Ionic Equilibria in Protic and in Dipolar Aprotic Solvents

Chemical interactions' as well as 'physical interactions' play an important part in solvation. Some equilibria are reversed on transfer froni protic to dipolar aprotic solvents and this behaviour can be interpreted and predicted through the solvent activity coefficients of the appropriate solutes. Some extrathermodynamic assumptions for the evaluation of single ion solvent activity coefficients are discussed. Trifluoroethanol seems to solvate anions more than does ethanol but solvates cations less than does ethanol. The Principle of Hard and Soft Acids and Bases is valid in dipolar aprotic solvents as well as in water. Solvation of transition states and of ion pairs is compared. Transition states are in equilibrium with their environment. Enthalpies of transfer parallel free energies of transfer, when a series of salts of the one cation are considered. Our interest in ionic equilibria in dipolar aprotic solvents began fifteen years ago, when we noted the enormous increase in rate of many important organic reactions on transfer from alcohols to solvents like dimethylformamide and dimethylsuiphoxide'. Our work since then has always been from the point of view of the physical organic chemist2, rather than of the electrochemist, analyst, or physical chemist, so that the work that I will describe contains all the strengths and weaknesses that this approach implies. It must be realised that solvent transfer frequently creates changes of one million or more in a rate or equilibrium constant, so that some uncertainties and approximations in available data can be tolerated. Rates of slow reactions, like the SN2 exchange between chloride ion and methyl iodide in solution, are best interpreted in terms of the absolute rate HH K Rate CH3I + C1 Cl—-C---I C1CH3 + 1