Decompositions of oxalates

Organic Solids A few organic compounds decompose before melting, mostly nitrogen compounds azides, diazo compounds, and nitramines. The processes are exothermic, classed as explosions, and may follow an autocatalytic law. Temperature ranges of decomposition are mostly 100 to 200°C (212 to 392°F). Only spotty results have been obtained, with no coherent pattern. The decomposition of malonic acid has been measured for both the solid and the supercooled liquid. The first-order specific rates at 126.3°C (259.3°F) were 0.00025/min for solid and 0.00207 for liquid, a ratio of 8 at II0.8°C (23I.4°F), the values were 0.000021 and 0.00047, a ratio of 39. The decomposition of oxalic acid (m.p. I89°C) obeyed a zero-order law at 130 to I70°C (266 to 338°F). 

Decompositions of oxalates containing the strongly electropositive metals yield an oxide product but the more noble elements yield the metal. Discussion of the mechanisms of these reactions and, in particular, whether metal formation necessarily involves the intermediate production of oxide which is subsequently reduced by CO has been extended to consideration of the kinetics of pyrolysis of the mixed oxalates [32]. 

The decomposition of oxalates is also used to prepare ferrites MFe204, which are important as magnetic materials (see Chapter 9). 

This group of initiators has no practical application. Nevertheless it is interesting from the theoretical point of view, due chiefly to the fact that the general equation for the decomposition of oxalates is ... 

Example of Application Large-Scale Actinometry. Neural network modelling was applied to large-scale actinometry in a continuous elliptical photochemical reactor with a concentric annular reaction chamber [2, 3,108, 148], Uranyl oxalate was used as an actinometer, which is based on the photosensitized decomposition of oxalate ions (Eq. 89) [2, 3] the experimental data were taken from the literature [108],... 

Loss of a carbon atom from the precursor need not always result. Barton and Crich have introduced a related procedure based on the chemistry of mixed oxalates, an example of which is provided in Scheme 49.159 Double decarboxylation is involved in the decomposition of oxalate precursors, such as (39). Unfortunately, there are indications that this method may to be limited to tertiary alcohols one secondary alcohol derived mixed oxylate did not fragment completely to the alkyl radical. 

A theoretical study of the thermal isomerization and decomposition of oxalic acid has attempted to account for the predominant formation of C02 and HCOOH from the vapour at 400-430 K.41 Transition-state theory calculations indicate that a bimolecular hydrogen migration from oxygen to carbon of intermediate dihydroxycarbene (formed along with C02) achieved through a hydrogen exchange with a second oxalic acid... 

Example Radiation of wavelength 250 nm was passed through a cell containing 10 ml of a solution which was 0.05 molar in oxalic acid and 0.01 molar in uranyle sulphate. After absorption of 80 joules of radiation energy, the concentration of oxalic acid was reduced to 0.04 molar. Calculate the quantum yield for the photochemical decomposition of oxalic acid at the given wavelength. 

Table 10.14. Comparison of BET areas derived from isotherms of nitrogen and cyclohexane on samples of MgO prepared by thermal decomposition of oxalate (Mikhail et al,. 1971).

The existence of 1,2-dioxetandione (6) is still arguable. It was postulated as an intermediate in the chemiluminescent decomposition of oxalates with basic peroxide (Eq. 9), a reaction first discovered for oxalic chloride. Although its isolation was claimed in this reaction, no characteristic spectral data could be documented. The report that this carbon dioxide dimer was detected by mass spectrometry was shown to be erroneous. Consequently, this elusive species has so far defied synthesis and detection, and it seems urgent and timely to reinvestigate this challenging problem. 

Therefore the final products of the decomposition of oxalic acid by heat are, carbon di-oxide, carbon mon-oxide and water. It will be recalled that, in elementary chemistry, the method of preparing carbon monoxide is by heating oxalic acid with sulphuric acid. The gaseous products are passed through a solution of potassium hydroxide to absorb the carbon di-oxide, and the resulting gas is pure carbon mon-oxide. 

The quantum efficiency is usually determined by using a chemical reaction of known efficiency (actinometer) as a standard. The most commonly used reaction is the uranyl-ion-catalyzed decomposition of oxalic acid into water, carbon dioxide, and carbon monoxide, which has a quantum efficiency of 0.55 and is essentially independent of wavelength between 200 and 400 m. The apparatus consists of a solution of uranyl sulfate (0.01 M) and oxalic acid (0.05 M) maintained at 25° and contained in a cell of sufficient thickness to permit adsorption... 

Correlation between selected values of E, for decompositions of oxalates and mellitates with the enthalpies of oxide formation [108] (some mean values of E, are included). The magnitudes of E, thus appear to be controlled by the strengths of the M-0 bonds. Two trends may be discerned. Reactions where breakdown of the oxalate anion are believed to be catalytically promoted by the metal product, have slightly lower E, values ( ), compared to reactions in which the residual product does not promote decomposition, including the mellitates where carbon deposition inhibits catalysis (+). 

