Substitution at saturated carbon. Part 26. A complete analysis of solvent effects on initial states and transition states for the solvolysis of the t-butyl halides in terms of G, H, and S using the unified method

Abstract

The influence of 20–30 solvents on ΔG^‡, ΔH^‡, and ΔS^‡ for the solvolysis of t-butyl chloride and t-butyl bromide has been dissected into initial-state and transition-state contributions. Using the unified method in which the general equation (i) is applied to both these contributions, it is shown that the decrease in ΔG^‡ due to solvent dipolarity (π₁*) and to solvent hydrogen-bond acidity (α₁) is primarily a transition-state effect, that there is little effect of solvent hydrogen-bond basicity (β₁) on either initial-state or transition-state, and that large effects of solvent Hildebrand solubility parameter (δ_H) on initial-state and transition-state partly cancel out. XYZ = XYZ₀+s(π₁*+dδ)+aα₁+bβ₁+hδ²_H/100 (i)

A similar analysis carried out on the transition-state transfer quantities, ΔH_t⁰ and ΔS_t⁰, yields the surprising results that the influence of solvent π₁* and α₁ values on the transition-state is primarily an entropic effect and that there is a very large influence of solvent hydrogen-bond basicity in increasing both ΔH_t⁰ and ΔS_t⁰ for the transition-state in an almost exactly compensatory way. It is suggested that this latter effect, previously unsuspected, may arise through solute/solvent lone pair/lone pair repulsions.