Calorimetric study of the ionization process for 2,5-dihydroxybenzoic acid. Resonance effect. A calorimetric acidity scale for dihydroxy-substituted benzoic derivatives

Abstract A calorimetric study of the ionization process of 2,5-dihydroxybenzoic acid, in water—DMSO mixtures, ranging from pure water up to 0.8 mole fraction is presented. This compound shows a very different behaviour with respect to the 2,3-dihydroxy derivative because of the two hydroxyl groups which, in the relative para position, stabilize and delocalize the charge on the anion. For this reason, 2,5-(OH) 2 PhCOOH is, with the exception of 2,6-(OH) 2 PhCOOH, the strongest acid in an acidity scale of dihydroxy derivatives.