Radical chain autooxidation

The final step is undoubtedly a typical radical chain autooxidation (p. 274), involving the propagation steps... 

Fig. 36 A Wagnerova Type II hydroperoxy radical chain initiated autooxidation.

Reactions that take place via radical intermediates are occasionally also begun by radical initiators, which are present unintentionally. Examples are the autooxidation of ethers (see later Figure 1.28) or one of the ways in which ozone is decomposed in the upper stratosphere. This decomposition is initiated by, among other things, the fluorochlorohydrocarbons (FCHCs), which have risen up there and form chlorine radicals under the influence of the short-wave UV light from the sun (Figure 1.10). They function as initiating radicals for the decomposition of ozone, which takes place via a radical chain. However, this does not involve a radical substitution reaction. 

The proposed free radical chain mechanism for this reaction is given in Scheme 3. The striking catalytic effect of the metal ions such as Cu2+ and Fe3+ is attributed to their ability to accept an electron from the enamine in the chain initiation step. The autooxidation of the SchifFs bases of a,/ -unsaturated ketones is thought to proceed similarly via the enamine form of the SchifFs bases. 

Organic peroxo compounds are also obtained by autooxidation of ethers, unsaturated hydrocarbons, and other organic materials on exposure to air. A free-radical chain reaction is initiated almost certainly by radicals generated by the interaction of oxygen and traces of metals such as copper, cobalt, or iron. The attack on specific reactive C—H bonds by a radical X" gives first R, then hydroperoxides, which can react further ... 

Each aromatic amine molecule, InH, terminates many free radical chains in autooxidation of alcohols and amines due to the ability of oxyperoxy and aminoperoxy radicals to oxidize InH as well as to reduce In to InH (JO. However, the coefficient of inhibition, f > 2, can be very often observed in oxidizing hydrocarbons too (2 ). Therefore, some reduction of aminyl radicals to InH proceeds in oxidizing hydrocarbons. To ellucidate the ways of such reduction we have studied the products and kinetics of the reactions of diphenylaminyl radical In. ... 

In commercial cumene oxidation processes, the radicals from the thermal decomposition of CHP are the initiators for the free-radical chain reaction. Therefore, the reaction towards CHP in the cumene oxidation is called autooxidation. 

Literature precedent suggests that SC-CO2 is inert to stabilized carbon-centered radicals (e.g., benzyl). For example, McHugh reported the autooxidation of cumene in SC-CO2 via the free radical chain process outlined in Scheme 3 19). This report is significant because it demonstrates for the first time that it is possible to conduct free radical chain reactions in SC-CO2. 

Also potentially hazardous are compounds that undergo autooxidation to form organic hydroperoxides and/or peroxides when exposed to the oxygen in air (see Table 3.12). Especially dangerous are ether bottles that have evaporated to dryness. A peroxide present as a contaminant in a reagent or solvent can be very hazardous and change the course of a planned reaction. Autoxidation of organic materials (solvents and otho" liquids are most frequently of primary concern) proceeds by a free-radical chain mechanism. For the substrate R—H, the chain is initiated by ultraviolet... 

A very striking example is provided by autooxidation. Autooxidation is a radical chain oxidation of alkyl derivatives RH by oxygen to hydroperoxides ROOH, involving the following propagation steps ... 

Furthermore, these hydroperoxides ean start a radical chain reaction, leading to additional aroma-active fat degradation products and enhanced autooxidation. The hydroperoxide degradation can happen spontaneously and may also be catalyzed by enzymes. Finally it should be emphasized that XO contributes to lipid oxidation in milk fat only if an appropriate substrate is present. 

The slow deterioration of rubber under ambient conditions was attributed to atmospheric oxygen by Hofmann in 1861. Subsequent research into this phenomenon was culminated in the pioneering work of Moureu and Dufriasse on antioxygens and by Backstrom on free radical chain theory for autooxidation. 

