Solvent and Pressure Effects on the Tautomeric Equilibrium of 4-Phenylazo-1-naphthol

The azo-hydrazone tautomerism of 4-phenylazo-1-naphthol has been studied in ethanol-water and acetone-water systems and at high pressures. The hydrazone form is favoured by adding water to organic cosolvents and by applying pressure. The effects of solvent polarity and pressure on the equilibrium constant (KT) between the tautomers were examined by using Kirkwood-type equations applied to chemical equilibrium. Remarkable deviations were observed in the Kirkwood plots for both types of mixtures. Solvent-induced polarization, especially of the hydrazone form in a high polarity region, was suggested. In ethanol-water mixtures, correlation is fair between ln KT and ET, while in acetone-water system the ln KT against ET plot was curved. The above findings, together with the difference in the volume data for the two systems, suggest an existence of special interactions between acetone molecules and hydrophobic parts of 4-phenylazo-1-naphthol.