Oxalic acid, oxidative decarboxylation

Oxalic acid does not form intramolecular hydrogen bonds. The decarboxylation of oxalic acid occurs by the direct reaction of the peroxyl radical with the carboxylic group. The rate constants of the peroxyl radicals of the oxidized cyclohexanol in co-oxidation with oxalic acid was found to be pi2 = 7.4 x 107exp(—60.2/RT) L mol-1 s-1 in cyclohexanol at 348-368 K [106],... 

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... 

In a series of transition metal oxide semiconductor powders, photochemical activity in the decarboxylation of oxalic acid was controlled by surface properties and the presence of recombination centers, which in turn depended on the preparation method Similar effects have also been noted in the photodecarboxylation of pyruvic acid and formic acid... 

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... 

At higher temperatures, carbon dioxide, formic acid, oxalic acid, glycolic acid, hydroxymalonic acid, glyceric acid, and other acids were shown to be formed. The formation of carbon dioxide is ascribed to decarboxylation of 38 oxalic acid and D-erythronic acid arise from cleavage of the C-2-C-3 bond compound 39 is cleaved to glyoxylic acid plus D-erythronic acid. Compound 40 is oxidized further to D-g7ycero-2,3-pentodiulosonic acid and is subsequently cleaved to oxalic and glyceric acids. 

The parent compound of this series, pyrazine, was first prepared in trace amounts by Wolff (30) by heating aminoacetaldehyde diethyl acetal [H2NCH2CH(OEt)2] with anhydrous oxalic acid at 110-190°C, and later in better yield by heating the mercuric or platinic chloride double salts (of the aminoacetaldehyde acetal) with hydrochloric acid (31) it was also obtained from aminoacetaldehyde with mercuric chloride in sodium hydroxide (23). Wolff in 1893 (22) also prepared pyrazine by decarboxylation of the tetracarboxylic acid, obtained by oxidation of tetramethyl-pyrazine and Stoehr (32) prepared it by the distillation of piperazine with lime and zinc dust. Brandes and Stoehr (33) in 1896 described the preparation of pyrazine by heating glucose with 25% aqueous ammonia at 100°C. 

Formic, oxalic and malonic acids were never detected during these oxidation experiments. It was verified that formic and oxalic acids were oxidized so rapidly, that they could not accumulate in the reaction mixture and be detected by HPLC. Malonic acid was decarboxylated very rapidly to yield acetic acid. On the other hand, as expected, acetic acid and propionic acid were much less reactive. The initial rates of TOC removal were 13 and 19 mol h mol, respectively, compared to 61 for succinic acid. After 6 h, TOC abatement was 65.9 and 68.5%, respectively. 

Other radical sources involve hydrogen abstraction from alkyl formates [Eq. (25)] and oxidation decarboxylation of semiesters of oxalic acid [Eq. (26)]... 

The dependence of the initial rate of oxidative decarboxylation of oxalic acid on the total acid concentration and the content of undissociated species of mono- and dianion in aqueous solutions in the presence of polymeric cobalt phthalocyanine has been examined [160]. Whereas the monoanion, [HC2O.C], of oxalic acid is shown to enter the reaction, the undissociated species does not. Furthermore, [C204 ] dianion retards the process. This is attributed to the fact that in catalyst A, active particles of CoPc(NaS03)4 are located mainly on the species surface while in catalyst B they are distributed within the species and are hardly accessible. 

Oxalic acid poses a problem to both leafy plants and vertebrates because these organisms cannot catabolize it [108]. Although accumulation of oxalate leads to stress in plants, in vertebrates this molecule can be metaboHzed by bacteria present in the intestinal tract [109]. Oxalate can be catabolized in different ways by oxidation, by decarboxylation of oxalyl-coenzyme A or by direct decarboxylation. Both oxidation and decarboxylation of oxalate are catalyzed by Mn-containing enzymes. Here we will discuss the oxalate decarboxylate reaction that produces formate and CO2. The crystal structure of oxalate oxidase from Bacillus subtilis... 

Oxalic and malonic acids, as well as a-hydroxy acids, easily react with cerium(IV) salts (Sheldon and Kochi, 1968). Simple alkanoic acids are much more resistant to attack by cerium(IV) salts. However, silver(I) salts catalyze the thermal decarboxylation of alkanoic acids by ammonium hexanitratocerate(IV) (Nagori et al., 1981). Cerium(IV) carboxylates can be decomposed by either a thermal or a photochemical reaction (Sheldon and Kochi, 1968). Alkyl radicals are released by the decarboxylation reaction, which yields alkanes, alkenes, esters and carbon dioxide. The oxidation of substituted benzilic acids by cerium(IV) salts affords the corresponding benzilic acids in quantitative yield (scheme 19) (Hanna and Sarac, 1977). Trahanovsky and coworkers reported that phenylacetic acid is decarboxylated by reaction with ammonium hexanitratocerate(IV) in aqueous acetonitrile containing nitric acid (Trahanovsky et al., 1974). The reaction products are benzyl alcohol, benzaldehyde, benzyl nitrate and carbon dioxide. The reaction is also applicable to substituted phenylacetic acids. The decarboxylation is a one-electron process and radicals are formed as intermediates. The rate-determining step is the decomposition of the phenylacetic acid/cerium(IV) complex into a benzyl radical and carbon dioxide. 

Although many hydrothermal experiments have been undertaken in an effort to delimit the probable rates of acetic acid and oxalic acid decarboxylation in natural settings, the results have merely been sufficient to define the nature of various factors which affect the decarboxylation of the n-C2 to n-C4 mono- and dicarboxylic acids. The subjects of the kinetics of oxidation and polymerization/condensation of these species remain virtually untouched. It will not be possible to speculate with certainty about the stability of these acids in basin brines until kinetic information is acquired allowing comparison of decarboxylation, oxidation, and condensation of the acids with the relevant mineral transformation, i.e., the rate at which mineral oxygen buffers can be expected to adjust/o2//h2- The following is a summary of some of the experiments that are necessary to provide such information. Hindsight applied to the previous experimental kinetic studies also provides suggestions for improved approaches to experimental and analytical procedures. 

Many oxidations (e.g., of oxalate) by the peroxodisulfate ion are catalyzed by Ag+ ion, and the kinetics are best interpreted by assuming initial oxidation to Ag2+, which is then reduced by the substrate. Decarboxylation of carboxylic acids are also promoted by Ag11 complexes, such as (18-I-IX) and others. 

Over-oxidation of the hydroxypyruvic acid to oxalic and glycolic acids reduces the selectivity in all cases and there appears to be a dependence on pH since increasing quantities of these by-products are evolved at higher pH. This may be because the rate of decarboxylation of hydroxypyruvic acid increases with pH and/or that the rate of the main reaction decreases with an increase in pH. At pH 2, levels of these by-products are negligible. 

---