NATURE OF THE ELECTRICAL EFFECT OF ALKYL-GROUPS .2. SIGNIFICANCE OF THE TAFT SIGMA-STAR VALUES FOR ALKYL-GROUPS

Values of σ* rel(σ* Ak/σ* Et) for alkyl groups are calculated from rate constants for C-substituted amide hydrolysis in water at three different temperatures for IV-acylimidazole hydrolysis and imidazole-catalyzed IV-acylimidazole hydrolysis in water. In all of these cases, the steric effect is the same in both the acid- and the base-catalyzed reactions. The range of σ* rel for primary, secondary, and tertiary alkyl groups shows absolutely no dependence on the degree of branching. The corrected or “true” σ* Ak constants were calculated from rate constants for C-substituted ester hydrolysis in the same medium at three different temperatures. It has been shown that in this case, steric effects in the acid- and base-catalyzed reactions are not the same. The corrected σ* Ak t constants are well correlated by σ* Ak t = miAk + c, which was derived previously. The Taft σ* Ak t parameters were successfully correlated with the equation σ* Ak t = anα + bnβ + i, where nα and nβ are measures of the degree of branching of the α and β positions of an alkyl group. On the basis of these results it is suggested that the Taft α* constants are probably a measure of a steric effect. Thus, log k = ρ*σAk + δES, Ak + h (Taft and Pavelich), log k = ρ*σAk + ρ*σAk + δES, Ak + ΔG + H (Hancock et al.) and log k = ρ*∑* Ak + d(n'H - 6) (Taft and Kreevoy) can be accounted for in terms of steric effects in reactions which differ in their sensitivity to branching at the α and β carbon atoms of an alkyl group. The correlations of ES and EC S with σAk and nH by Koppel can be explained in the same manner. © 1979, American Chemical Society. All rights reserved.