Autooxidation. Liquid-phase oxidation of hydrocarbons, alcohols, and aldehydes by oxygen produces chemiluminescence in quantum yields of 10 to 10 ° ein/mol (128—130). Although the efficiency is low, the chemiluminescent reaction is important because it provides an easy tool for study of the kinetics and properties of autooxidation reactions including industrially important processes (128,131). The light is derived from combination of peroxyl radicals (132), which are primarily responsible for the propagation and termination of the autooxidation chain reaction. The chemiluminescent termination step for secondary peroxy radicals is as follows ... 

Because the chemiluminescence intensity can be used to monitor the concentration of peroxyl radicals, factors that influence the rate of autooxidation can easily be measured. Included are the rate and activation energy of initiation, rates of chain transfer in cooxidations, the activities of catalysts such as cobalt salts, and the activities of inhibitors (128). 

There are still some non-explained observations. For example, syndiotactic PP was reported [45,46] as being more stable than isotactic polymer. At 140°C, the maximum chemiluminescence intensity was achieved after 2,835 min for syndiotactic PP, while isotactic polymer attained the maximum after only 45 min. Atactic PP was reported to be more stable than the isotactic polymer [46]. An explanation has been offered that the structure of isotactic PP is much more favourable for autooxidation, which proceeds easier via a back-biting mechanism where peroxyl radicals abstract adjacent tertiary hydrogens on the same polymer chain. 

We call this type of reaction autooxidation because it is a an autocatalytic process (the reaction generates radical intermediates that propagate chain reactions) and it is an oxidation that converts alkanes into alkyl peroxides. 

The core of the crystalline region of irradiated PE contains residual free radicals. These diffuse slowly to the interface with the amorphous region, where, in the presence of dissolved oxygen, whose equilibrium concentration is maintained by diffusion, they initiate an autooxidative chain of degradation.89 Postirradiation annealing in an inert atmosphere at a temperature above the alpha-transition temperature (85°C) leads to a rapid mutual reactions of the free radicals and eliminates the problem.90... 

Formation of an enamine radical cation 45 was proposed as the chain initiation step in the autooxidation of enamines and SchifFs bases of a,/ -unsaturated ketones to give unsaturated 1,4-diones37. Pyrrolidine enamine of 10-methyl-A1(9)-octal-2-one (44) reacts with oxygen at room temperature to produce, after acid hydrolysis, 10-methyl-A1 (9)-octalin-2,8-dione (47) in 20% yield. Addition of a catalytic amount of FeCl3, Cu(OAc)2 or CuCl2 causes a pronounced enhancement in the oxidation rate and increases the yield to 80-85% after 1 h. 

Light during processing, handling, and use. Typically are radical scavengers which interrupt the chain propagation steps of polymer autooxidation Absorb UV light to prevent photooxidation Tinuvin 327, 328, 384, 440, etc. (derivatives of... 

The reactions responsible for the deviation are, in Gautron s opinion, of the autooxidation type, i.e., oxidative chain reactions of free-radical intermediates with the initiators being, most likely, peroxides from the solvent or solute. This... 

Moreover, formation of radical transients with S.-.O bonds is kinetically preferred, but on longer time scale they convert into transients with S.-.N bonds in a pH dependent manner. Ultimately transients with S.-.N bonds transform intramolecularly into C-centred radicals located on the C moiety of the peptide backbone. Another type of C-centred radicals located in the side chain of Met-residue, a-(aikylthio)alkyl radicals, are formed via deprotonation of MetS +. C-centred radicals are precursors for peroxyl radicals (ROO ) that might be involved in chain reactions of peptide and/or protein oxidation. Stabilization of MetS +through formation of S.-.O- and S.-.N-bonded radicals might potentially accelerate oxidation and autooxidation processes of Met in peptides and proteins. Considering that methionine sulfoxide, which is the final product coming from all radicals centred on sulphur, is restored by the enzyme methionine sulfoxide reductase into MetS, stabilization of MetS +appears as a protection against an eventual peroxidation chain that would develop from a carbon centred radical. 

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