Chemical mechanism of the radical feedback loop in the classical BZ reaction. Malonyl bromite and oxalic acid as flow-through intermediates
Abstract
High-pressure
liquid chromatography (HPLC) and measurements of the CO2 produced were performed in the induction
period of the classical Belousov–Zhabotinsky (BZ) reaction (malonic acid–bromate–cerium catalyst in sulfuric
acid medium). It was found that oxalic acid is a flow-through intermediate of the reaction. This was confirmed
with an independent qualitative test with thiobarbituric acid. The concentration of oxalic
acid grows in the induction period together with that of bromomalonic acid and dibromomalonic acid intermediates.
It is known that there are two negative feedback loops in the BZ reaction: one is ia bromide and
the other
ia organic free radicals. Oxalic acid and also CO2 are products of this second loop where organic
radicals react with BrO2
radicals. The induction period was chosen for the present experimental
studies because the above radical–radical reactions are most intense during that time. Based on the experimental
results mechanistic proposals are made for the radical feedback loop. A method to accumulate multivalent
organic acids present in very low concentrations in the BZ reaction was also developed. Applying this
and a thermal decomposition method ethenetetracarboxylic
acid (EETA) was
identified as an oxidation product of ethanetetracarboxylic acid (ETA).