Ce4+-malonic acid reaction in the presence of O-2. Reaction channels leading to tartronic and oxalic acid intermediates

The effect of oxygen on the Ce4+-malonic acid reaction was studied in a sem ibatch reactor. That effect is important for the Belousov-Zhabotinsky (BZ) chemical oscillator. The Ce4+ reagent inflow and consequently the rate of t he reaction itself was controlled by a peristaltic pump. The reaction produ cts were analyzed by HPLC. With this technique two major oxidation pathways were identified. One is significant at high Ce4+ inflow rates only; the pr oduct of this channel is tartronic acid. The other pathway leading to oxali c acid is active at all flow rates but dominant when the feed is slow. A gr eat part of oxalic acid is oxidized further to carbon dioxide and water. A reaction mechanism compatible with these findings is presented. A key step of this mechanism is the fate of the peroxymalonyl radical which is the fir st intermediate for both channels. It is proposed that at high Ce4+ concent rations a fast reaction of this intermediate with Ce4+ leads to tartronic a cid. At low Ce4+ concentrations. however, the peroxymalonyl radical has a l onger lifetime to decarboxylate before reacting with a second Ce4+ and givi ng oxalic acid this way. Two mechanistic schemes proposed fur this low Ce4 channel were tested with further HPLC and kinetic experiments. From the hi gh and low Ce4+ channels it is only the low one which plays a significant r ole in oxygen-perturbed BZ systems, The effect of that channel and its inte rmediates on BZ oscillators is discussed briefly.