Oxalic acid, oxidation stability

In acidic solution, the degradation results in the formation of furfural, furfuryl alcohol, 2-furoic acid, 3-hydroxyfurfural, furoin, 2-methyl-3,8-dihydroxychroman, ethylglyoxal, and several condensation products (36). Many metals, especially copper, cataly2e the oxidation of L-ascorbic acid. Oxalic acid and copper form a chelate complex which prevents the ascorbic acid-copper-complex formation and therefore oxalic acid inhibits effectively the oxidation of L-ascorbic acid. L-Ascorbic acid can also be stabilized with metaphosphoric acid, amino acids, 8-hydroxyquinoline, glycols, sugars, and trichloracetic acid (38). Another catalytic reaction which accounts for loss of L-ascorbic acid occurs with enzymes, eg, L-ascorbic acid oxidase, a copper protein-containing enzyme. 

In context with the formation of peraminosubstituted 1,4,5,8-tetraazaful-valenes of type 85 it must be mentioned that the bis-vinylogous compounds 94 can be easily prepared by reaction of acetamidine with bisimidoylchlo-rides derived from oxalic acid (96S1302). In the course of a complex reaction a cyclic ketene aminal was produced it immediately underwent an oxidative dimerization to yield deeply colored TAFs. Tlieir high chemical stability can be compared with that of indigoid dyes and manifests itself, for example, by the fact that they are soluble in hot concentrated sulfuric acid without decomposition. Tire same type of fulvalene is also available by cy-... 

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. ... 

Except for phthalic acid, all other carboxylic acids studied induce considerable increases in the light compared to the dark values (the relatively high rate of iron oxide dissolution induced by oxalic acid has been extensively studied (5,8). Phthalic acid actually appears to stabilize the iron oxide against photodissolution despite the solution phase complex exhibiting some photoactivity. 

MnPc-SAMs have been employed for the detection of thiocyanate [86] on SAMs formed by coordination of MPc complexes to preformed SAMs. On MnPc-4-MPy-SAM the oxidation of SCN- occurred at 0.50 V (Table 3). The stability of the electrode was less on MnPc compared to CoPc preformed SAM. Analysis of SCN-in the presence of possible interfering species (uric acid, oxalic acid, and ascorbic acid) in biological samples revealed insignificant effects from these compounds [86], Thus, SCN- can be analyzed in the presence of ascorbic acid. An analysis of the urine of smokers and nonsmokers showed clearly that the SAM electrode could be used to differentiate between the two groups. 

The above discussion clearly revealed that the covalently bound oxalic acid dinitrate ester is the most promising candidate as a high oxidizer and potential replacement for AP in this series. In order to evaluate its thermodynamic and kinetic stability we calculated the decomposition into C02 and N02. The reaction enthalpy of AH = 56.5 kcal mol1 clearly indicates that oxalic acid dinitrate ester is (as expected) thermodynamically unstable with respect to its decomposition into C02 and N02. 

The distinctive stability of the ginkgolide core was also encountered by Nakanishi et al. For example, treatment of GA with 50% NaOH at 160 °C for 30 minutes resulted in the loss of two carbons as oxalic acid to give a hemiacetal bis-nor GA. Treatment of GA with sodium dichromate in concentrated sulfuric acid simply oxidized the hydroxyl lactone, which subsequently undergoes a photocycli-zation to give photodehydro-GA. 

In an excellent work by Reuter et al. (1983), serious shortcomings in the interpretation of previous oxidation data were demonstrated. They established that mild oxidation of stream humic substances produced large quantities of oxalic acid indicating stream humic substances to be predominantly aliphatic in nature, whereas strong or severe oxidation produced structures stabilized by a 4n -I- 2 overlapping 7r-electron system, attributed mainly to a series of benzene carboxylic acids. Structural interpretation based on the products of severe oxidation would lead one to falsely conclude a greater degree of aromaticity in stream humic substances than actually exists in the unoxidized material. 

Chelation of metals by certain compounds decreases their prooxidant effect by reducing then-redox potential and stabilizing the oxidizing form of the metal. A few natural acids (citric, phosphoric, tartaric, oxalic, etc.) and ethylenediamintetraacetic acid (EDTA) can chelate metals and thereby increase oxidation stability in oil model systems. ... 

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