Triple-ion formation and acid–base strength of ions in protophobic aprotic solvents

Abstract

The formation of symmetrical triple ions [2M++ X–⇌ M2X+, (K2); M++ 2X–⇌ MX–2, (K3); K2=K3] and the quadrupole [M2X++ X–⇌ M2X2, (K4); M++ MX–2⇌ M2X2, (K5); K4=K5], in addition to ion-pair formation [M++ X–⇌ MX, (K1)] from various uni–univalent salts were examined by means of conductometry in acetonitrile, benzonitrile and propylene carbonate. The salts are made from bases (B) and acids (HX) of varying strength. For diethylcyclohexylammonium chloride [(0.4–6.0)× 10–3 mol dm–3 in benzonitrile], the calculated molar conductivities (Λ/S cm2 mol–1) were fitted to the observed ones within 0.36% of the standard deviation of the relative error, considering the symmetrical formation of the triple ions. In the higher concentration range [(0.4–12.0)× 10–2 mol dm–3] of the salt a minimum was observed in the relation between Λ and C1/2. The observed minimum could be reproduced in terms of the large formation constants of the triple ions and the quadrupole complex by computer simulations. However, a larger formation constant of the quadrupole above the critical value caused the minimum to disappear. Weaker basicities of amines tended to give higher formation constants of the triple ion; however a levelling-off was observed below pKBH+= 18 in acetonitrile. On the other hand, the formation constants for the ion pair and the triple ions from salts with different anion basicities (from perchlorate to 3,5-dinitrobenzoate) were proportional to the acidities of the corresponding acids.