Ionisation of dimethyl sulphoxide in highly basic media

Abstract

Measurements are reported on the rates of exchange of dimethyl sulphoxide at 25° in three systems (Me₂SO–OH^––H₂O, Me₂SO–OMe^––MeOH, Me₂SO–OEt^––EtOH) over an H_– range of more than eight units with tritium as tracer. The exchange has been measured with the tritium label in the protic solvent as well as in dimethyl sulphoxide. The ionisation rate coefficients depend only on the equilibrium basicity of the solution and for all three systems the logarithm of the ionisation rate coefficient is linearly related to the H_– of the medium with a slope of +0·93. The results as well as giving indirect validity to the H_–scale also suggest that both nitrogen and carbon acids, employed in setting up these scales, respond in a similar manner to solvent changes. The results are interpreted in terms of a Brønsted β value near unity and a highly unsymmetrical transition state.