The acid–base function in non-aqueous solution. Part V. Entropy changes due to intramolecular and solvation effects in aprotic solvents

Abstract

The free energies, enthalpies, and entropies of proton transfer reactions for the following systems are discussed: (i) Bromothymol Blue + pyridine and substituted pyridines in chlorobenzene and (ii) picric acid + substituted pyridines and n-butylamine in chlorobenzene, benzene, and dioxan. The reactions occurring are of the type AH + B ⇌ AHB. The results for the first indicate that negative contributions to entropy changes can be caused by restricted rotations in the hydrogen bonded ion-pair due to substituents in the base. In the second system there are no marked differences in solvation between the solvents benzene and chlorobenzene but in dioxan large positive entropy changes arise from solvent exclusion when the ion-pair is formed. For these systems therefore the thermodynamic functions do not simply reflect intramolecular effects alone despite the solvents being aprotic and of low dielectric constant.