TRIPLE-ION FORMATION AND ACID-BASE STRENGTH OF IONS IN PROTOPHOBIC APROTIC-SOLVENTS

The formation of symmetrical triple ions [2M+ + X- half arrow rightover half arrow left M2X+, (K2); M+ + 2X- reversible MX2-, (K3); K2 = K3] and the quadrupole [M2X+ + X- half arrow right over half arrow left M2X2. (K4); M+ + MX2- half arrow right over half arrow left M2X2, (K5); K4 = K5], in addition to ion-pair formation [M+ + X- half arrow right over half arrow left 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) x 10(-3) mol dm-3 in benzonitrile], the calculated molar conductivities (LAMBDA/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) x 10(-2) mol dm-3] of the salt a minimum was observed in the relation between LAMBDA 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 pK(BH+) = 18 in acetonitrile. On the other hand, the formation constants tor 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.