Conductance study of ion-pairing of alkali tetrafluoroborates and hexafluorophosphates in acetonitrile containing some substituted macrocyclic polyethers

Abstract

The conductance of sodium and potassium tetrafluoroborates and hexafluorophosphates in acetonitrile solutions at 25°C in the presence of bis(4-tert-butylbenzo)-18-crown-6(mtbDBC), bis(3,5-di-tert-butylbenzo)-18-crown-6 (dtbDBC) and bis(4-hexadecylbenzo)-18-crown-6 (hdDBC) has been investigated. The analysis of data has been performed by means of the Fuoss—Hsia conductance equation. Values of the limiting conductance, Λ 0, and ion-pair dissociation constants, K D , are reported. The results obtained are compared with those obtained for the conductance of sodium and potassium tetrafluoroborates and hexafluorophosphates in pure acetonitrile. Addition of macrocyclic polyethers in the salt solutions gives a decrease in the conductivity in the order dtbDBC < mtbDBC < hdDBC. All systems were found to be associated, and the trends in association are discussed in terms of the polyether interactions with alkali cation which are controlled by their size, the steric hindrance effects caused by the alkyl substituents in the polyether molecule, size and charge density of the anion, and the interaction forces of the solvent molecules on the ionic species present in solution.