Basic properties of Anions in mixed solvents

Abstract

The thesis is primarily concerned with the effect of solvent on the kinetic and thermodynamic properties of anions. The solvent systems used are mixtures of water with dimethylsulphoxide, trifluoroethanol and acetonitrile. There are two main sections of the thesis. The first deals with thermodynamic aspects of the work and includes values for the free energies of transfer of silver hydroxide and silver fluoride from water to the mixed solvents together with the pKfl values of many acids in the solvent mixtures. (It is shown that the fluoride ion selective electrode is well suited for use in all the solvent systems studied). Variations in the thermodynamic parameters are discussed in terms of changes in anionic solvation. The second section is concerned with the effect of solvent on the rates of several proton-transfer reactions involving carbon acids and anion bases. Rate constants for such reactions were found to be very sensitive to solvent composition and, in particular, increased rapidly with the DMSO content of DMSO-HjO mixtures. The corresponding equilibrium constants were often largely independent of solvent composition. The rates and equilibria are discussed in terms of the Brtinsted relationship and it is shown that apparent anomalies in Brttnsted coefficients for several of the reactions studied disappear upon transfer to DMSO-HjO mixtures of sufficiently high DMSO concentration. It is argued that anomalous values of Brbnsted coefficients in water result from specific solvation effects. The primary kinetic hydrogen isotope effects (i.e. k /k ) for several of the above reactions were also studied. Comparisons of k^/k^1 with AG° of reaction lead to the conclusion that variations in the latter are not solely responsible for the observed changes in the isotope effects; this is discussed in terms of solvent involvement in the proton/deuteron-transfer reaction. A section dealing with the mechanism of the observed oxidation of dimethylsulphoxide by bromine in aqueous solution is also included.