Towards a solvent acidity scale: the calorimetry of the N-methyl imidazole probe

Abstract

This paper reports a rather straightforward calorimetric method for the precise determination of the acidity of organic solvents. From the calculated enthalpies of solvation (ΔH_solv) of the probe compounds N-methylimidazole and N-methylpyrrole and the known relative permittivity (ε) of the solvent ΔH^acid is obtained through the equation: ΔH^acid=–[ΔH⁰ _solv(N-methylimidazole)-ΔH⁰ _solv(N-methylpyrrole)]+ 18.760 f(ε)+ 1.69. The proposed method allowed us to determine the acidity of 36 solvents, including some slightly acidic ones, whose acidity is difficult to obtain by existing methods.