KINETICS AND MECHANISM OF DECARBOXYLATION OF ANTHRANILIC ACID IN AQUEOUS SOLUTION

The kinetics of decarboxylation of anthranilic acid (HA) has been studied at 70 and 85° in weakly acidic solutions (pH 4.7-1.4) of a constant ionic strength of μ = 0.1 N, and at 85° in 0.1-3.9 N aqueous hydrochloric acid. The results in weakly acidic solutions supply evidence for two parallel reactions of H3O+ with HA and of H3O+ with A-. The mechanisms of both pathways involve predominant rate-determining proton transfer from H3O+ to the carbon atom in the position 1 of the aromatic ring. In strongly acidic solutions, above [H3O+] = 0.5 N, the pseudo-first-order rate constant, k, decreases with increasing [H3O+], which indicates a change of mechanism. A quantitative treatment of the pH dependence of k has been carried out on the basis of the stationary state principle and with the aid of the Hammett acidity function. For sufficiently high [H3O+], there is a linear relationship between 1/k and h0 with a high precision. It may be derived solely from a mathematical analysis of the experimental rate data that in weakly acidic solutions the total rate is mainly governed by the rates of the proton-transfer steps; while with increasing acidity of the solution, a gradual change takes place to complete rate control by the C-C bond-cleavage steps.