TAUTOMERISM OF 4-PHENYLAZO-1-NAPHTHOL IN ORGANIC SOLVENT-WATER MIXTURES

Abstract

The azo-hydrazone tautomerism of 4-phenylazo-l-naphthol (PAN) in the mixtures of water and organic solvents (alcohols such as methanol, ethanol, n-propanol and t-butanol, ethylene glycol and glycerine, and nonhydroxylic solvents such as dioxane and acetone) was studied spectrophotometrically. The equilibrium constants (KT) of the tautomerism were obtained. The effects of the composition of the mixture solvent on the KT value were examined. The KT values were found to increase with an increase in water contents in the mixtures. In monohydric primary alcohol-water mixtures, the KT values were smaller in the mixtures of less polar alcohols. In ethylene glycol- and glycerine-water mixtures, the KT values were supposed to be dependent upon the sum of water and OH concentrations. In nonhydroxylic solvent-water mixtures log KT was proportional to the water concentration in the mixtures. In the mixtures of predominantly aqueous compositions the KT values sharply increase with an increase of aqueous compositions. These results can be explained by assuming that the KT values are not dependent upon the bulk solvent properties but dependent upon the microenvironmental properties of PAN molecules. The tautomerism of PAN in the mixtures is almost athermic. The results suggest that the disperse dye in organic solvent-water mixtures is supposed to be in the more hydrophobic environment than the bulk.