Partition and dimerization of ion-pair tetraalkylammonium salts in xylene

Abstract

The partition equilibria of symmetric [TAAn⁺; (C_nH_{2n+ 1})₄N⁺, n= 5–8] and asymmetric [TOMA⁺; (C₈H₁₇)₃(CH₃)N⁺] tetraalkylammonium ions, R⁺, with inorganic anions X^–(= Cl^–, Br^–, BF₄^–, ClO₄^–, NO₃^–, SCN^–) in xylene has been studied at 25.0 °C. The tetraalkylammonium in xylene is present in the form of an ion pair, R⁺· X^–, and the ion pair is partially dimerized as (R⁺· X^–)₂. The distribution constants, K_dist, and dimerization constants, K_dim, of the ion pairs in xylene were determined. By taking into consideration the ion-pair formation constant in organic solvent, K_{ip, org}, the distribution constant of ion pairs was interpreted by the partition of the dissociated cations and anions. The logarithmic distribution constants corrected for ion-pair formation, log K_dist– log K_{ip, org}, of each tetraalkylammonium are proportional to the reciprocal of the ionic radius of the anion except for NO₃^– and SCN^–. The slope of the plot of log K_dist– log K_{ip, org}vs. the number of carbon atoms of R⁺ is almost constant irrespective of the counter-anion except for the TOMA ⁺ system. The distribution constants of asymmetric ions in the TOMA ⁺ system are considerably larger than those of the TAAn⁺ system for a comparable number of carbon atoms. On the other hand, the dimerization constants of ion pairs in the TOMA ⁺ system are considerably smaller than those of the TAAn⁺ system. This peculiar behaviour of the ion pairs of the asymmetric TOMA ⁺ cation is due to the relatively large ion-pair formation constant caused by the shorter closest approach between TOMA ⁺ and the anion.