Oxidation of oxalate

Many reactions catalyzed by the addition of simple metal ions involve chelation of the metal. The familiar autocatalysis of the oxidation of oxalate by permanganate results from the chelation of the oxalate and Mn (III) from the permanganate. Oxidation of ascorbic acid [50-81-7] C HgO, is catalyzed by copper (12). The stabilization of preparations containing ascorbic acid by the addition of a chelant appears to be negative catalysis of the oxidation but results from the sequestration of the copper. Many such inhibitions are the result of sequestration. Catalysis by chelation of metal ions with a reactant is usually accomphshed by polarization of the molecule, faciUtation of electron transfer by the metal, or orientation of reactants. 

Here the change in oxidation number per atom of iron is from +2 to +3, or by 1 unit of oxidation, hence the equivalent of iron(II) sulphate is 1 mole. Another important reaction is the oxidation of oxalic acid to carbon dioxide and water ... 

Inverse dependences are seen with reactions with rapid prior equilibria. If they are rapid enough, a single denominator term is found over a wide range of the concentrations (even though this form, too, is the limit of a sum of two terms). One example is the oxidation of oxalic acid by chlorine according to the equations6... 

Early work of Dhar established that oxidation of oxalic acid by chromic acid occurs readily, but some of his kinetic data are unreliable as the substrate itself acted as the source of hydrogen ions. The reaction is first-order in oxidant and is subject to strong manganous ion catalysis (as opposed to the customary retardation), the catalysed reaction being zero-order in chromic acid. This observation is related to those found in the manganous-ion catalysed oxidations of several organic compounds discussed at the end of this section. 

The permanganate oxidation of oxalic acid has been studied exhaustively and has been reviewed by Ladbury and Cullis . It is characterised by an induction period and a sigmoid dependence of rate upon time. Addition of manganous ions eliminates the induction period and produces first-order decay kinetics . Addition of fluoride ions, however, practically eliminates reaction . ... 

Dhar noted that the oxidation of oxalic acid by chromic acid is markedly accelerated on adding manganous ions, the reaction order in Cr(VI) changing from one to zero. Bobtelsky and Glasner ° found the oxidation of bromide ions by chromic acid in aqueous sulphuric acid to follow kinetics... 

The oxidation of oxalic acid by mercuric chloride to give CO2 and mercurous chloride is a classic example of an induced reaction. This reaction is extremely slow unless small quantities of chromic acid and manganous ions are added, whereon facile reduction takes place Addition of permanganate or persulphate and some reducing agents is also effective and the oxidation proceeds readily under photo- or X-irradiation (Eder s reaction). The large quantum yield points to a chain mechanism , which could also function with an inducing oxidant, viz. 

The oxidation of oxalic acid by AuCl4. represents one of the few examples of a kinetic study of Au(IIl) oxidation falling within the present category, viz. 

The kinetics of the Ce(IV) sulphate oxidation of oxalic acid are simple second order although the rate coefficient is inversely proportional to both hydrogen and bisulphate-ion concentrations, and it is also reduced at very high oxalic acid concentrations. Values of the activation energy from 13.4+1.5 (ref. 411) to 16.5+0.4 (ref. 409) kcal.mole have been reported. An intermediate has been detected spectroscopically " this decays in first-order fashion with E = 10.5+0.5 kcal.mole" and with a rate independent of acidity. However, the extent of formation of this complex is reduced as the acidity is increased ", and it appears that a less reactive dioxalato complex is formed at higher substrate concentrations ". 

The oxidation by Mn(lII) chloride involves three complexes and the kinetic data of Taube " are summarised in Table 15. The greater thermal stability of the /m-complex is considered to result from the lowering of the free energy relative to the transition state as compared with bis- and mono-complexes. The study of MnC204 was based on the Mn(III)-catalysed chlorine oxidation of oxalic acid. ... 

The Np02 oxidation of oxalic acid in aqueous perchloric acid provides one of the few examples of redox kinetic studies of Np(VI) . The rate law is... 

The absence of retardation by V(IV) rules out a mechanism analogous to that of Mn(III) oxidation. Mn(II) ions strongly catalyse reaction , altering the kinetics to those observed for the Mn(II)-catalysed oxidation of oxalic acid by V(V) (preceding sub-section) except that the [V(V)] dependence has a Michaelis-Menten, form rather than being first-order. E is reduced from 19.7 to 6.9 kcal.mole , and a similar mechanism is believed to operate. 

It was observed by Gopala Rao and Sastri that the reaction between hydro-quinone and chromic acid leads to the induced oxidation of oxalic acid, glycerol, lactic acid, glucose, citric acid, and malic acid. If the concentrations of the above acceptors are cen times that of that of the hydroquinone inductor, the values of F found are, respectively, 0.51,0.46,0.35,0.27 and 0.17. The numerical values of the induction factor do not permit us to discuss the nature of coupling intermediate. 

