Mechanism of decarboxylation of substituted salicylic acids. I. Kinetics in quinoline solution

Abstract

First-order rate constants for the decarboxylation of fourteen 4- and 5-substituted salicylic acids have been determined in quinoline solution in the temperature range 90–230 °C. Substituents have almost no effect on the rate constants, except those with large negative σ-constants: p-amino, p-hydroxy, p-ethoxy. The enthalpies and entropies of activation do not fit the isokinetic relationship, with the same three substituents deviating. It is suggested that the decarboxylation involves a preliminary ionization of the carboxyl group, followed by protonation of the aromatic ring of the anion so formed, and then loss of carbon dioxide. The isokinetic relationship fails because substituents affect all three steps differently, and the Hammett relationship fails because the substituent effect on the ionization is related to σ while that on the other two steps follows σ + . The three substituents which deviate are those for which σ and σ + differ widely.