Free radicals, atmospheric oxidation

Whilst their molecular structure gives polyimides outstanding thermal properties, under certain conditions some degradation may still occur. At temperatures below 300°C, polyimides are susceptible mainly to hydrolytic types of reaction, whilst above 300°C in air the material decomposes by a free radical initiated oxidation process. In inert atmospheres at high temperatures, pyrolysis occurs yielding carbon monoxide and carbon dioxide as the major decomposition products. The nature of any substrate materials (particularly metals) in contact with a polyimide may have a profound influence on the decomposition rate. 

Saturated fatty acids are very stable but unsaturated acids are susceptible to oxidation the more double bonds the greater the susceptibility. Unsaturated fatty acids, therefore, have to be handled under an atmosphere of inert gas (e.g. nitrogen) and kept away from (photo) oxidants or substances giving rise to free radicals. Anti-oxidant compounds have frequently to be used in the biochemical laboratory just as organisms and cells have to utilize similar compounds to prevent potentially harmful attack of acyl chains in vivo (section 8.11). 

The reaction follows a free radical mechanism and gives a hydroperoxide a compound of the type ROOH Hydroperoxides tend to be unstable and shock sensitive On stand mg they form related peroxidic derivatives which are also prone to violent decomposi tion Air oxidation leads to peroxides within a few days if ethers are even briefly exposed to atmospheric oxygen For this reason one should never use old bottles of dialkyl ethers and extreme care must be exercised m their disposal... 

The most important reactions of trichloroethylene are atmospheric oxidation and degradation by aluminum chloride. Atmospheric oxidation is cataly2ed by free radicals and accelerated with heat and with light, especially ultraviolet. The addition of oxygen leads to intermediates (1) and (2). 

A large proportion (30-90% in tropical waters) is absorbed by bacteria and oxidized to FfjS in order to allow the sulfur to be used by these organisms. Once in the atmosphere, DMS is oxidized by various free radicals such as hydroxyl and nitrate ions. In the presence of low concentrations of NO the hydroxyl reaction... 

The principal components of atmospheric chemical processes are hydrocarbons, oxides of nitrogen, oxides of sulfur, oxygenated hydrocarbons, ozone, and free radical intermediates. Solar radiation plays a crucial role in the generation of free radicals, whereas water vapor and temperature can influence particular chemical pathways. Table 12-4 lists a few of the components of each of these classes. Although more extensive tabulations may be found in "Atmospheric Chemical Compounds" (8), those listed in... 

The oxides are gaseous and do not undergo reactions in the atmosphere that produce aerosol particles. Carbon monoxide is a relatively inert material with its main sinks in the atmosphere via reactions with free radicals, e.g.,... 

When catechol was oxidized with Mn04 under aprotic conditions, a semiquinone radical ion intermediate was involved. For autoxidations (i.e., with atmospheric oxygen) a free-radical mechanism is known to operate. 

Also autooxidation or auto-oxidation. A slow, easily initiated, self-catalyzed reaction, generally by a free-radical mechanism, between a substance and atmospheric oxygen. Initiators of autoxidation include heat, light, catalysts such as metals, and free-radical generators. Davies (1961) defines autoxidation as interaction of a substance with molecular oxygen below 120°C without flame. Possible consequences of autoxidation include pressure buildup by gas evolution, autoignition by heat generation with inadequate heat dissipation, and the formation of peroxides. 

The chief precursors for both oxidant and suspended particulate matter formation in the atmosphere, which are directly emitted into the atmosphere, are nitrogen oxides, hydrocarbons and their derivatives, ammonia, and sulfur dioxide. The measurement of particulate components is discussed in Chapter 2. This section describes briefly the measurement of nitrogen oxides, hydrocarbons, free radicals, and other precursors. 

Photolytic. Plimmer and Hummer (1969) studied the irradiation of chloramben in water (2-4 mg/L) under a 450-W mercury vapor lamp (7, >2,800 A) for 2-20 h. Chloride ion was released and a complex mixture of colored products was observed. It was postulated that amino free radicals reacted with each other via polymerization and oxidation processes. The experiment was repeated except the solution contained sodium bisulfite as an inhibitor under a nitrogen atmosphere. Oxidation did not occur and loss of the 2-chloro substituent gave 3-amino-5-chlorobenzoic acid (Plimmer and Hummer, 1969). 

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