Solute-solvent interaction in pyridine

Abstract

The nature of the interaction between MA and solvent pyridine to form the pyridinium species, pyrMA, is being investigated on the basis of the electrochemical reduction of the pyridinium moiety; M is a Lewis acid, e.g., a proton, a metal ion, a tetraalkylammonium ion or an alkyl group. The anion A influences the various equilibria involving the pyridinium species through (a) its chemical type, e.g., whether derived from a carboxylic acid, a phenol or a heterocyclic ring, (b) the contribution of the dispersion effect to the medium effect on free energy in connection with specific solute-solvent interactions, (c) the association of M and A in the MA species, e.g., as measured in pyridine, water and other solvents, and (d) interactions involving other ions present such as those of the background electrolyte in polarography. Ion association and ion exchange involving competitive Lewis acids are important and can be evaluated by spetrophotometric, potentiometric and conductometric measurements, as well as by polarography. The overall influence of the solvent on the pyridinium equilibria is illustrated by the effect of the addition of water.