Peroxy alkyl radicals, fragmentation

The fragmentation reaction of peroxy alkyl radicals is the key step in the formation of oxiranes during the oxidation of alkenes. This reaction may be understood as an intraradical decomposition of peroxides. The more nucleophile the alkyl radical, the more quickly fragmentation occurs. Thus the rate constant of fragmentation of P-peroxyalkyl radical ... 

A mechanism was proposed in which the perferryl iron-oxeme, resulting from heterolytic cleavage of the 0-0 bond of the iron-peroxy intermediate, abstracts an electron from the 0=0 double bond of the carbonyl group of the aldehyde. The reduced perferryl attacks the 1-carbon of the aldehyde to form a thiyl-iron-hemiacetal diradical. The latter intermediate can fragment to form an alkyl radical and thiyl-iron-formyl radical. The alkyl radical then abstracts the formyl hydrogen to produce the hydrocarbon and C02 (Reed et al 1995). 

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. 

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... 

The formation of this ketone is believed to proceed via internal abstraction of H in the initial peroxy radical (128 cf. p. 328), followed by migration of Me. It may be that the vigorous conditions employed now make a 1,2-alkyl shift feasible, or that the shift of Me may involve fragmentation followed by re-addition, rather than direct migration. 

The rate constant for the reaction of ethyl radicals and oxygen increases with the increasing concentration of inert helium [57]. This indicates the possible participation of the molecules in the process of stabilization of peroxy radicals formed. The excited alkyl peroxy radicals do not fragmentate only to original reactants but rearrange to products through separate pathways [58]. 

This reaction has been put forward to explain the observed fact that the number of chain scissions corresponds to the number of carboxyl groups formed in the oxidation of polyethylene. Activation energy of both processes is 140 kj/mol. The mechanism of such an elementary fragmentation reaction remains however uncertain. The reactions of a chain scission are likely to precede the isomerization of original secondary alkyl peroxy radicals. 

Reactions (5)-(8) and (10) lead to the formation of stable molecules (hydrocarbons and aldehydes). Subsequent reactions of peroxy- (CnH2n+i02) and oxy-radicals (CnH2n+iO) formed in reactions (9) and (11) lead to the formation of oxygenates (alcohols, aldehydes, etc.), carbon oxides, and/or olefins. The fractions of radicals transformed into different final products depend on the reaction conditions (temperature, oxygen pressure) and on the number of carbon atoms in the alkane molecule. For example, the stability of peroxy radicals decreases with increase of the number of carbon atoms in the alkyl fragment, that is why the probability of total oxidation via their subsequent transformations decreases from methane to... 

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