The objective of a mechanistic study is to identify all contributory steps that are involved in the transformation of the reactants into products. There are difficulties of investigating behaviour in the highly localized laminar zones (reaction interfaces) within which the major activity is located. Identifications of rate limiting steps in carboxylate reactions have usually been based upon comparisons between related salts. Many factors must be considered and little is known of the character of bonding (i.e. covalent or ionic) in the anhydrous reactant structures. Acheson and Galwey [108] concluded that the activation step in the decompositions of oxalates and mellitates involved the rupture of the bond between the cation and anionic oxygen (see Figure 16.2.). 

Many studies have been intentionally concerned with relatively simple rate processes to minimize stoichiometric problems. However, even relatively simple reactions do not always give a single product, for example barium azide has been reported to give about 70% BajNj together with the metal [26]. The extensively studied decompositions of oxalates require that the identity of the residual solid be identified, for each constituent cation, as either carbonate, oxide or metal, together with any change of cation valency. The chemistry of oxalate breakdown can, however, be much more complicate as has been shown for the Y, Eu and Yb salts [27]. 

Dollimore [72], specifically referring to the decompositions of oxalates, states that... 

Dollimore, D. Griffiths, D.C. Nicholson, D. The thermal decomposition of oxalates (11). Thermo-gravimetric analysis of various oxalates in air and nitrogen. J. Chem. Soc. 1963, 2617-2623. 

Thermal Decomposition of Oxalate Oxalate was obtained from 1 M CufNO,), and a 10% excess of oxalic acid, washed with water, and calcined at 600°C, washed, and then calcined at 800-1400°C for 6 h. The oxide was washed with water until constant conductivity. 

PZC/IEP of PuOj Obtained by Thermal Decomposition of Oxalate, Ground... 

Thermal Decomposition of Oxalate Precipitated from Nitrate... 

Photoreduction of oxalato-Ti complexes in aqueous oxalic acid gave oxalato-Ti" complexes which catalysed the photosensitized decomposition of oxalic acid to CO2 and CO. Hydrolysis also occurred in the initial stages of irradiation e.p.r. signals characteristic of oxalate radicals were observed. Photochemical reduction of Ti compounds in alcohol solution is characterized by phototransfer of an electron followed by removal of H from the alcohol to give RCHOH (R = H or Me) and rearrangement of the first co-ordination sphere to give different Ti complexes depending on the temperature. ... 

The oxalic-acid dosimeter has some substantial advantages over the ceric-sulfate dosimeter, which made us investigate it more closely (1) it is quite insensitive to impurities, (2) it has very good energy-absorption characteristics, and (3) the system is very stable to normal storage before and after irradiation. The system also has some drawbacks (1) the decomposition of oxalic-acid does not proceed linearly with the absorbed dose, and (2) the chemical yield is not fully independent of the radiation conditions. Other difficulties have been reported and have hampered the practical use. 

NaOH Titoation. The only problem with this method is the quantitative removal of C02, produced by the decomposition of oxalic acid. Purging of the solution with water-saturated air works well. An easier method is to take an aliquot from the irradiated sample, weigh it, dilute with distilled water, and heat the solution for 30 minutes before titration. 

Examination of the data reported by Gilbert and Lindsey (1289) on the pyrogenesis of PAHs from various tobacco constituents would appear to contradict this statement. Gilbert and Lindsey reported the generation of a series of PAHs in the pyrolysate (see Table XXV-10) from the dicarboxy-lic acid oxalic acid [(COOH)j] which obviously has no carbon-hydrogen bond. However, it is known that a major product of the thermal decomposition of oxalic acid is formic acid [H-COOH] via decarboxylation. Formic acid does have the requisite carbon-hydrogen bond. 

Perfluoroalkanes are much less reactive but the classic 1959 work by Tadow in Nature attracted attention by showing that partial reduction to give definite products could occur (equation 26). In this work, perfiuorodecalin (PFD) was reduced to perfluoronaphthalene (PFN) by NaSAr. Since then, several PFD conversions have been reported with transition metal reagents, either via photochemical or thermal routes. " For example, hot sodium oxalate (465 °C) aromatizes perfiuorodecalin to give perfluorotetralin and perfluoronaphthalene (equation 27). Decomposition of solid sodium oxalate at temperatures above 450 °C involves initial formation of elemental carbon that catalyzes the subsequent decomposition of oxalate, leading to the presence of an induction period in the overall decomposition reaction. Prior incipient decomposition of sodium oxalate to carbon and sodium carbonate is necessary for this reagent to effect aromatization. 

It has been mentioned above that heat alone brings about a partial decomposition of oxalic acid in this way. In the presence... 

The decomposition of oxalic acid photosensitized by uranyl ion is a common actino-metric reaction. The light may have any wavelength between 250 and 450 nm. The absorption of the light quantum activates the uranyl ion, which transfers its energy to a molecule of oxalic acid, which then decomposes. The reaction may be written... 

The decomposition of oxalic acid by sulfuric acid or heat is analogous to reaction (a) with formic acid ... 

By comparing the weights involved in the equations for decomposition of oxalates, carbonates, and bicarbonates, the utility of each can be assessed. The best glitter will be achieved with those compounds which contain the least weight per metal ion equivalence, if all other factors are equal. 

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