On the analysis of substituent effects. Part 2. Brönsted and related plots

Abstract

Conclusions reached in Part 1 concerning rate–structure relationships are used in the construction of a theoretical model for the mechanism of covalent bond formation and/or cleavage. In this model the changes in the structure of the reaction complex as it proceeds along the reaction co-ordinate are controlled by a requirement to make charge-transfer between the species joined by the covalent bond as facile as possible. The changes in the electronic structure do not smoothly follow the changes in the geometry of the atomic nuclei, and as a consequence the global pattern of rate–equilibrium and rate–rate relationships is not expected to be very simple. However, the permissible changes in electronic structure suggest that this pattern should exhibit certain simple local features, notably straight lines with slopes close to particular values. Reports of experimental observations which are in accord with the predicted features are cited.