Alkylperoxy radical isomerization theory

The alkylperoxy radical isomerization theory was developed primarily as a result of studies of the oxidation of alkanes of carbon number greater than four during the later stages of the reaction, namely just prior to Emd at the cool flame. 

The main chain-propagating cycle in this theory may be summarized in general terms by the following reactions 

In contrast to the alkene theory the predominant mode of oxidation of the alkyl radicals is by oxygen addition and the alkylperoxy radical so formed then undergoes homogeneous intramolecular rearrangement (reaction (14)). Decomposition of the rearranged radical (reaction (16)) usually leads to a hydroxyl radical and stable products which include O-heterocycles, carbonyl compounds and alcohols with rearranged carbon skeletons relative to the fuel and alkenes. The chain-cycle is then completed by unselective attack on the fuel by the hydroxyl radical (reaction (12)). 

Reaction (3) is a second-order reaction (except for methyl and ethyl radicals) and has an activation energy close to zero. The rate coefficient will therefore be approximately equal to the pre-exponential factor. Various estimations and experimental determinations of its value have been made, however, a value of 10 - 1. mole . sec. appears to be an acceptable mean value for most workers [38, 58]. 

The importance of alkylperoxy radicals as intermediates had long been realized (see Sect. 2) and their subsequent reaction to yield the alkyl-hydroperoxide or decomposition products such as aldehydes and alcohols had been reasonably successful in describing the mechanism of the autocatalytic oxidation of alkanes. However, even though 0-heterocycles (which cannot be derived from intermediate aldehydes) had been found in the products of the oxidation of n-pentane as early as 1935 [66], the true extent of alkylperoxy radical isomerization reactions has been recognized only recently. Bailey and Norrish [67] first formulated the production of O-heterocycles in terms of alkylperoxy radical isomerization and subsequent cyclization in order to explain the formation of 2,5-dimethyl-tetrahydrofuran during the cool-flame oxidation of n-hexane. Their mechanism was a one-step process which involved direct elimination of OH. However, it is now generally formulated as shown in reactions (147) and(I67) 

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Direct compEirison of the theoretical and experimental yields is complicated by the fact that further reactions of the products have not been taken into account. Even so, some excellent agreements are obtained and in most cases where there is disparity it is clear that this is due to competing propagating steps not embraced by the scheme [78]. The alkylperoxy radical isomerization theory provides a good account, then, of the mechanism of chain-propagation, at least semi-quantitatively. 

The alkylperoxy radical isomerization and alkene-hydroperoxy radical addition theories of chain propagation in the oxidation of alkyl radicals at ca. 300°C. have been combined into a single comprehensive reaction scheme. 

In 1968 Fish [119] proposed an alkylperoxy isomerization theory that explains the temperature dependence of the ignition delays and the complex spectrum of products that can be obtained in hydrocarbon oxidation. Once formed the ROO radical can under go an internal isomerization. Cleavage of the resulting R OOH (or QOOH) radical is a chain propagating step leading to generation of a number of different products and an OH radical. Alternatively at low temperatures the addition of O2 can compete with unimolecular dissociation. 

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