Solvent effects on ion-pair distribution and dimerization of tetraalkylammonium salts

The distribution of tetraalkylammonium ions (CnH2n+1)4N+ (R+, TAAn+, n=4-7) with picrate ion (pic-) and inorganic anions X-, (Cl-, Br-, ClO4-), into various inert organic solvents was studied at 25.0°C. The distribution data were analyzed by taking into consideration the distribution of ion pairs, R+·X-, and the dimerization of the ion pairs, (R-·X-)2, in the organic phase. The ion-pair, distribution constant, Kdist, increases with increasing chain length of the tetraalkylammonium ion and with increasing ionic radius of the anion. The values of Kdist show a good correlation with the ET value of solvent, i.e. the solvation ability with respect to the anion, and smoothly increase with increasing ET. The effect of the solvent on the dimerization constants, Kdim, is markedly different between the ion pairs of picrate ion and inorganic anions. In the case of picrate, Kdim significantly decreases with decreasing length of the alkyl chain of the tetraalkylammonium ion, but hardly changes by changing the solvent. On the other hand, in the case of ion pairs of inorganic anions the value of Kdim decreases with decreasing ET and is almost constant for all anions. These results were reasonably explained by the difference of the solvation of the anion moieties of the monomeric and dimeric ion pairs.