Thermodynamics of proton-ligand dissociation equilibria in mixed solvents. acid dissociation of 2-isopropyltropolone in mixtures of water and dioxane

Abstract

Thermodynamic acid dissociation constants, TpKa, of 2-isopropyltropolone (HIPT) have been determined by pH titration in aqueous and aqueous dioxane media at 25 and 35 ± 0.01°C applying an empirical pH correction for mixed aqueous media. HIPT is a very weak acid with TpKa between 7 and 9. The TpKa values do not vary linearly with the reciprocal of the dielectric constant of the medium, but a plot of TpKa 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 this reagent have been evaluated. Mean ionic radii, r ±, for the tropolonate ion, being in the region of 2.0–2.4 Å, slightly increases with dioxane percentages. Values of ΔG0, ΔH0 and ΔS0 have been evaluated. The effects of temperature as well as medium effects are briefly discussed.