Proton-transfer equilibria in isodielectric acetonitrile–ethylene glycol mixtures at 298.15 K

Abstract

The dissociation constants (K_a) of tris(hydroxymethyl)methylammonium (TrisH⁺) and p-nitroanilinium (pNAH⁺) ions and p-nitrophenol (pNP) have been determined at 298.15 K both in ethylene glycol (EG) and its isodielectric mixtures with acetonitrile (ACN), an e.m.f. method being used for TrisH⁺ and a spectrophotometric method for the other two acids. These K_a values have been used to derive the solvent effect on the free energies of dissociation of these acids. From independent solubility measurements on the neutral species: tris(hydroxymethyl)methylamine (Tris), p-nitroaniline (pNA), and pNP, the free energies of transfer of these species from EG to ACN–EG mixtures have been determined at 298.15 K. By a combination of the results of K_a and solubility measurements and with a knowledge of the free energies of transfer of the hydrogen ion evaluated earlier, the free energies of transfer of TrisH⁺, pNAH⁺, and pNP^–(p-nitrophenoxide ion) in these solvents have been computed. Plots of free energies of transfer for the individual species against solvent composition reflect a wide range of solvation behaviour which has been interpreted in terms of the tendency of protic solvents like EG to solvate specifically hydrogen-bonding solutes and the tendency of dipolar aprotic solvents like ACN to solvate specifically solutes undergoing primarily dispersion interactions.