Conductivity studies in benzotrifluoride

The variation of conductivity with concentration of some onium salts in benzotrifluoride, (1′, 1″, 1″'-trifluoromethylbenzene), BTF, has been studied in the 10−6−10−2 M concentration range. All salts studied display a distinct maximum in the Λ− c 1 2 plots around 1 × 10−5 M preventing Λ° and the degree of dissociation to be determined. The concentration at which Λmax is observed decreases with increasing size of the tetraalkylammonium cations, with decreasing coordinating ability of the anions and with increasing temperature; i.e. with decreasing association constants as deduced from conductivity studies in other solvents. It is suggested that the solvent molecules form pairs or larger aggregates inducing a unique type of solvent-structure-enforced ion-pairing which at very low concentrations will subdue the anticipated increase in the molar conductivity upon further dilution. The presence of head-to-tail anti-parallel dimers in BTF, linked together by F…H interactions, has also been arrived at from quantum chemical calculations. The crystal structure of BTF is not yet known but the crystal structures of some higher-melting compounds like 4-(trifluoromethyl)benzonitrile and 4-(trifluoromethyl)phenylacetonitrile have revealed the presence of weak intermolecular contacts between the fluorine atoms and the aromatic hydrogen atoms but no ππ interactions.