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).