Malonic acid, periodate oxidation

Thus, if triose reductone is, in fact, the first intermediate in the periodate oxidation of malonaldehyde, the total consumption of periodate per mole of malonaldehyde should be four molar equivalents two moles of formic acid and one mole of carbon dioxide should be formed, in accordance with the sequence proposed by Fleury and his collaborators (22). As in the case of the periodate oxidation of malonic acid (32) the rate determining step should be the hydroxylation step. 

The oxidation of some anhydroaldopento-benzimidazoles13 was found to entail uptake of more than the calculated amount of oxidant per mole. This apparent anomaly was further explored by Huebner, Ames and Bubl,14 and their work culminated in the discovery that periodate, under the usual conditions, oxidizes certain methylene carbon atoms, namely, those activated by two flanking carbonyl groups. This type of oxidation (a-hydrogen oxidation) was simultaneously observed (by Fleury and Courtois16) to occur on the periodate oxidation of malonic acid. A satisfactory reaction was obtained under the following conditions. 

A. M. Zhabotinsky, Periodic process of the oxidation of malonic acid in solution. Study of the kinetics of Belousov s reaction. Biofizika 9, 1306 (1964). 

A typical chemical system is the oxidative decarboxylation of malonic acid catalyzed by cerium ions and bromine, the so-called Zhabotinsky reaction this reaction in a given domain leads to the evolution of sustained oscillations and chemical waves. Furthermore, these states have been observed in a number of enzyme systems. The simplest case is the reaction catalyzed by the enzyme peroxidase. The reaction kinetics display either steady states, bistability, or oscillations. A more complex system is the ubiquitous process of glycolysis catalyzed by a sequence of coordinated enzyme reactions. In a given domain the process readily exhibits continuous oscillations of chemical concentrations and fluxes, which can be recorded by spectroscopic and electrometric techniques. The source of the periodicity is the enzyme phosphofructokinase, which catalyzes the phosphorylation of fructose-6-phosphate by ATP, resulting in the formation of fructose-1,6 biphosphate and ADP. The overall activity of the octameric enzyme is described by an allosteric model with fructose-6-phosphate, ATP, and AMP as controlling ligands. 

Numerous versions of the Belousov-Zhabotinsky system differ by chemical compounds used. The typical reaction involves oxidation of some organic compound by bromate ion (BrOj ) occurring in acid medium with metal catalyst (Ce3+, Mn2+, as well as complexes of Fe2+, Ru2+). As an example, a particular reaction [4] could be mentioned, where an organic reductor is malonic acid CH2(COOH)2 and Ce3+ ions serve as a catalyst. In this reaction a solution changes periodically its colour due to oscillations in Ce3+ concentration. Generally speaking, the reaction consists of two stages. At the first one metal is oxidized... 

Oxidation of arylmethyl ketoximes by phenyliodoso diacetate in glacial acetic acid was second order overall, first order each in substrate and oxidant.145 Iodine allowed the oxidative dimerization of glycine ester enolates with low to moderate diastereoselec-tivity that is consistent with kinetic control.146 Although malonic acid is not oxidized by iodate under acidic conditions, oxidation proceeds in the presence of catalytic ruthenium(III). A mechanism is put forward to account for the observed orders of reaction.147 The rate of periodate oxidation of m-toluidine in acetone-water increases with ionic strength.148... 

A. M. Zhabotinsky, Periodic Kinetics of Oxidation of Malonic Acid in Solution (Study of the Belousov Reaction Kinetics). Biofizika 1964, 9, 306-311 A. N. Zaikin, A. M. Zhabotinsky, Concentration Wave Propagation in Two-dimensional Liquid-phase Self-oscillating System. Nature 1970, 225, 535-537. See, also, a conversation with Anatol M. Zhabotinsky, I. Hargittai, Candid Science III More Conversions with Famous Chemists. (ed. M. Hargittai.) Imperial College Press, London, 2003, pp. 432-447. 

Huebner et al. studied the stoichiometry and approximate rates of oxidation of a number of compounds containing an active methylene group. They found that not all such compounds are oxidised by periodate, and that in general one of the activating groups must be -CHO or -COOH for oxidation to occur. Thus diethyl malonate, ethyl acetoacetate, and cyanoacetic acid are not oxidised. Acetylacetone and other acyclic 1,3-diketones are oxidised very slowly, but cyclic 1,3-diketones are readily oxidised (Wolfrom and Bobbitt ). The first step in the oxidation of a compound containing an active methylene group is hydroxylation, then this is followed by further oxidation, e.g. malonic acid and 1,3-cyclohexanedione react as follows... 

The pH dependence may be due to the reactive periodate species being lOJ, but the mechanism of hydroxylation is uncertain. The exceptions noted above show that enolisation cannot be the sole factor determining whether or not hydroxylation occurs, furthermore some weakly enolised compounds (e.g. malonic acid) are readily oxidised. Bose et suggested a cyclic mechanism, but such a mechanism cannot be extended readily to malonic acid, or, for steric reasons, to 1,3-cyclohexanedione (Sklarz ). Bunton has suggested that hydroxylation may occur by IO4 acting as an electrophilic oxidant transferring oxygen to the substrate, viz. 

