Temperature-jump study of the mechanism of base catalysed removal of the intramolecularly hydrogen-bonded proton from a substituted salicylate ion in aqueous solution

Abstract

Rate coefficients and equilibrium constants for proton transfer from 4-(3-nitrophenylazo)salicylate ion to hydroxide ion and six amines have been measured in aqueous solution. The rate coefficients are approximately three orders of magnitude below the diffusion limited value and this is explained by the presence of a moderately strong intra-molecular hydrogen bond in the salicylate ion. Although the amine bases vary in strength by over 1.5 pK units, the rate coefficients are the same within experimental error and this is accounted for by a two-step mechanism for proton transfer. The first step involves a rapid equilibrium between hydrogen-bonded and non-hydrogen-bonded forms of the salicylate ion and this is followed by base catalysed proton removal from the non-hydrogen-bonded form. A mechanism consisting of direct attack by base on the intramolecularly hydrogen-bonded species will not explain the results.