PARTITION AND DIMERIZATION OF ION-PAIR TETRAALKYLAMMONIUM SALTS IN XYLENE

The partition equilibria of symmetric [TAAn(+); (CnH2n+1N+, n = 5-8] and asymmetric [TOMA(+); (C8H17)(3)(CH3N+] tetraalkylammonium ions, R(+), with inorganic anions X(-) (= Cl-, Br-, BF4-, ClO4-, NO3-, SCN-) in xylene has been studied at 25.0 degrees 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,K-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,K-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,K-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.