Solvent effect on the dissociation of o-nitroanilinium ion in methanol-propylene glycol media at 25°C

Abstract

The dissociation constants ( K s ) of o-nitroanilinium ion have been determined at 25° in methanol, propylene glycol and mixtures of the two components containing 10, 30, 50, 70 and 90 wt. % propylene glycol by spectrophotometric measurements. Standard free energies of transfer, Δ G° t (B) of o-nitroaniline from methanol to the other solvents have been evaluated from the measurement of solubilities at 25°C. The solvent effect on the proton transfer process of this acid: Δ G° t [BH +−B] sys = 2.303 RT[p( K s −p( K m )] has been discussed in terms of the free energies of transfer Δ G° t from methanol to other solvents, of the uncharged based B, of the hydrochloride of the base (BHCl) computed from the corresponding values for HCl and also of the individual ions assessed from the previous knowledge of Δ G° t (Cl −). The corresponding data for the protonated tris(hydroxymethyl)methyl amine [TrisH +] and p-nitroanilinium ion in the same solvent system obtained from the literature are also compared. Δ G° t [BH +−B] sys values for the dissociation of o-nitroanilinium ion have been found to increase less readily with wt.% propylene glycol compared to p-nitroanilinium ion and are in contrast with the decreasing nature of the corresponding values for the TrisH + ion of different chemical nature. Electrostatic effect being negligible in this solvent system, the overall behaviour of these acids is found to be dictated by specific chemical nature of solute-solvent interactions besides the effect of relative solvent basicities.