Recently, Yadawa and Krishna studied the oxidation of malonic acid by periodate at 30° C in the range pH 2-8. The kinetics are second-order, and the rate reaches a maximum at pH 6. Yadawa and Krishna concluded that the malonate dianion is reactive, but their data are insufficient to allow details of the mechanism to be inferred. 

IIIC) 1964-1 Zhabotinskii, A. M. Periodic Course of Oxidation of Malonic Acid in Solution, (An Investigation of the Kinetics of the Reaction of Belousov), Biophysics, vol. 9, 329-335, (Biofizika, 9, 306-311)... 

Treindl and Fabian (1980) studied the effect of oxygen on parameters of the B-Z reaction in the presence of Ce(IV)/Ce(III) redox catalyst and malonic acid, citric add or 2,4-pentanedione as substrates. They concluded that the effect of oxygen was in its catalytic influence on the oxidation of the substrate with Ce(IV) ion. Under this influence, the number of oscillations as well as the induction period and the first oscillation period diminished. 

A reaction may be periodic if its network provides for restoration of a reactant or intermediate that has been depleted, while conversion of main reactants to products continues. Periodic behavior often results from competition of two or more contending mechanisms. Predator-prey fluctuations in ecology (Lotka-Volterra mechanisms) provide an easily visualized example. The Belousov-Zhabotinsky reaction—catalyzed oxidation of malonic acid by bromate—involves a similar competition between two pathways. 

As a certain concentration of CHBr (C02H)2 is needed for reaction 9 to occur long induction period for oscillations is expected, a phenomenon, which is also observed experimentally. During this induction period, the concentration of Br" is small and mechanism II dominates due to the slow conversion of Ce4+ into Ce3+ and the accumulation of brommalonic acid (reaction 8). Step 9 (8.71) results in the change of the blue color of solution to red resetting the chemical clock for the next oscillation. In fact, the oxidized form of the catalyst can also react directly with malonic acid, so there may be less than one bromide ion per cerium (III) ion produced. 

Given the importance of the BZ reaction in nonlinear chemical dynamics, it is not surprising that polymers and polymerizations would be coupled to it. Pojman et al. studied the BZ reaction to which acrylonitrile was added and showed that the polyacrylonitrile was produced periodically in phase with the oscillations (41). Given that radicals are produced periodically from the oxidation of malonic acid by ceric ion, it seemed reasonable to assume the periodic appearance of polymer was caused by periodic initiation. However, Washington et al. showed that periodic termination by bromine dioxide caused the periodic polymerization (42). 

Zhabotinskii, A. Mo (1964). "Periodic course of oxidation of malonic acid in solution (investigation of the kinetics of the reaction of Belousov)." Biophysics 329-335. 

In this section we will focus our attention on the chemical mechanism of the Belousov-Zhabotinsky (B-Zh) reaction. As mentioned above, the B-Zh reaction is one of the most extensively studied among the oscillating chemical reactions [15-47], The classical B-Zh oscillating reaction system presents the oxidation of malonic acid (MA) by bromate ion (potassium bromate) in an acid medium (sulfuric acid), catalyzed by the single-electron Ce /Ce redox couple. The oxidized form of the catalyst, the Ce" ion, is yellow colored, and the reduced one, Ce, is colorless. The periodicity of the B-Zh reaction is detected as a periodic yellow coloring of the solution. 

We now wish to test the above gel-reaction-diflfusion approach (Eqs 9.7-9.10) using the BZ reaction, which consists in the oxidation of malonic acid by bromate ions in acidic medium. The reaction proceeds only when catalyzed by a suitable metal ion. In the experiments, the chosen catalyst is the ruthenium tris(2,2 -bipyridine) that intervenes through its oxydo-reduction couple (Ru(bpy)3 /Ru(bpy)j ). In a batch reactor or a CSTR, this reaction exhibits well-documented periodic oscillations of concentrations in some region of the parameter space. 

While concentration oscillations have been observed in a variety of systems, only two homogeneous chemical oscillators have been unambiguously established. When malonic acid (HMa) is oxidized by BrOg- in sulfuric acid solution using a Ce(IV)/Ce(III) couple as catalyst, Belousov observed that the tCe(IV)]/[Ce(III)] ratio oscillated periodically. Further studies by Zhabotinskii demonstrated oscillatory behavior when Ce(IV)/Ce(III) was replaced by Fe(III)/Fe(II) or Mn(III)/Mn(II) and... 

Without doubt the main interest here lies in the oscillating reactions involving bromate (the Belousov-Zhabotinskii reaction), i.e., the catalytic oxidative bromination with acidic bromate of (usually) aliphatic polycarboxylic acids or polyketones. The best-studied system involves malonic acid and a catalyst, usually cerium. The oxidation of Ce(III) by acidic bromate is inhibited periodically by bromide whenever the concentration of bromide exceeds a critical value. 

---