Alternative Reaction Channels and Carbene Intermediates in the Ce 4+ -Malonic Acid and Ce 4+ -Bromomalonic Acid Reactions. 1. CO 2 Measurements Szilveszter Nagygyo _ ry, Ma ´ ria Wittmann, Szilveszter Pinte ´ r, Andra ´ s Visegra ´ dy, Andra ´ s Dancso ´ , Nguyen Bich Thuy, and Zolta ´ n Noszticzius* Center for Complex and Nonlinear Systems and the Department of Chemical Physics, Technical UniVersity of Budapest, H-1521 Budapest, Hungary La ´ szlo ´ Hegedu _ s and Horst-Dieter Fo 1 rsterling Fachbereich Chemie, Philipps-UniVersita ¨ t Marburg, D-35032 Marburg/Lahn, Germany ReceiVed: January 27, 1999; In Final Form: April 15, 1999 The Ce 4+ -malonic/bromomalonic acid reactions play an important role in the oscillatory Belousov- Zhabotinsky reaction. In this work CO 2 evolution from these reactions was studied with a sensitive and quantitative method, by converting the CO 2 to methane and measuring it with a flame ionization detector. It was found that the stoichiometries depend on the initial conditions in batch experiments or on the mixing rate of reagents in a semibatch reactor. These findings are explained by two main reaction channels: one for recombination and another for decarboxylation. Decarboxylation itself has two separate routes, one is dominant at low while the other at high Ce 4+ concentrations. In the latter, formation of more than two CO 2 molecules from one malonic/bromomalonic acid molecule was observed. Novel reaction schemes containing carbenes are proposed for these “high Ce 4+ ” decarboxylation channels. To check the new mechanism, HPLC measurements are planned as a continuation of the research. Introduction The Belousov-Zhabotinsky reactionsthe oxidation of ma- lonic acid by acidic bromate in the presence of Ce 4+ catalyst and related reactionssis the focus of interest in the past decades because of its interesting dynamics. 1 Currently, its chemical mechanism, the reaction steps, and rate constants are elaborated so that kinetic simulations show a rather good agreement with various types of the observed dynamical phenomena. Though these mechanisms give a detailed description of the inorganic subset including elementary steps, they involve hypothetical composite reactions in the organic subset of the whole BZ reaction. It would be important to understand more details of the latter reactions because these are crucial ones in the negative feedback loop of the autocatalytic process. The first step in investigating the organic steps of the whole BZ reaction is to examine the reaction of the catalyst, e.g., Ce 4+ ions, with malonic acid and bromomalonic acid, respectively. Recently a series of papers has identified some primary products of these reactions using the HPLC technique. 2-4 These were 1,1,2,2-ethanetetracarboxylic acid (ETA) and malonyl mono- malonate (MAMA) in the Ce 4+ -malonic acid reaction and bromoethenetricarboxylic acid (BrEETRA) in the Ce 4+ -bromo- malonic acid reaction, products that emerge in the recombination of the primarily formed malonyl or bromomalonyl radicals. Experimental observations suggest, however, that recombina- tion of the radicals is not the only possibility. For example, it is known that in the Ce 4+ -malonic acid reaction formic acid can also be an end product. Actually, for a long time it was believed that it is the only one. It is clear that formic acid cannot be a product of a recombination route, as recombination alone can only generate molecules with more than one carbon atom. Formic acid should be formed in a different reaction channel starting also from the malonyl radical but continuing with decarboxylation steps. Such a decarboxylation channel should exist in the Ce 4+ -bromomalonic acid reaction as well. In that case no formic acid is produced but about 60% of the bromomalonic acid is converted directly into CO 2 when Ce 4+ is in excess, 4 although BrEETRA does not react further with Ce 4+ . All these observations suggest that the recombination reaction channel is not unique; it dominates only when the organic radical concentration is relatively high and the Ce 4+ concentration is relatively low. The other reaction pathway, where the main product is carbon dioxide, involves probably very reactive intermediates that cannot be easily detected. In this study our aim was to get a better insight into these processes by measuring the CO 2 evolved in the reaction of Ce 4+ and malonic/bromomalonic acid. CO 2 evolution is followed continu- ously by transferring it into an inert gas stream, converting to methane with H 2 , and detecting with a flame ionization detector. 5-7 Experimental Section Materials. Malonic acid (Fluka, puriss.), oxalic acid (Reanal, puriss.), Ce(SO 4 ) 2 (Riedel-de-Haen, pro analysi), (NH 4 ) 2 Ce- (NO 3 ) 6 (Fluka), and H 2 SO 4 (Carlo Erba, RPE) were used as received. Bromomalonic acid was produced following the procedure of Fo ¨rsterling et al. 8 Reactions. The reaction of malonic acid and bromomalonic acid with Ce 4+ was examined both in sulfuric acid and in perchloric acid media at room temperature (22 ( 2 °C). Perchloric acid was used because sulfate ions establish a complex with ceric ions so in sulfuric acid medium the oxidation reactions are slower. The acid concentration was 1 M in all 4885 J. Phys. Chem. A 1999, 103, 4885-4892 10.1021/jp990330+ CCC: $18.00 © 1999 American Chemical Society Published on Web 06/09/1999