The Decarboxylation of Anthranilic Acid

Anthranilic acid is known to decarboxylate on being heated above its melting point, or on being boiled in water. We have found that the aqueous decomposition can be acid catalyzed, but that, after the concentration of added mineral acid approximates that of the anthranilic acid, the reaction rate decreases with increasing mineral acid concentration. A mass spectrometer study of the carbon dioxide produced in the decomposition has shown that C12-carboxyl anthranilic acid decomposes at the same rate as C13-carboxyl anthranilic acid. Thus, unlike all other organic acid decarboxylations in which an "isotope effect" has been searched for thus far, the decarboxylation of anthranilic acid does not show an "isotope effect". From the experimental facts available, it appears that the mechanism of the decarboxylation is best explained as bimolecular electrophilic substitution, with the attack of a proton being the rate determining step. While other possibilities are not entirely excluded, a proton attack on the α carbon of the zwitterion is the detailed mechanism suggested as being most probable.