Primary decomposition processes

Three primary processes have to be considered in the photolysis of acetaldehyde, viz. 

Step I was first suggested by Leighton and Blacet and has been confirmed several times since. Flash photolytic studies also support this primary process. The occurrence of step II is demonstrated by the effect of radical inhibitors which do not suppress the formation of CH4 and CO beyond a certain limit. 

It has been observed - that a considerable amount of acetone is formed at very high light intensities (lO -lO quanta.P. sec ) under these conditions acetyl radicals are a product of primary process III and not of the secondary reactions. There was no acetyl iodide detectable in the presence of iodine at medium and low light intensities -Primary process III is likely to play only a minor part at these intensities. 

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In solution, chlorine dioxide decomposes very slowly at ambient temperatures in the dark. The primary decomposition process is hydrolysis of chlorine dioxide into chlorite and chlorate ions. The hydrolysis rate is a function of the concentration of hydroxyl ions and temperature, proceeding rapidly at solution pH values above 10 ... 

Clearly, the major primary decomposition processes are loss of SO and loss of CO, both of which require prior formation of a C—0 bond. This virtually demands isomerization to an internal sulphinate ester (7),... 

Acetic Acid.—Besides hydrogen, carbon mon- and dioxides, Maquenne 3 also obtained methane, ethylene, and acetylene. With increasing pressure he found an increase in hydrogen and carbon monoxide, a decrease in-carbonic acid and hydrocarbons. Hemptinne observed similar results with his experimental arrangement. He accepts the following as the primary decomposition process, corresponding to that of the alcohols ... 

The other primary decomposition processes for the excited propylene have all been recently found in the mercury-photosensitized decom-f)osition of this molecule. It is interesting to note that in both cases, while vinyl radicals are postulated (and in the work of Avrahmi and Kebarle the decomposition of these radicals amounts to 11% of the primary process), no butarl,3-diene was detected. This indicates the rapid destruction of vinyl radicals, either by abstraction of hydrogen or by addition to propylene, the former reaction leading to ethylene, which was in fact detected in the system. 

Decay products of the principal radionuclides used in tracer technology (see Table 1) are not themselves radioactive. Therefore, the primary decomposition events of isotopes in molecules labeled with only one radionuclide / molecule result in unlabeled impurities at a rate proportional to the half-life of the isotope. Eor and H, impurities arising from the decay process are in relatively small amounts. Eor the shorter half-life isotopes the relative amounts of these impurities caused by primary decomposition are larger, but usually not problematic because they are not radioactive and do not interfere with the application of the tracer compounds. Eor multilabeled tritiated compounds the rate of accumulation of labeled impurities owing to tritium decay can be significant. This increases with the number of radioactive atoms per molecule. 

Organic peroxides and hydroperoxides decompose in part by a self-induced radical chain mechanism whereby radicals released in spontaneous decomposition attack other molecules of the peroxide.The attacking radical combines with one part of the peroxide molecule and simultaneously releases another radical. The net result is the wastage of a molecule of peroxide since the number of primary radicals available for initiation is unchanged. The velocity constant ka we require refers to the spontaneous decomposition only and not to the total decomposition rate which includes the contribution of the chain, or induced, decomposition. Induced decomposition usually is indicated by deviation of the decomposition process from first-order kinetics and by a dependence of the rate on the solvent, especially when it consists of a polymerizable monomer. The constant kd may be separately evaluated through kinetic measurements carried out in the presence of inhibitors which destroy the radical chain carriers. The aliphatic azo-bis-nitriles offer a real advantage over benzoyl peroxide in that they are not susceptible to induced decomposition. 

Pyrolysis. In this context it is relevant to consider initially the effect of hydrogen contents on tar yields during pyrolysis (carbonization). This is particularly so, since, in all coal conversion processes little happens until the coal is at a temperature above that where active thermal decomposition normally sets in. In other words, all coal conversion processes may be regarded as pyrolysis under a variety of conditions which determine the nature of the primary decomposition and the reactions which follow. 

The primary decompositions occurring by parallel first-order processes yield cyclohexene, butadiene plus ethylene, and four cyclopropyl-propenes. The cyclopropylpropenes formed initially decompose to yield various Ce dienes at rates comparable with those of the primary reactions, and this leads to a complex reaction mixture that contains at least seventeen products. 

The primary step in the decomposition of ethyl nitrate (C2H5ONO2) is again the breaking of the C2H5O-NO2 bond, and the decomposition rate obeys a first-order law. The decomposition process of ethylene glycol dinitrate can be written as [29]... 

Photochemical decomposition of cyclic compounds proceeds by two different primary photolytic processes. The molecule may decompose either by direct fragmentation or by intramolecular rearrangement. Thietanes have not been investigated in as much detail as the thiophenes. 

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