Oxidative decarboxylation of benzoic acid by peroxyl radicals.

ABSTRACT

A chemical model based on the thermal decomposition of AAPH (2,2'-azobis(2-amidinopropane) dihydrochloride is used for the production of peroxyl radicals. Peroxyl radicals induces the decarboxylation of [7-13C]benzoic acid and the production of 13CO2, which is measured by gas chromatography-isotope ratio mass spectrometry (GC-IRMS). The decarboxylation depends on temperature, AAPH, and benzoic acid concentrations. The decarboxylation also depends on the presence of oxygen. Electron spin resonance studies are performed to confirm the presence of peroxyl radicals under oxygen and of carbon-centered radicals in the absence of oxygen. Decarboxylation rates are measured in the presence of various antioxidants ascorbate, dimethylsulfoxide, mannitol, and uric acid. It turns out that the decarboxylation is inhibited by each of these antioxidants. The ratio of decarboxylation rates, with and without the antioxidant, varies linearly with the antioxidant concentration. HPLC and GC-MS analyses of reaction products between benzoic acid and AAPH-derived radicals do not detect the presence of radical substitution products on the aromatic ring or the products derived from benzoic acid. There is no doubt that GC-IRMS is a powerful technique to investigate the effects of peroxyl radicals on benzoic acid. In addition, it is possible to follow the degradation of 13C-labeled chemical targets exposed to peroxyl radicals through the production of 13CO2.