Molecular mechanics and molecular shape. Part 4. Size, shape, and steric parameters

Abstract

To deal with steric effects of substituents on reaction rates, two types of descriptor are proposed. Shape is characterised by the ratio (G) of the substituent's surface area to its volume. Bulk is characterised by the volume (V^a) of the portion of the substituent that is within 0.3 nm of the reaction centre. For alkyl substituents, rate constants for several series of reactions (including the classical series, acid hydrolysis of the esters RCO₂Et) correlate fairly well with V^a and G. Taft's steric parameters (E_s) correlate less well. Values of E_s for hetero-substituents are found to be contaminated by non-steric factors. It is possible to construe whether the contaminant acts to hinder or to enhance reaction.