Thermodynamics of proton-ligand dissociation of some fluorinated β-diketones in dioxane-water mixtures

Abstract

The thermodynamic proton-ligand dissociation constants (Tp K a) of the three fluorinated β-diketones namely 1,1,1-trifluoro-2,4-hexanedione, 1,1,1-trifluoro-5-methyl-2,4-hexanedione and 1,1,1-trifluoro-5,5-dimethyl-2,4-hexanedione have been determined pH-metrically in various dioxane-water mixtures at 25 and 35±0.1°C by applying an empirical pH correction for mixed organo-aqueous media. Both extrapolation and the least-squares methods were used to obtain Tp K a in pure water (0%). All the compounds investigated are weak monoprotic acids. The Tp K a values do not vary linearly with the reciprocal of the dielectric constant of the medium whereas a plot of dioxane mole fraction ( n 2) versus Tp K a is linear at a given temperature. An expression relating the solvation number and ‘true ionization constant’ has been mathematically evaluated. The values of changes in the standard free energy, enthalpy and entropy associated with their proton-ligand dissociation have been calculated. Temperature and medium as well as substituent effects are briefly discussed.