Isomerization alkylperoxy radical

Hydroperoxyalkyl radicals can react in several ways. First, of course, alkylperoxy radical isomerization is reversible (Reaction —4). Secondly, several modes of decomposition (Reaction 5) occur, giving O-hetero-cycles, alkenes, and saturated and unsaturated aldehydes and ketones. (Alcohols can also be formed by the decomposition of the alkylperoxy-alkyl radicals which result from isomerization by group transfer in alkylperoxy radicals.) These modes of decomposition have been enumerated (24, 25) only examples need be given here. Each mode of decomposition gives one or more product molecules plus one free radical, acting, therefore, as a propagation step. Moreover, the radical produced... 

Reaction 8 may, therefore, be the major chain-propagating reaction of H02 between 250° and 400°C. The radicals produced will, of course, undergo the same fates as those produced in Reaction 4, regenerating (eventually) alkyl radicals. The main difference between the alkene-H02 addition route and the alkylperoxy radical isomerization route is that in the former case the hydroperoxyalkyl radicals formed are necessarily a-radicals—i.e., radicals in which the unpaired electron is borne by a carbon atom adjacent to that bearing the hydroperoxy group, such as... 

Since Reactions 4 and 8 have a common product, the scheme also incorporates, and hence in principle reconciles, the apparently contradictory alkylperoxy radical isomerization and alkene-hydroperoxy radical addition schemes for propagating chains during the oxidation of alkyl radicals. If this reconciliation is to hold in practice as well as in principle, however, two conditions are necessary. 

It appears, then, that alkylperoxy radical isomerization is capable of producing hydroperoxyalkyl radicals during the oxidation of all alkanes and that alkene-hydroperoxy radical addition will serve a similar function during the oxidation of those alkanes which contain a high proportion of primary C—H bonds. It remains to determine the proportion of hydroperoxy alkyl radicals arriving by each route as equilibrium is approached. 

The mechanism of formation of the cracking products had not been resolved at the time of publication of Shtern s review [2]. Semenov [3], however, had pointed out that the direct decomposition of prop-l-yl and prop-2-yl radicals at 300 °C was most unlikely, due to the large activation energies involved (27—29 and 40 kcal. mole", respectively). He therefore suggested the following alkylperoxy radical isomerization and decomposition reactions to explain the formation of propene and ethylene in propane oxidation, viz. 

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 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)... 

The addition of [2- " C] but-2-ene to reacting n-butane + oxygen mixtures at 315 °C [56] showed that the reverse isomerization reaction (— 14a) does not occur to any appreciable extent, since the 2-methyloxetan found in the products was inactive. It can be safely concluded, therefore, that the formation of derivatives of oxetans, furans and pyrans is diagnostic of alkylperoxy radical isomerization and subsequent decomposition, reactions (14) + (16). Table 1 thus presents a considerable volume of evidence for the wide occurrence of this chain-propagation step. 

It is clear from the wide variety of intermediate products formed that the initial attack on the alkane is extremely unselective. Consideration of the mode of formation of the major products via alkylperoxy radical isomerization shows that OH is the radical predominantly formed in reaction (16) and spectroscopic studies have confirmed the presence of OH radicals in the oxidation of aldehydes [93] and methyl radicals [94]. Furthermore, Haskell and Read [95] have convincingly shown that the inhibition of the oxidation of 2-methylpentane by hydrogen is due to the participation of reactions (18) and (19)... 

Thermodynamic and kinetic parameters for alkylperoxy radical isomerization at 600... 

Products resulting from group shifts during alkylperoxy radical isomerization... 

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. 

Alkylperoxy radical isomerization has a rather higher probability when 1-propyl radicals are involved in a corresponding competition because the activation energy for the primary H atom abstraction is higher than that for a secondary H atom abstraction. 

Even if it does not occur very readily, alkylperoxy radical isomerization may play an important part because the decomposition products, which include aldehydes, lead to propylhydroperoxide via H atom abstraction reactions of appreciably lower activation energy, for example. 

The production of 2,2,4,4-tetramethyltetrahydrofuran as a major product of /-octane oxidation points to the predominance of the alkylperoxy radical isomerization R02 Q(1,6)00H involving either,... 

R + O2/RO2 equilibria and on the modes of alkylperoxy radical isomerization and further oxidation in this scheme. 

A cyclic transition-state model has been proposed for the alkylperoxy radical isomerization reaction. Literature ab initio calculation data are used to determine the frequencies of the C—H—O group of the cyclic transition state. H-atom transfer occurs from the carbon atom to the oxygen atom. 

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