Solvent effects on ion pairing of tetra-n-butylammonium cyanide. A conductometric study

Tetra-n-butylammonium cyanide (n-Bu4NCN) is a commonly used reagent, for example, for the synthesis of nitriles. Recently n-Bu4NCN has been used as the nucleophilic reagent in kinetic isotope effect studies of nucleophilic aliphatic substitution reactions. The present research concerns the aggregation status (dissociated ions, ion pairs, higher aggregates) and transport properties of n-Bu4NCN in water, dimethyl sulfoxide (DMSO), and tetrahydrofuran (THF), at 25 C as studied by means of precision conductometry. These properties are of great importance since both non-polar and dipolar aprotic solvents are commonly used in the applications. In water as solvent the equilibrium constant for ion-pair formation, K-p =10.1 and the limiting molar conductivity, Lambda(o)= 102.4 cm(2) Omega(-1) mol(-1). The corresponding values for DMSO are K-p =1.98 +/- 0.19 and Lambda(o) = 34.59 +/- 0.03 cm(2) Omega(-1) mol(-1). These data imply that the degree of dissociation, in contrast to the expectations, is higher in DMSO than in water at the same salt concentration. In THF, the conductance as a function of concentration shows a minimum typical for solvents with low relative permittivity, indicating the formation of higher aggregates. The equilibrium constant for ion-pair formation and conductivity in THF is K-p=58.4 X 10(3) and Lambda(o)=9.81 cm(2) Omega(-1) mol(-1). Copyright (c) 2007 John Wiley & Sons, Ltd.