Effect of intramolecular hydrogen bonds on the kinetics of proton exchange of thiocarboxylic acids with phenols

The kinetics and mechanism of proton exchange between thiocarboxylic acids and o-substituted phenols with intramolecular hydrogen bonds (IMHB) were studied by dynamic PMR spectroscopy. Fast uncatalyzed exchange in cyclic binary complexes is observed for phenols with weak IMHB. In systems with strong IMHB fast exchange occurs only in the presence of an alkaline catalyst, the reaction of which with the phenol leads to cleavage of the IMHB. Concerted transfer of two protons and a cation is realized in the final step of the reaction. According to the proposed mechanism, the measured energy of activation of proton exchange is the sum of the energies of activation of the steps involving cleavage of the IMHB and strictly proton transfer.Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 24, No. 1, pp. 49–56, January–February, 1988.