The dependence of vanadate phenyl ester formation on the acidity of the parent phenols

The condensation of vanadate (VO4H21−) with a variety of substituted phenols in aqueous acetone solution has been studied by 51V nuclear magnetic resonance spectroscopy. The formation constants for the various product esters were determined, as were their respective pKa values. Inductive and resonance contributions to vanadate ester formation were investigated using the dual substituent parameter approach. This procedure did not succeed particularly well, although trends were clearly observed. Correlations with Hammett constants were compared with those of the dual parameter approach. The observation of some linear free-energy relationships was interpreted to mean that the inductive and resonance parameters generally ascribed to the various phenols are not directly applicable to vanadate ester formation where d orbitals are probably involved. The results did indicate that resonance effects have a significantly greater influence on ester formation than do inductive effects.