Peroxy alkyl radicals transfer reaction

On the other hand, because of their inertness, alkanes probably oxidize by a radical mechanism52 shown in Fig. 6.12. If this is so, then it is uncertain how methanol would increase the rate of this reaction. Perhaps by complexing to Ti as in Fig. 6.8, it facilitates homolytic cleavage of the peroxy species or transfer of the OH radical to the alkyl radical. 

Mention has already been made of the relatively small reactivity of allyl peroxy radicals compared with other alkyl peroxy radicals. Jost (88, 96) has reasoned that paraffins react by a small number of long chains, whereas olefins oxidize by a large number of short chains. Olefins are thus attacked more readily than paraffins but form less reactive allyl radicals. In addition, during oxidation chain transfer occurs in which alkyl radicals are replaced by allyl radicals. Shorter chains would then be expected. Comparison of the precombustion products of iso-octane and diisobutylene (154) indicates that marked self-inhibition of reaction chains was occurring in the latter case. 

The P-scission process with peroxy radicals that are highly resonance stabilized, such as with triphenyl methyl peroxyls may be observed even at a markedly lower temperature. The regeneration of alkyl radicals in a reaction system may however proceed through numerous fragmentation and transfer reactions which makes the correct determination of ceiling temperature of peroxy radicals backward decomposition more complicated. 

Phenolic inhibitors in this case exhibit a so called oxygen synergy. This reaction is much more rapid than the transfer reaction on an alkyl radical. Moreover, the slightest trace of oxygen will very rapidly form a peroxy radical from an alkyl radical. 

Mechanism. Degradation of a-linolenic acid (a-lin) as proposed by (29,50,21) is demonstrated in Figure 6. The initial step is a hydrogen abstraction from an a-linolenic molecule by a radical species that was formed as a result of herbicidal action. In the following radical-chain reaction the w-3 alkyl peroxide is formed via the peroxy radical. Subsequently, this peroxide is decomposed in a Fenton-type reaction to the oj-3 alkoxy radical in the presence of transition metals that can undergo one-electron transfer reaction, e.g., Cu(I/II), Fe(Il/III), Ti(IIl/IV), or Ce(III/IV). The w-3 alkoxy radical can split by 8-sclssion to an unsaturated aldehyde and the ethyl radical. The latter is either oxidized to ethylene or reduced to ethane. 

Jovanovic SV, Jankovic I, Josimovic L. Electron-transfer reactions of alkyl peroxy radicals. J Am Chem Soc. 1992 114 9018-9021. 

Reactions (2b) and (3b) formally involve H-atom transfer reactions from the —OH and —CHO groups, respectively, to the peroxy group and fragmentation of the hydroperoxide so formed. This type of chemistry is common in reactions of alkyl radicals in hydrocarbon combustion, but at temperatures in the 700-800 K range (Walker and Morley, 1997) this chemistry is responsible for the phenomenon... 

Taken together, the various reactions and interconversions of these manganese porphyrin complexes have allowed the examination of each step in the activation of molecular oxygen by the mechanism suggested for P-450. Detailed mechanistic studies of the 0-0 bond cleavage event in 29 by kinetics, substituent effects, and product analysis showed that the reaction proceeds via heterolysis to produce 27 when acid is present, whereas homolysis is predominant in the absence of acid but in the presence of hydroxide ion (95). Under basic conditions, homo-lytic cleavage of the 0-0 bond of 29 forms Mn (=0)TMP (28) and an acyl-oxyl radical. Thus, when an alkyl peroxy acid is employed, decarboxylation competes with electron transfer, as shown in Scheme IX, to afford a mixture of 27 and 28. Yuan and Bruice have proposed a similar heterolysis mechanism based on the kinetic analysis for the reaction of mCPBA with catalytic amounts ofMn TPP(/(W). 

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