Partition and dimerization of ion-pair tetraalkylammonium salts in xylene
Abstract
The partition equilibria of symmetric [TAAn+; (CnH2n+ 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, Kdist, and dimerization constants, Kdim, of the ion pairs in xylene were determined. By taking into consideration the ion-pair formation constant in organic solvent, Kip, 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 Kdist– log Kip, 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 Kdist– log Kip, orgvs. 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.