Reaction Routes Leading to CO2 and CO in the Briggs−Rauscher Oscillator: Analogies between the Oscillatory BR and BZ Reactions

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2009-08, Vol.113 (32), p.9102-9108
• Main Authors: Muntean, Norbert, Szabó, Gabriella, Wittmann, Maria, Lawson, Thuy, Fülöp, János, Noszticzius, Zoltán, Onel, Lavinia
• Format: Article
• Language: English
• Subjects: A: Kinetics, Spectroscopy
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp905239w

With Fenton-type experiments, it is shown that the intense CO2/CO evolution in the Briggs−Rauscher (BR) reaction is due to decarboxylation/decarbonylation of organic free radicals. The metal ion applied in the Fenton-type experiments was Fe2+ or Ti3+ or Mn2+ combined with H2O2 or S2O8 2− as a peroxide, whereas the organic substrate was malonic acid (MA) or a 1:1 mixture of MA and iodomalonic acid (IMA). Experiments with a complete BR system applying MA or the MA/IMA mixture indicate that practically all CO2 and CO comes from IMA. The decarboxylation/decarbonylation mechanisms of various iodomalonyl radicals can be analogous to that of the bromomalonyl radicals studied already in the Belousov−Zhabotinsky (BZ) reaction. It is found that an intense CO2/CO evolution requires the simultaneous presence of H2O2, IO3 −, Mn2+, and IMA. It is suggested that the critical first step of this complex reaction takes place in the coordination sphere of Mn2+. That first step can initiate a chain reaction where organic and hydroperoxyl radicals are the chain carriers. A chain reaction was already found in a BZ oscillator as well. Therefore, the analogies between the BR and BZ oscillators are due to the fact that in both mechanisms, free radicals and, in most cases, also transition-metal complexes play an important role.