Taft σ* constants, particularly for halogen-substituted alkyl groups, and their utility in mechanistic studies

Abstract

To verify the Taft σ* values, data from several reaction series were fitted to the equation log k=ρ*σ*+ log k₀. The calculated σ* values were in good agreement with each other, but certain substituents, especially hydrogen, behave differently in some reaction series. On the basis of the σ*(CCl₃) values obtained from the different reaction series, it is concluded that the acid-catalysed esterification of trichloroacetic acid in an alcohol solution takes place by the normal A_AC2 mechanism, although the acid-catalysed hydrolysis of ethyl trichloroacetate in aqueous solutions mainly proceeds by the exceptional A–B_AC3 mechanism. In the case of simultaneous reactions, the Ingold–Taft equation can be employed to estimate the contribution of the normal A_AC2 mechanism.