Peroxy radical isomerization

Data on alkyl radical oxidation between 300° and 800°K. have been studied to establish which of the many elementary reactions proposed for systems containing alkyl radicals and oxygen remain valid when considered in a broad framework, and the rate constants of the most likely major reactions have been estimated. It now seems that olefin formation in autocatalytic oxidations at about 600°K. occurs largely by decomposition of peroxy radicals rather than by direct abstraction of H from an alkyl radical by oxygen. This unimolecular decomposition apparently competes with H abstraction by peroxy radicals and mutual reaction of peroxy radicals. The position regarding other peroxy radical isomerization and decomposition reactions remains obscured by the uncertain effects of reaction vessel surface in oxidations of higher alkanes at 500°-600°K. 

As E- and do not differ greatly, the relative importance of (-6) and (9) is not markedly influenced by temperature, and the relative yields of O-heterocyclic compounds (from OOOH homolysis) and conjugate al-kenes are similarly little affected between 600 and 800 K. This is particularly noticeable [17] with the larger alkanes where O-heterocyclic compounds and conjugate alkenes are formed in similar quantities. Fish [76] was the first to develop the extensive peroxy radical isomerization and decomposition (PRID) theory which was used to explain the many oxygenated compounds in alkane oxidation. Fish also considered the possibility of group transfer in RO2 radicals, for example... 

There have been extensive measurements of other alkyl radical systems. All are compatible with a mechanism in which the peroxy radical isomerizes to form a QOOH radical which then dissociates in a variety of ways. 

Fig. 1. Simplified outline mechanism for low-temperature alkane oxidation. Peroxy radical isomerization, an internal hydrogen abstraction, plays a crucial role at two points in the mechanism and is illustrated on the left...

The main oxidation products of the methyl esters of aliphatic acids containing n C atoms are methyl esters of dicarboxylic acids C4—C 3, aliphatic acids Ci— Cn-i, and keto- and hydroxy compounds [301—307]. Oxidation of acetates (140—160° C) yield acids, carbon dioxide, hydroxy, and keto compounds (see Table 17). Hydroperoxide is the primary product of oxidation. Oxidation of dimethyl esters of dicarboxylic acids gives monoesters with a lower number of C atoms in the acidic group (see Table 17). Carbon dioxide is formed in parallel with acids and monoesters [308]. All monoesters C 1 Cn 2 etc. are also formed in parallel. This suggests several mechanisms of C—C bond scission in the oxidation, an a-mechanism with only one C—C bond broken to form Cm and C — products, a /3-mechanism with two C—C bonds broken in the /3-position to form Cm, C —m— and C02, etc. The a, /3, and 7-mechanisms of C—C bond scission may be regarded as a result of peroxy radical isomerization to form labile dihydroperoxides, e.g. 

Crounse JD, Knap HC, 0ms0 KB, J0rgensen S, Paulot F, Kjaerdaard HG, Wennberg PO (2012) Atmospheric fate of methacrolein 1 peroxy radical isomerization following addition of OH and O2. J Phys Chem A 116(24) 5756-5762... 

Crounse JD, Paulot F, Kjaergaard HG, Wennberg PO (2011) Peroxy radical isomerization in the oxidation of isoprene. Phys Chem Chem Phys 13 13607-13613... 

A high-level ab initio study of related reactions of alkyl nitrates (RO—NO2) at the G3 and B3LYP/6-311-I— -G(d,p) levels has revisited the reactions of alkyl peroxy radicals (ROO") with nitric oxide. Activation barriers for the isomerization of RO—ONO to RO—NO2 were found to be too high to account for the formation of alkyl nitrates... 

The formation of oxidation products a-c in a range of G values (0.7-3.8) during the 7-R of S in 02-saturated DCE suggests that a-c would be produced from complicated reactions of peroxy radicals with S (Table 5). On the other hand, the regioselective formation of 3d with large G values (2.6-3.0) in oxidation of 3 with O2 is explained by spin localization on the p-olefinic carbon because of the contribution of (B) in 3. The results of products analyses are essentially identical with prediction based on k and ko for S measured with PR. It should be emphasized that the reactivities of c-t unimolecular isomerization and reaction of S with O2 can be understood in terms of charge-spin separation induced by p-MeO. 

Zaikov (9) showed that isomerization and decomposition of a methyl ethyl ketone peroxy radical may occur as shown below. 

Below the temperatures for the NTC regime, the peroxy radical (ROO) may be involved in a chain-branching sequence of reactions that is responsible for the positive temperature dependence. The oxidation rate varies significantly between different hydrocarbons or hydrocarbon isomers, depending on their structure. The first step is an internal isomerization,... 

The Negative-Temperature-Coefficient Region The equilibrium constant for the reaction R + O2 ROO (R64) is strongly temperature dependent, and as the temperature increases, the equilibrium shifts in favor of R + O2. The shift in equilibrium is the primary reason for the existence of the region where the conversion decreases with an increase in temperature (i.e., where there is a negative temperature coefficient). Above about 650 K, the alkyl peroxy radical becomes less thermally stable, and alternative reaction paths for ROO begin to compete with the isomerization reaction (R65). A new product channel opens up for the R + O2 reaction... 

The asymmetric rearrangement of peroxy radical (5) has recently been used as the key step in the asymmetric synthesis of Plakorin (Scheme l).17 The thermal isomerization of buta-1,2- to buta-1,3-diene has been studied using ab initio calculations and the mechanism concluded to proceed stepwise via radical intermediates.18 The competition between cyclopropyl formation and the homoallyl-homoallyl radical rearrangement has been studied in the radical (6) and found to give the 3-exo cyclization product (7) and the rearranged product (8) in a 1 5 ratio, respectively, under the conditions shown (Scheme 2).19... 

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. 

The resulting hydroxy alkyl radical then adds 02 to become a hydroxy alkyl peroxy radical which reacts with more NO to give more N02 and another alkoxy radical capable of undergoing further isomerization ... 

As discussed in section 4, reaction of the peroxy radicals with N02 gives thermally unstable peroxy nitrates. Reaction with H02 gives hydroperoxides and possibly carbonyl compounds. Reaction with other peroxy radicals (R 02) gives alkoxy radicals, carbonyls, and alcohols. The alkoxy radicals will then either isomerize, react with 02, or decompose (see Sect. 3). Thus, the NO3 radical-initiated atmospheric degradation of alkenes leads to oxiranes (generally in small yield), nitrooxy hydroperoxides, nitrooxy carbonyls, and nitrooxyalcohols. For a detailed listing of products from individual alkenes the reader should consult Calvert et al. [55]. 

From Table 3 it can be seen that methyl substitution at one end of the allyl radical has only a slight effect on the distribution of the unpaired electron. In fact the effect is to produce a slightly lower spin density at the terminal carbon atom, which is contrary to that proposed to explain the preferential reactivity of this position. Moss and Steiner (1965) have suggested that there is a rapid interconversion between the two isomeric peroxy-radicals. 

At temperatures of approximately 700 K, oxirane is formed from channel (49c), in competition with channel (49b), with a yield which is much lower than that of ethene. The mechanism is presumed to involve the the formation of the peroxy radical and subsequent isomerization to the QOOH radical. 

---