Solvation of ions. Part X. Kinetic and thermodynamic properties of bimolecular reactions between anions and cations and between dipoles in protic and dipolar aprotic solvents

Abstract

Rate data are reported for two anion–cation reactions, the SN2 decompositions of trimethylsulphonium thiocyanate and trimethylsulphonium bromide, and for a dipole–dipole reaction between methyl bromide and dimethyl sulphide in a protic and a dipolar aprotic solvent of the same dielectric constant. The entropy and enthalpy of ion-pair formation are compared with the entropy and enthalpy of activation of the anion–cation reaction and with the overall entropy and enthalpy change for the decomposition of trimethylsulphonium bromide in dimethylacetamide. The anion–cation reaction has a high activation energy and a large positive entropy of activation, and the reverse dipole–dipole reaction has a low activation energy and a large negative entropy of activation. This situation is less pronounced in 88% methanol–water than in dimethylacetamide. Solvent and ionic strength influence the rate of the anion–cation reaction much more than that of the dipole–dipole reaction, although both reactions pass through the same transition state. Changes in the equilibrium and rate constants for anion–cation reactions, on transfer from dipolar aprotic to protic solvent, can be predicted from medium effects on anion–dipole reactions, on trihalide equilibria, and on the e.m.f. of halogen–halide cells. All are influenced by the much greater hydrogen-bonding solvation of anions and the weaker solvation of dipolar molecules, transition states, or cations, by protic solvents relative to dipolar aprotic solvents.