Thermodynamics of ionization of some fluorinated β-diketones

Abstract

Thermodynamic ionization constants, Tp K a, of several fluorinated β-diketones, viz. 2-furoyltri-fluoroacetone, heptafluorobutrylpivaloyl methane, hexafluoroacetylacetone, 2-thenoyltrifluoroacetone and trifluoroacetylacetone have been determined pH-metrically in 0-70vol.% dioxane-water mixtures at 25 and 35±0.01°C applying an empirical pH correction for mixed hydro-organic media. The p K as in aqueous medium (at 0% dioxane) are obtained following the method of extrapolation and least-squares. All of them are very weak monoprotic acids. Tp K as do not vary linearly with the reciprocal of dielectric constant of the medium, but a plot of Tp K a vs the mole fraction of dioxane is linear at a given temperature. The “true constant” K′ a and solvation number, n, in the expression (H +)(A −)/(HA)= K′ a ( S) n= K * a for these reagents have been evaluated by analysing the experimental data mathematically. Values of Δ G°, Δ H° and Δ S° are also computed. Positive values of Δ H° found in aqueous-dioxane media for the above fluorinated derivatives, except HFOD, indicate the endothermicity of the proton enolization reactions. Temperature, medium as well as substituent effects are briefly discussed.