Calorimetric Determination of Thermodynamic Parameters for the Dissociations of Acids in Dipolar Aprotic Solvents

Abstract

A calorimetric method has been investigated for the determination of the thermodynamic parameters of acid dissociations in dipolar aprotic solvents. For the dissociations of monoprotonated bases BH+ (B: aniline, pyridine, triethylamine, tributylamine, triethanolamine, 1,3-diphenylguanidine and 1,1,3,3-tetramethylguanidine) in N,N-dimethylformamide, dimethyl sulfoxide (DMSO), acetonitrile and propylene carbonate, the solutions of the bases were titrated with small volumes of a “strong acid,” trifluoromethanesulfonic acid. In each solvent, there was an approximately linear relation of unit slope between ΔH298° and ΔG298° showing that the difference in pKa between different BH+’s can mainly be attributed to the difference in the enthalpy term. As for the solvent effect on pKa, however, the entropy term seems to play an important role. A preliminary study has shown that the calorimetric method is also applicable to the dissociations of HX-type weak acids (benzoic and salicylic acids in DMSO, for example), if the dissociation equilibria are not complicated by such reactions as homoconj ugation.