TOWARDS A SOLVENT ACIDITY SCALE - THE CALORIMETRY OF THE NORMAL-METHYL IMIDAZOLE PROBE

This paper reports a rather straightforward calorimetric method for the precise determination of the acidity of organic solvents. From the calculated enthalpies of solvation (DELTA-H(solv)) of the probe compounds N-methylimidazole and N-methylpyrrole and the known relative permittivity (epsilon) of the solvent DELTA-H(acid) is obtained through the equation: DELTA-H(acid) = - [DELTA-H0solv(N-methylimidazole)-DELTA-H0solv(N-methylpyrrole)] + 18.760 f(epsilon) + 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.