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 S2O82- as a peroxide, whereas the organic substrate
was malonic acid (MA) or a H 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.
