Nonlinear Brønsted Correlations: The Roles of Resonance, Solvation, and Changing Transition-State Structure

Brønsted-type correlations for nucleophilic attack of the anions of weakly basic fluorinated alcohols and carbonyl hydrates on the esters 1-acetoxy-4-methoxypyridinium ion, 2,4-dinitrophenyl acetate, and p-nitrophenyl acetate exhibit slopes of β_nuc ⁼ 0.5–0.8. Similar slopes are observed for catalysis of proton removal from carbon acids by these bases, which is less subject to steric hindrance. The results do not support a specific role of resonance delocalization as a cause for the relatively large Brønsted slopes for phenolate anions in these reactions. There is suggestive evidence for a modest change in transition-state structure with changing basicity of the anion, but this is insufficient sufficient to account for the observed curvature of Brønsted plots for catalysis by oxygen anions. The curvature may be accounted for, in part, by a requirement for partial desolvation of basic aliphatic oxygen anions that decreases the observed Brønsted slope for these compounds.