Steric effects-II: Relationship between topology and the steric parameter. E's—topology as a tool for the correlation and prediction of steric effects

Abstract

The steric effect of alkyl groups as characterized by the revised Taft E' s parameter is analysed using an approach based on the DARC topological system and its PELCO correlation method. This approach involves an analysis of the systemativ variation of E' s in a topological dequencing of alkyl groups and shows the existence of three regions of distinct behaviour: R I, a “normal” behaviour region ( ca 6 E' s units) in which the contribution of the introduction of successive Me groups to the overall steric effect increases monotonically (groups with 1 to 7 carbons); R II, a region, in which a “levelling” effect is observed, i.e. the contribution diminishes and becomes nil (groups with 8 and 9 carbons); and R III, where this contribution changes sign, “inversion” effect (groups with 10 carbons). Using a series of successive approximations, topological models are developed and tested. The conditions under which the topology may be used to represent the topography (i.e. the real 3-dimensional structure) are considered. The correlation of existing E' s values and the reliable prediction of experimentally unavailable steric effects are direct consequences of this treatment.