Oxidation of Methylmalonic Acid by Cerium(IV). Evidence for Parallel Reaction Pathways Per Olav Kvernberg, ² Eddy W. Hansen, ‡ Bjørn Pedersen, ² Asbjørn Rasmussen, ² and Peter Ruoff* ,§ Department of Chemistry, UniVersity of Oslo, P.O. Box 1033 Blindern, N-0315 Oslo, Norway, SINTEF Oslo, P.O. Box 124 Blindern, N-0314 Oslo, Norway, and School of Technology and Science, StaVanger College, P.O. Box 2557 Ullandhaug, N-4004 StaVanger, Norway ReceiVed: October 24, 1996; In Final Form: January 21, 1997 X The reaction between methylmalonic acid (MeMA) and Ce(IV) ion in 1 M sulfuric acid/D 2 SO 4 has been studied by means of 1 H- and 13 C-NMR. When MeMA is in excess, acetic acid, hydroxymethylmalonic acid, and pyruvic acid are formed as stable end products. When Ce(IV) is in stoichiometric excess, acetic acid is the only product. Approximately 70-80% of the MeMA forms rapidly acetic acid with hydroxymethylmalonic acid and pyruvic acid as reactive intermediates. The remaining MeMA reacts along parallel pathways and forms two intermediates, which slowly convert to hydroxymethylmalonic acid and pyruvic acid, respectively. Introduction Ce(IV) compounds are important oxidants in many fields of chemistry. 1-5 In the Belousov 6 -Zhabotinsky 7 (BZ) reaction, 8 Ce(IV) oxidation of aliphatic organic substrates that contain a reactive C-H bond, as in malonic acid and derivatives, is a necessary component process for obtaining sustained concentra- tion oscillations. While the inorganic mechanism for the oscillatory BZ reaction is well established 9,10 and has success- fully been modeled by the Oregonator 11 and related models, 12 still little is known about the reactions of the organic subset. Considerable “detective work” is necessary to identify the various components in this subset. 13,14 The methylmalonic acid (MeMA) BZ reaction 15 has shown a variety of unusual dynamical behaviors such as excitabilities of oxidized and reduced steady states and even bistability in a closed system. 16 A kinetic study of the reaction between MeMA and Ce(IV) has been performed by Ruoff and Nevdal 17 mainly by using initial rates and UV spectroscopy. Although the stoichiometry of the Ce(IV)-MeMA reaction is expected to be iodometric analysis showed that during a 20 h period only 4.7 mol of Ce(IV) were consumed per mole of MeMA. 17 Ruoff and Nevdal also found that under certain conditions a precipitate appeared, while the recorded 1 H-NMR spectrum of the reaction mixture showed an unidentified product together with pyruvic acid as a probable intermediate. Earlier NMR investigations on MeMA-BZ systems and subsystems 18,19 encouraged us to study in further detail the MeMA-Ce(IV) system. In this paper we report new results where intermediates and products of the MeMA-Ce(IV) reaction were followed quantitatively with different initial concentrations. It was found that most of the MeMA (∼70- 80%) reacts rapidly to form acetic acid (HOAc). For this main pathway NMR spectra indicate the presence of small amounts of hydroxymethylmalonic acid (MeMAOH) and pyruvic acid (Pyr), as reactive intermediates. The remaining MeMA reacts by another route, where two additional intermediates form. These intermediates react slowly by first-order processes to form respectively MeMAOH and Pyr with half-lives of 6-8 h. Experimental Section MeMA was purchased from Fluka (>99%) and dissolved in 1MH 2 SO 4 (Merck) or D 2 SO 4 /D 2 O solution (Norsk Hydro). The purity of MeMA was checked by 1 H-NMR. Reacting solutions were made by dissolving (NH 4 ) 2 Ce(NO 3 ) 6 (Merck, analytical quality) in H 2 SO 4 or D 2 SO 4 and adding a correspond- ing solution of MeMA. The reaction volume was 5 mL. 1 H- and 13 C-NMR spectra were recorded on a Varian XL 300 spectrometer. Sixty four scans were accumulated with a sweep width of 4000 Hz, a repetition time of 3.752 s, and a pulse angle of 17°. All NMR measurements were performed at 23 ( 2 °C. Because the chemical shifts are affected by the presence of paramagnetic Ce(III) ions, peaks were identified by adding the compound in question to the solution and observing a corresponding increase in the peak intensity. Partly overlapping peaks were resolved with the program PEAKFIT. 20 In the experiments reported here we have not excluded atmospheric oxygen from the reaction solution. A test where oxygen was removed by Ar-bubbling through initial reagent solutions and through the reaction mixture during a NMR run showed very similar kinetics of the same intermediates and final products as under aerobic conditions. Although this indicates that oxygen probably has little effect on the Ce(IV)-MeMA reaction, more detailed studies are necessary to look for possible oxygen effects. MeMAOH was synthesized using the method by An et al. 21 The purity of the MeMAOH was tested with 1 H- and 13 C-NMR. The presence of Ce(III) in a formed precipitate was found by electron-emission spectroscopy using a Philipps 515 scanning electron microscope. Simulation calculations were done by using the programs FACSIMILE/CHECKMAT 22 and the FORTRAN subrouine LSODE. 23 Experimental Observations Stoichiometric Excess of MeMA. In a Ce(IV)-MeMA solution the main product is acetic acid (HOAc). Besides HOAc two new intermediates “preMeMAOH” and “prePyr” have been found (Figure 1). After about 1.5 h ([Ce(IV)] 0 ) 0.2 M, ² University of Oslo. ‡ SINTEF Oslo. § Stavanger College. X Abstract published in AdVance ACS Abstracts, March 1, 1997. CH 3 CH(COOH) 2 + 6Ce(IV) + 2H 2 O f CH 3 COOH + 6Ce(III) + 6H + + 2CO 2 (R1) 2327 J. Phys. Chem. A 1997, 101, 2327-2331 S1089-5639(96)03316-6 CCC: $14.00 © 1997 American Chemical Society