonance could be retained but some effects currently attributed to through-resonance would have to be reattributed to a field-modifying major rearrangement of electronic structure within the benzene ring. From our analysis, the ratio of values of the resonance parameter for corresponding meta and para substituents is not even approximately independent of the nature of the substituent, which indicates a serious flaw in commonly used dual substituent parameter expressions.