Alkylperoxy radicals, transformations

At elevated temperature alkyl hydroperoxides undergo thermal decomposition to alcohols [Eqs. (9.9)—(9.11)]. This decomposition serves as a major source of free radicals in autoxidation. Because of side reactions, such as p scission of alkylperoxy radicals, this process is difficult to control. Further transformation of the alkoxy... 

In the presence of dioxygen, the carbon radical R- produced by reactions (201) and (202) ar transformed into alkylperoxy radicals ROO, reacts with Co or Mn species to regenerate th Co " or Mn " oxidants, and produce primary oxygenated products (alcohol, carbonyl compounds which can be further oxidized to carboxylic acids. This constitutes the basis of several Industrie processes such as the manganese-catalyzed oxidation of n-alkenes to fatty acids, and the cobal catalyzed oxidation of butane (or naphtha) to acetic acid, cyclohexane to cyclohexanol-on mixture, and methyl aromatic compounds (toluene, xylene) to the corresponding aromatic monc or di-carboxylic acids. ... 

C. Reactions between Alkyl Radicals and Oxygen and Transformations of Alkylperoxy Radicals... 

Alkylperoxy radicals are direct precursors of hydroperoxides the key intermediates causing a degenerate chain-branching thus, transformations of alkylperoxy species determine the overall kinetic features of the process. 

At the very beginning it was found that HALS do not absorb UV light, and so they cannot be classified as UV-absorbers or quenchers of excited states. The first mechanism proposed to explain HALS stabilization activity is known as the Denisov cycle. This mechanism attributes the key role in stabilization to HALS transformation products (nitroxyl radicals and hydroxylamino ethers), and it fits the generally accepted pathway of hydrocarbon degradation through alkyl- and alkylperoxy-radicals (Figure 3) ... 

The phenoxy radicals are relatively long-lived and can undergo self-disproportionation, recombination vith alkylperoxy radicals, or isomeric rearrangement followed by recombination. The resulting compounds may have some stabilization activity. Propionate-type hindered phenols constitute a special class in this category during reaction, the phenol is transformed into phenolic cinnamates, which are known to be efficient chain-breaking antioxidants [Eq. (85)]. 

The aUcyl radicals formed by the abstraction reaction react exclusively with O2 in the atmosphere to form alkylperoxy radicals (pathways (c) (h)), and most of them are then transformed to alkoxy radicals by oxidizing NO to NO2 in the presence of NOx (pathways (d) and (i)). However, some alkylperoxy radicals react with NO by the recombination isomerization reactions (e) and (j), to form alkyl nitrate (see Sect. 5.3.3). Although the yields of 2-butyl and 1-butyl nitrate are not large in the case of 2-butyl and 1-butyl radicals, 0.083 and <0.04, respectively, the production yields of alkyl nitrates increase with the increase of carbon number of alkyl radicals as described in Sect. 5.3.3, and it is as large as >0.2 for the Cg, and C7 secondary n-aUcyl radicals (2-hexyl, 3-hexyl, 2-heptyl, and 3-heptyl radicals) (Lightfoot et al. 1992 Arey et al. 2001). Since these reactions act as termination reactions for the OH chain reaction, they are important as parameters in determining the ozone formation efficiency in model calculations. The production yields of RONO2... 

The last statement of the previous section needs a more precise definition in some important cases, one of which will be discussed here. The central role of reactions between alkyl radicals and molecular oxygen and further transformations of alkylpreoxy species for combustion chemistry has been extensively discussed elsewhere (see, for instance, Miller and Klippenstein, 2001 Slagle et al., 1986, and literature cited therein). Let us mention just some aspects of alkylperoxy chemistry and kinetics, which are of crucial importance for partial oxidation processes. 

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