Complex formation between sulfur dioxide and halide ions

Abstract

Complex formation between sulfur dioxide and iodide ions in acetonitrile (AN) has been studied by vapour pressure measurements. The enthalpy changes ΔH0 for 1:1 association reactions between SO2 and halide (X−) ions in AN and between SO2 and Cl− in dimethylsulfoxide (DMSO) have been determined at 25 °C by calorimetry. The ΔH0 values (kcal mol−1) are respectively −4.1 (Cl−), −3.4 (Br−), −3.0 (I−) in AN and −0.7 (Cl−) in DMSO. In contrast to previous literature data there is a linear relationship between these ΔH0 and reported ΔG0 values for the formation of the SO2X− complexes in AN. The difference between the ΔH0 values for SO2Cl− in the solvents AN and DMSO is accounted for by the more exothermic enthalpy of solution of SO2 in DMSO which is the more basic solvent, and by the expected minor difference in the enthalpies of transfer of Cl− and SO2Cl− from AN to DMSO, both dipolar aprotic solvents. Some of the problems connected with the stability order of the SO2X− complexes are discussed in relation to solvent effects and properties of X−.