Prediction of Taft’s σ* parameter for alkyl groups and alkyl groups containing polar substituents

Abstract

X¹, X², and X³ are polar groups, hydrogens or alkyl groups bound to a carbon atom in a group CX¹X²X³. The value of σ*(CX¹X²X³) has been found to be equal to the sum of the contributions of all groups X¹, X², and X³. The contribution of a polar group X to the value of σ*(CX¹X²X³) is b × Σσ*(X) and that of an alkyl group R is a × Σ[σ*(R) – σ*(H)] where a = 0.202 and b = 0.326. The additivity of the contributions of the groups is good, giving σ* values usually as good as those obtained by experiments.