Thermodynamic proton-ligand dissociation constants of substituted 3-hydroxy-4-pyrones in dioxane-water mixtures

Abstract

Thermodynamic proton-ligand dissociation constants Tp K a were determined pH-metrically in 10–50 vol.% dioxane-water mixtures at 25 and 35 ± 0.1° C for two substituted 3-hydroxy-4-pyrones (3-hydroxy-2-methyl-4-pyrone (maltol) and 3-hydroxy-5-(hydroxymethyl)-4-pyrone (kojic acid). Extrapolation and least-squares methods were used to obtain Tp K a in pure water (0%). Both of the 3-hydroxy-4-pyrones are monoprotic and they are very weak acids. The Tp K a values do not vary linearly with the reciprocal of the dielectric constant of the medium, but a plot dioxane of mole fraction n 2 vs. Tp K a is linear at a given temperature. The “true constant” Tp K a t and solvation number n (in the expression (H +)(A −)/(HA = Tp K a t(S) n = Tp K a ∗ for these compounds were evaluated by analysing the experimental data mathematically. Values of the changes in standard free energy, enthalpy and entropy associated with the dissociation were calculated. The effects of temperature, medium and substituent are briefly discussed.