Alkoxy radicals from alkyl hydroperoxides

Alkoxy radicals from hydroperoxides can undergo a -scission reaction (eq. 2) to yield an alkyl radical and a ketone. The higher stabiUty of the generated alkyl radical compared to that of the parent alkoxy radical provides the driving force for this reaction, and the R group involved is the one that forms the most stable alkyl radical. 

Figure 17.2 Lipid peroxidation scheme. LH, a polyunsaturated fatty acid LOOM, lipid hydroperoxide LOH, lipid alcohol L, lipid radical LOO, lipid hydroperoxyl radical LO, lipid alkoxyl radical. Initiation the LH hydrogen is abstracted by reactive oxygen (e.g. lipid alkyl radical, lipid alkoxy radical, lipid hydroperoxyl radical, hydroxy radical, etc.) to produce a new lipid alkyl radical, L. Propagation the lipid alkyl, alkoxyl or hydroperoxyl radical abstracts hydrogen from the neighbouring LH to generate a new L radical.

Competition between metal ion-induced and radical-induced decompositions of alkyl hydroperoxides is affected by several factors. First, the competition is influenced by the relative concentrations of the metal complex and the hydroperoxide. At high concentrations of the hydroperoxide relative to the metal complex, alkoxy radicals will compete effectively with the metal complex for the hydroperoxide. Competition is also influenced by the nature of the solvent (see above). Contribution from the metal-induced reaction is expected to predominate at low hydroperoxide concentrations and in reactive solvents. The contribution from the metal-induced decomposition to the overall reaction is readily determined by carrying out the reaction in the presence of free radical inhibitors, such as phenols, that trap the alkoxy radicals and, hence, prevent radical-induced decomposition.129,1303 Thus, Kamiya etal.129 showed that the initial rate of the cobalt-catalyzed decomposition of tetralin hydroperoxide, when corrected for the contribution from radical-induced decomposition by the... 

Two different cases may occur. If this radical is formed in a succession of styrene units (1), it reacts in the same way as in PS. If it is formed on a styrene unit linked to an acrylonitrile unit (2), three reaction pathways may be envisaged. The alkoxy radical resulting from the decomposition of the hydroperoxide formed on this polystyryl radical may react by 3-scission. Scissions (a) and (b) yield chain ketones, acetophenone end-groups and phenyl and alkyl radicals as previously observed in the case of PS photooxidation mechanism. Scission (c) leads to the formation of an aromatic ketone and an alkyl radical. This alkyl radical may be the precursor of acrylonitrile units (identified by IR spectroscopy at 2220 cm-1), or may react directly with oxygen and after several reactions generates acid groups, or finally this radical may isomerize to a more... 

The decomposition may occur either uni- or bimolecularly to form alkoxy and peroxy radicals. These oxy radicals abstract a labile hydrogen from the hydrocarbon to produce either an alcohol or a hydroperoxide. The alkyl radical thus formed readily adds oxygen to reform a peroxy radical, and the process continues. When the autoxidation occurs In the absence of an antioxidant, the termination of the kinetic chain occurs chiefly by the combination of two peroxy radicals. This termination Is a source of alkoxy radicals which can undergo chain scission and give rise to volatile products and carbonyl groups. 

In thermal oxidation, initiation (1) results Irom the thermal dissociation of chemical bonds that may arise Irom intrinsically weak links formed as by-products of the polymerization reaction (e.g. head-to-head links) or impurities formed in the polymerization reactor such as hydroperoxides, POOH, or in-chain peroxides as occur in polystyrene from oxygen scavenging. Reaction (1 ) shows that POOH may produce peroxy and alkoxy radicals that may subsequently form alkyl radicals via reaction (3). 

The use of additives to control lubricant degradation requires a focus on alkyl radicals (R), alkylperoxy radicals (ROO) and hydroperoxides (ROOH). Primary alkoxy radicals (RCH2O) and hydroxy radicals (HO) rapidly abstract hydrogen from the substrate. It is therefore very unlikely that they can be deactivated by natural or synthetic antioxidants. In practice, three additive types have proven to be successful in controlling the degradation of lubricating oils ... 

On the other hand, the persistence of the dialkyl peroxide indicates that it must not derive from a mechanism involving such oxidants. Indeed such dialkyl peroxides are readily formed by metal-catalyzed decomposition of alkyl hydroperoxides and involve alkoxy and alkylperoxy radicals. The mechanism for dialkyl peroxide formation shown below is adapted for FeTPA from previously proposed schemes ... 

Polymer is degraded by heat, energy, UV or residues of catalyst and generates alkyl radicals. This alkyl radical reacts with oxygen and form peroxy radicals. These peroxy radicals abstract hydrogen from other polymer and forms alkyl radicals and hydroperoxide. The decomposition of hydroperoxide to alkoxy and hydroxyl radicals induces additional decomposition of the polymer chain. In order to stop the radical chain reaction of degradation, stabilisers such as phenolic antioxidant, phosphites, thioether and hindered amine light stabilisers (HALS) are added. 

In the case of aryl analogs, products may be derived either from the carboxyl radical or the radical formed by decarboxylation. Alkyl hydroperoxides give alkoxy radicals and the hydroxyl radical. t-Butyl hydroperoxide is easily available, and has often been used as a radical source. Detailed studies have been reported on the mechanism of the decomposition, which is somewhat more complicated than simple unimolecular decomposition." Dialkyl peroxides give two alkoxy radicals ... 

In the solid phase, the recombination of radicals in the cage is highly efficient, whereas the escape of NO and RO radicals from the cage is quite low. Alkoxy radicals RO generated in reactions (Equation 3.77) decompose or enter into substitution reactions with neighbouring macromolecules to form chain R and end R alkyl macroradicals as well as low-molecular alkyl radicals r. The appearance of the latter radicals in the reaction of PP hydroperoxide with NO was verified in [66] ... 

The alkoxy radical is usually described as a typical product of the thermal decomposition of hydroperoxides. Nevertheless, in the post-irradiation oxidation process at room temperature, it cannot originate from this reaction because all the formed products follow a kinetic similar to that of ketone formation [21]. The reaction between the alkyl macroradical and the peroxy macroradical forms peroxides (Scheme 9, Reaction 20), but we can also hypothesize Reaction 21, Scheme 9. Literature studies demonstrate that the alkoxy radical can give beta-scission (Reaction 28) forming a primary alkyl radical and CO, a product that is found during the irradiation of PE (Scheme 10, Reaction 29) [24]. The activation energy of this reaction is around 50kJ/mole. 

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