PROTONATION EQUILIBRIA AND HYDRATION IN DISUBSTITUTED BENZOIC-ACIDS

The protonation constants log k(app) of a series of disubstituted benz oic acids in aqueous solution at different temperatures between 5 degr ees and 55 degrees C have been determined potentiometrically. The data of log k(app) have been analyzed under the light of a statistical the rmodynamic model. The curvature of the function log k(app) = f (1/T) i s related to the number n(w) of water molecules involved in the proton ation and hydration reaction. The upward concavity of the curves of di nitro compounds are steeper that those for monosubstituted acids and i mply higher number of water molecules. The curves of polyalkyl-substit uted benzoic acids as determined by other authors show opposite (downw ard concavity) curvatures corresponding to negative numbers n(w) of wa ter molecules. The values of log k(app) at 25 degrees C of disubstitut ed and polyalkyl-substituted benzoic acids plotted against the Hammett substituent constants sigma(Ham) deviate significantly from the line of the Hammett model. (C) 1998 Elsevier Science B.V. All rights reserv ed.