Role of solvent nucleophilicity in excess-acidity plots for equilibria in sulfuric acid – water mixtures

Abstract

For protonation of a base B to give BH+ and for reaction of carbocations R+ with water to give ROH in a range of sulfuric acid – water mixtures, slopes of excess-acidity plots of equilibrium data are discussed. Formal connections between the excess-acidity approach and the Grunwald–Winstein (GW) and extended GW equations, noted previously by Bagno et al., are evaluated. It is proposed that a major factor is variations in the solvent dependence of the rates of reactions of the cations (BH+ or R+) with water in the sulfuric acid – water mixtures. Rates of reactions of more stable cations (e.g., Ph3C+) with water are more sensitive to changes in solvent nucleophilicity, and the lower nucleophilicity of the water in sulfuric acid – water mixtures leads to an increase in [BH+]/[B] or [R+]/[ROH] concentration ratios (and hence to greater slopes m* in excess-acidity plots).Key words: excess acidity, sulfuric acid, kinetics, solvent nucleophilicity, solvolysis.