Ultraviolet–visible spectrophotometric determination of ion-association constants for alkylpyridinium iodides. N-ethyl-4-cyanopyridinium iodide in mixed solvents containing ethanol

Abstract

A new method of analysis of spectrophotometric data has been developed to deal with the problem of ion-pair formation represented by the scheme contact ion pair⇌solvent-shared/solvent-separated ion pair⇌free ions for alkylpyridinium iodides. Variation of the composition of a mixed solvent enables the position of each equilibrium to be measured. It has been observed that solvent-shared ion pairs are distinct entities in ethanol solution (ca. 80% of the associated species) and the extent of solvent-shared ion pairing decreases as the polarity of the solvent decreases. The primitive model can explain successfully the formation of an ion pair from the free ions in mixed solvents as long as the percentage of polar component is high, but effects caused by solute—solvent interactions appear at the non-polar end of the mixed-solvent system. Finally the association constants have been correlated with a suitable solvent-polarity parameter.