Gopala Rao and Venkateswara Rao found that the oxidation of indigo to isatin by chromic acid is accelerated by the presence of oxalic acid, and at the same time the extent of the oxidation of oxalic acid by chromic acid is increased. This observation is an example of mutual induction. 

For oxalate detection, authors proposed a similar detection approach for recognition of oxalate via an immobilized oxalate oxidase/peroxidase couple and dye precursors MBTH (3-methyl-2-benzothiazolinone hydrazone) and DMAB (3-dimethylaminobenzoic acid). The peroxide generated by oxidation of oxalate to CO2 reacted with the dye precursors in a peroxidase-catalyzed reaction to yield an indamine dye with absorption maximum at 590 nm. The concentration of oxalate was correlated with increased absoiption from dye. 

In aerated suspensions no measurable (in the time-frame of typical experiments) reductive dissolution takes place and hematite acts as a photocatalyst for the oxidation of oxalate by 02 ... 

A well known example of a CL reaction is the oxidation of luminol (63) with strong oxidants like permanganate, hypochlorite, and especially hydrogen peroxide in alkaline medium (Fig. 24). A representative example of sensitized CL is the oxidation of oxalates with hydrogen peroxide in the presence of a fluorophore [158],... 

The effect of pH on the periodate oxidation of seven anilines has been investigated. " The kinetics of periodate oxidation of aromatic amines have been studied. " - " Periodate oxidation of oxalic acid is catalysed by Mn(II). " The reaction of ethane-1,2-diol with periodate has been investigated under a variety of conditions and the results compared with those of earlier work and analogous studies on pinacol. " The 104 ion is the primary reactant, with H5IO6 as a secondary reactant the reverse is true for pinacol. The complex observed in previous work is shown not to be an intermediate, but rather to deactivate the reactants. 

A differentiation in the activities of surfaces may likewise be witnessed in a variety of chemical and catalytic processes. Thus we find that charcoal will undergo slow autoxidation when exposed to air it will also cataljrtically accelerate the oxidation of a number of organic subtances such as oxalic acid. By processes of selective poisoning of the charcoal it can be demonstrated quite readily that the portion of the surface which can accelerate the oxidation of, oxalic acid is but a small portion of the surface which is available for say the adsorption of methylene blue and that but a minute fraction of the surface (less than 0 5 % foi a good sugar charcoal) is capable of undergoing autoxidation. 

Ghem. xxxi. 161, 1902) obtained 8 = 095 mm. from measurements on the rate of the electrolytic oxidation of oxalic acid. Fischer (Elektrochemie, p. 60) obtained the following values from data on the limiting current densities in the precipitation of copper at a rotating cathode ... 

As an aside, it should be noted that a large intrinsic ET barrier does not necessarily imply an increase of the bond length upon radical anion formation. A substantial angle deformation also may contribute to the internal intrinsic barrier, AGqj. This was shown in the case of the stepwise dissociative oxidation of oxalate ( O2C-CO2 — 02C-C02 4-e—CO2-F CO2 ), where the data suggest that the oxalate undergoes a substantial increase of... 

In the field of organic ligands the oxidation of oxalic acid ligated to different central atoms provides us with almost any pattern of behavior of oxidized ligands. The oxidation of oxalate by permanganate in the presence of manganous ions (/, 68) proceeds according to Mechanism 10... 

Since the oxidation of oxalic acid involves the transfer of two electrons before forming a stable product, the oxidized intermediate may either break down to give a C2O4-- radical, or it may be long lived enough to undergo a second reaction with the oxidant. The latter mechanism has been demonstrated in the oxidation of chromic oxalate by ceric ions (88). 

If the oxidant is a double equivalent reactant like H2O2 or CI2, the oxidation of oxalate takes place without the formation of any long lived intermediate. This has been demonstrated in the oxidation of Co(NH3)5(C204)+ by H2O2, where C02 and... 

The proposed mechanism agrees completely with that suggested by Allen for the copper-catalyzed oxidation of oxalate by peroxydisulfate (2). 

Secondly, we studied the oxidation of oxalate by cerium(IV) in sulfuric acid. This reaction is analagous to the classical manganese (111) oxalate reaction studied by H. Taube and also by F. Duke. Indeed, it proceeds through an intermediate containing one or more oxalates per cerium. This decomposes to give cerium(III) and other products. The point I want to raise is that we have been able to detect free radicals in this reaction, using EPR experiments and a flow system. So far, it has not been possible to say whether the radical is the oxalate or CO2 radical, both of which have been proposed in other studies. 

