Thermodynamic constants for tautomerism and ionization of pyridoxine and 3-hydroxypyridine in water–dioxane

Abstract

Thermodynamic values of the ionization constants and tautomeric equilibrium constants of pyridoxine and 3-hydroxypyridine were determined in dioxane–water at 25 °C. The Born model is unable to explain the effect of the solvent on these constants. The logarithms of the equilibrium constants calculated change linearly according to the logarithm of the molar water content in the mixture, up to 40% dioxane in the case of pyridine ionizations, and up to 70% dioxane (maximum content investigated) for the phenol ionizations. The effect of protonation of either the phenolate or pyridinium nitrogen groups on the pK of the other group in these substances increases when the water content in the medium is decreased.