The interpretation of quantitative linear correlations. Studies on the substituent shell concept

Abstract

The substituent shell concept is that in series of compounds XGY with a common class of group G, the effects of changing X on various properties of Y ought to be linearly related to one another irrespective of the nature of the property or the nature of Y. The concept is examined both empirically and theoretically. It is concluded that substituent shell behaviour is very common even when G is unsaturated. This result is not compatible with theories of electronic effects in which polar effects are complemented by an independent resonance effect, but it is shown to be in accord with a theory in which coulombic field effects and charge-transfer effects are modified by a noncoulombic polarisation effect, so that they tend to become directly proportional to one another as the distance from X increases. The meanings of field, inductive, and resonance effects are discussed in terms of the new theory.