Hydrogen peroxy radicals

Nitrogen oxides (NOx= N02 and nitrogen monoxide NO) sources are mainly emitting NO into the troposphere. Thai, NO may be converted to N02 by reaction with hydrogen peroxy radical (H02) or with higher peroxy radicals (R02), produced from hydrocarbon oxidation. 

Antioxidants markedly retard the rate of autoxidation throughout the useful life of the polymer. Chain-terminating antioxidants have a reactive —NH or —OH functional group and include compounds such as secondary aryl amines or hindered phenols. They function by transfer of hydrogen to free radicals, principally to peroxy radicals. Butylated hydroxytoluene is a widely used example. 

Oxidation begins with the breakdown of hydroperoxides and the formation of free radicals. These reactive peroxy radicals initiate a chain reaction that propagates the breakdown of hydroperoxides into aldehydes (qv), ketones (qv), alcohols, and hydrocarbons (qv). These breakdown products make an oxidized product organoleptically unacceptable. Antioxidants work by donating a hydrogen atom to the reactive peroxide radical, ending the chain reaction (17). 

Carbon-centered radicals generally react very rapidly with oxygen to generate peroxy radicals (eq. 2). The peroxy radicals can abstract hydrogen from a hydrocarbon molecule to yield a hydroperoxide and a new radical (eq. 3). This new radical can participate in reaction 2 and continue the chain. Reactions 2 and 3 are the propagation steps. Except under oxygen starved conditions, reaction 3 is rate limiting. 

Bimolecular reactions of peroxy radicals are not restricted to identical radicals. When both peroxy radicals are tertiary, reaction 15 is not possible. When an a-hydrogen is present, reaction 15 is generally the more effective competitor and predominates. 

Reaction 36 may occur through a peroxy radical complex with the metal ion (2,25,182). In any event, reaction 34 followed by reaction 36 is the equivalent of a metal ion-cataly2ed hydrogen abstraction by a peroxy radical. 

The reaction rate of molecular oxygen with alkyl radicals to form peroxy radicals (eq. 5) is much higher than the reaction rate of peroxy radicals with a hydrogen atom of the substrate (eq. 6). The rate of the latter depends on the dissociation energies (Table 1) and the steric accessibiUty of the various carbon—hydrogen bonds it is an important factor in determining oxidative stabiUty. 

Radical Scavengers Hydrogen-donating antioxidants (AH), such as hindered phenols and secondary aromatic amines, inhibit oxidation by competing with the organic substrate (RH) for peroxy radicals. This shortens the kinetic chain length of the propagation reactions. 

In the presence of propane (C3H8), the reaction mechanism is initiated by hydrogen abstraction from C3H8 by OH radicals, producing alkyl radicals, which then rapidly react with 02 to form peroxy radicals [88], The peroxy radicals react with NO and oxidize it to N02 ... 

We can also produce direct crosslinks by the action of peroxy radicals, as shown in Fig, 18.8. In this process, we blend an organic peroxide, such as dicumyl peroxide, into molten polyethylene at a temperature below that at which the peroxide decomposes. Once we have formed the molten blend into the required shape, we increase its temperature until the peroxide decomposes into peroxy radicals, as shown in Fig, 18.8 a). The peroxy radicals abstract hydrogen atoms from the polyethylene chains to create free radicals, as shown in Fig. 18.8 b). Crosslinking takes place when two radicals react to form a covalent bond, which is shown in Fig. 18.8 c). 

As mentioned in the introduction, there are conflicting views as to the contributions made to polymer degradation by various initiating species. Among these species, in addition to ketones, hydroperoxides are some of the more important chromophores. As it is known, the photolysis of hydroperoxides yields alkoxy and hydroxy radicals. In polymers, in the presence of oxygen, these radicals lead to the secondary formation of peroxy radicals. The latter in turn are converted by hydrogen abstraction into new hydroperoxides (Scheme I) ... 

Ethylene-propylene and silicone rubbers are crosslinked by compounding with a peroxide such as dicumyl peroxide or di-t-butyl peroxide and then heating the mixture. Peroxide cross-linking involves the formation of polymer radicals via hydrogen abstraction by the peroxy radicals formed from the decomposition of the peroxide. Crosslinks are formed by coupling of the polymer radicals... 

Peroxy radical recombination appears to be the most important source of the electronic excitation energy emitted during hydrocarbon autoxidation. In addition to the above-mentioned energetic considerations, this is clear from the following experimental facts the termination rate for secondary peroxy radicals is 103 times faster than for tertiary peroxy radicals due to their having no a-hydrogen 14> the termination rate constant decreases by 1.9 with a-deuteration 39 40>. 

The active enzyme abstracts a hydrogen atom stereospecifically from the intervening methylene group of a PUFA in a rate-limiting step, with the iron being reduced to Fe(II). The enzyme-alkyl radical complex is then oxidized by molecular oxygen to an enzyme-peroxy radical complex under aerobic conditions, before the electron is transferred from the ferrous atom to the peroxy group. Protonation and dissociation from... 

However in low NOx conditions peroxy radicals primarily react through self and cross peroxy-peroxy reactions to form methyl hydrogen peroxide (CH3OOH) and hydrogen peroxide (H202). H02 is also recycled back to OH through the reaction with 03 (Reaction 9). 

Sulfenic acids undergo hydrogen atom transfer to free radicals extremely readily (Koelewijn and Berger, 1972), rate constants of at least 107 M-1 s-1 being observed for (18a) when R is a peroxy radical. Block and O Connor (1974a) believe the marked antioxidant activity of most thiolsulfinates is... 

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