The second important class of reducing agents is generated by means of oxidative decarboxylation of carboxylic acids. Electrochemical oxidation of oxalate ion C2042 produces, in aqueous as well as in acetonitrile solutions containing Ru... 

Experimental observations indicate that the oxidation of cobalt (II) to cobalt (III) and the formation of ethylenediamine from N-hydroxyethylethylene-diamine occur simultaneously. This is quite the opposite to what is usually assumed in other instances of transition metal catalysis of organic reactions—for example, the catalytic effect of manganese in the oxidation of oxalic acid (7, 8), of iron in the oxidation of cysteine to cystine (22) and of thioglycolic acid to dithioglycolic acid (5, 23), of copper in the oxidation of pyrocatechol to quinone and in the oxidation of ascorbic acid (29, 30), and of cobalt in the oxidation of aldehydes and unsaturated hydrocarbons (4). In all these reactions the oxidation of the organic molecule occurs by the abstraction of an electron by the oxidized form of the metal ion. 

If compounds already react very fast with ozone, the addition of hydrogen peroxide is nearly ineffective, which was shown by Brunet et al. (1984) in the case of benzaldehyde and phthalic acid. The functional groups on the aromatic ring are relatively reactive towards molecular ozone. The advantage of this process lies in the removal of compounds relatively non-reactive with ozone. It was shown that the oxidation of oxalic acid, which is often an end product in the case of molecular ozone reactions, was significantly accelerated with the addition of hydrogen peroxide. 

Figure 2-11. Formation of H202 via oxidation of oxalate by oxalate oxidase.

Photocatalytic oxidation of 2,4-dichlorophenoxyacetic acid (2,4-D) was investigated (Sun and Pignatello, 1995). In addition to the dominant hydroxyl radical mechanism, Sun and Pignatello found evidence that direct hole oxidation may be the mechanism for the photocatalytic degradation of some organic compounds. The assumed mechanism for this oxidation is H+ acting as an electron-transfer oxidant, while O behaves like a free OH and abstracts H or adds to C=C multiple bonds. Hole oxidation has been used to explain the oxidation of oxalate and trichloroacetate ions, which lack abstractable hydrogens or unsaturated C-C bonds. Whether the reaction... 

Oxidation of oxalic acid with dimethyl-V,V-dichlorohydantoin and dichloroisocya-nuric acid is of first order with respect to the oxidant. The order with respect to the reductant is fractional. The reactions are catalysed by Mn(II). Suitable mechanisms are proposed.129 A mechanism involving synchronous oxidative decarboxylation has been suggested for the oxidation of a-amino acids with l,3-dichloro-5,5-dimethylhydantoin.130 Kinetic parameters have been determined and a mechanism has been proposed for the oxidation of thiadiazole and oxadiazole with trichloroiso-cyanuric acid.131 Oxidation of two phenoxazine dyes, Nile Blue and Meldola Blue, with acidic chlorite and hypochlorous acid is of first order with respect to each of the reductant and chlorite anion. The rate constants and activation parameters for the oxidation have been determined.132... 

The oxidation of oxalate by Pb(IV) does not proceed readily although it is strongly favored thermodynamically. This reaction is catalyzed by Ru(bpy)j2+ with bipy = 2,2 bipyridine according to the following mechanism (13) ... 

In this reaction C02 can be produced at the anode by anodic oxidation of oxalic acid derivatives 369). In particular cases, the reaction can also be carried out in the presence of small amounts of water 370) ... 

The limited reversibility of some electrode reactions might require consideration of consumable (cheap) ionic liquids in the anode compartment for technical applications and commercial electroplating. For example, the electrochemical oxidation of oxalate delivers carbon dioxide, hydride could be oxidized to hydrogen, halides to the halogen or trihalide salt in the case of iodide ionic liquids and so on. Since ionic liquids can readily form biphasic systems an alternative may be to have the anodic reaction in an immiscible solvent. In that case a common ion would be needed that can be transferred from one phase to the other. 

Writing half-reaction for the oxidation of oxalic acid. Balancing (i) the atoms in the order carbon-oxygen-hydrogen and in (ii) Equalising the charge ... 

Wright and his co-workers [141] found mercury plus a small amount of aluminium (ca. 2%) and manganese (ca. 5%) to be a more efficient catalyst than mercury alone. The experiments have shown that mercury increases the reaction rate while manganese, though it has no influence on the principal reaction, assists in the complete oxidation of oxalic acid which would otherwise contaminate the reaction product. 

A scries of researches, concerning the relation between the oxidation of oxalic acid and the electrical conditions have been made. Oettel 1 discovered that the current consumption required for an oxidation process is greater when a smaller current density is used than when a higher density is employed. 

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