Ionization of sulfonic acids and dimethylsulfoxide basicity. A 13C nuclear magnetic resonance and calorimetric study

Abstract

The ionization of trifluoromethane-, methane-, and benzene-sulfonic acids in Me2SO and in water has been studied by 13C nuclear magnetic resonance. While the protonation shifts of CF3SO3− and CH3SO3− in both solvents are too small to make a quantitative interpretation possible, the C6H5SO3− shift data in H2O–H2SO4 mixtures, when treated by the method of Cox and Yates, gave a pKa value of −2.5 for the ionization of C6H5SO3H. Our results are discussed in relation to those of other workers. Enthalpies of solution and of ionization of these acids and related compounds have been obtained in both solvents. Single ion enthalpies of transfer from water to Me2SO have been calculated from our data and from a transfer value for CF3SO3− (+1.9 kJ mol−1) based on the tetraphenylarsonium–tetraphenylborate extrathermodynamic assumption. The transfers are endothermic processes for the anions CH3SO3− (+29.6 kJ mol−1), C6H5SO3− (+ 20.2 kJ mol−1), and p-CH3C6H4SO3− (+ 21.8 kJ mol−1), and exothermic for H+(−27.4 kJ mol−1). These results are discussed in terms of the properties of the ions and solvents. The transfer value for H+ is a valuable measure of the basicity difference between bulk water and Me2SO. Keywords: sulfonic acids, ionization, dimethylsulfoxide, 13C NMR, calorimetry.