TRIPLE ION FORMATION BY THE HYDROGEN-BONDING FORCE IN PROTOPHOBIC APROTIC-SOLVENTS

A large deviation was observed between the observed value of the equivalent conductivity (Λcalcd) and the calculated value (Λcalcd) only with ion-pair formation constant (K1) for tributylammonium chloride (n-Bu3NHCl, (0.4-6.0) × 10-3 M) in nitrobenzene and benzonitrile. The error was completely corrected by the symmetrical formation of triple ions (2M + X ⇄ M2X (K2), M + 2X ⇄ MX2 (K3), K2 = K3, where M = n-Bu3NH+, X = Cl-): K1 = 2.65 × 106, K2 = K3 = 1.1 × 109, -2.0 to +1.6% in nitrobenzene; K1 = 2.45 × 106, K2 = K3 = 4.5 × 108, -0.8 to +0.5% in benzonitrile. In acetone, the triple ion formation constant was smaller (K1 = 1.85 × 106, K2 = K3 = 1.5 × 108) than that in the above two solvents. Even in propylene carbonate with a high dielectric constant (ε = 64.4), the triple ion formation could be observed to some extent. On the other hand, only the ion-pair formation corrected by the activity coefficients of simple ions explained the conductivity data in ethanol which is a hydrogen bond donor and acceptor. As for anions, the ion-pair formation constants of tributylammonium halides decreased in the order Cl > Br > I, and so did the triple ion formation constants. The triple ion formation from the iodide was very small even in nitrobenzene. The triple ion formation from MX decreased in the following order: nitrobenzene > benzonitrile > acetone > acetonitrile > propylene carbonate. In the higher concentration range (4.0 × 10-3 to 0.1 M) of n-Bu3NHCl in nitrobenzene or benzonitrile, the minimum was observed in the relation of Λ vs C1/2. The ion-pair and the triple ion formation constants caused a large relative error for the Λcalcd to the Λobsd (K1 = 2.4 × 106, K2 = K3 = 1.1 × 109, +69.5% at the initial concentration of the salt, Cs = 0.08 M in nitrobenzene; K1 = 2.55 times; 106, K2 = K3 = 4.5 × 108, +50.0% at Cs = 0.12 M in benzonitrile). The association between the triple ion and the simple ion (the formation of the quadrupole: M2X + X ⇄ M2X2 (K4), MX2 + M ⇄ M2X2 (K5)) reduced the relative error within ±2% (K4 = K5 = 1.8 × 104 in nitrobenzene; 2.0 X 104 in benzonitrile). The quadrupole formation was also observed for the bromide at the higher concentrations in these solvents. © 1990 American Chemical Society.