Solvent effect on the dissociation of protonated tris(hydroxymethyl)-methylamine and p-nitroanilinium ion in water–ethylene glycol media at 25 °C

Abstract

The dissociation constants (_sK)_BH⁺ of two BH⁺-type acids, protonated tris(hydroxymethyl)methylamine (tris,H⁺) and p-nitroanilinium ion, have been determined at 25 °C in a series of aqueous mixtures of ethylene glycol from electrometric and spectrophotometric measurements respectively. The solvent effects on the dissociation behaviour of these acids, ΔG_t°[BH⁺–B]_sys= 2·303RT[p(_sK)^N_BH⁺– p(_wK)^N_BH⁺], in these media result in the characteristic minima with the solvent composition. The solvent effects have been discussed in terms of the free energies of transfer (ΔG_t°) from water to the aqueous glycol solvents, of the uncharged bases (B) evaluated from the measured solubilities, of the hydrochlorides of the bases (B,HCl) computed from the corresponding values for HCl, and also of the individual ions assessed from the previous knowledge of ΔG_t°(Cl^–). The corresponding data for tris,H⁺ and and m-nitroanilinium ion in methanol–water solvents, obtained from the literature and from the present work, are also compared. The individual contributions of the different species concerned are discussed in terms of the interaction energies of the species with positive or negative charge centres of the isolated solvent dipoles. The overall behaviour of these protonated amines with different chemical nature and also with different solvophilism is found to be largely dictated by the specific solute–solvent interactions besides the effect of the relative solvent ‘basicities’.