Hydrocarbons alkylperoxy radical reactions

Higher Hydrocarbons. The VPO of higher hydrocarbons is similar to that of the lower members of the series with two significant additional comphcations (/) the back-bitiag reactions of alkylperoxy radicals (eq. 32), particularly at positions 2 or 3 carbons removed from the peroxy position, and (2) above the NTC region, radical fragmentation (eq. 28). 

Alkyl radicals, R, react very rapidly with O2 to form alkylperoxy radicals. H reacts to form the hydroperoxy radical HO2. Alkoxy radicals, RO, react with O2 to form HO2 and R CHO, where R contains one less carbon. This formation of an aldehyde from an alkoxy radical ultimately leads to the process of hydrocarbon chain shortening or clipping upon subsequent reaction of the aldehyde. This aldehyde can undergo photodecomposition forming R, H, and CO or, after OH attack, forming CH(0)00, the peroxyacyi radical. 

Variable valence transition metal ions, such as Co VCo and Mn /Mn are able to catalyze hydrocarbon autoxidations by increasing the rate of chain initiation. Thus, redox reactions of the metal ions with alkyl hydroperoxides produce chain initiating alkoxy and alkylperoxy radicals (Fig. 6). Interestingly, aromatic percarboxylic acids, which are key intermediates in the oxidation of methylaromatics, were shown by Jones (ref. 10) to oxidize Mn and Co, to the corresponding p-oxodimer of Mn or Co , via a heterolytic mechanism (Fig. 6). 

This difficulty has now been overcome. Howard, Schwalm, and Ingold (24) show that the rate constant for reaction of any alkylperoxy radical with any hydrocarbon can be determined (by the sector method) by carrying out the autoxidation of the hydrocarbon in the presence of >0.1 M hydroperoxide corresponding to the chosen radical. All the absolute propagation and termination constants for the co-oxidation of cumene and Tetralin were thus determined. Our Tetralin-cumene work suggests that their results agree well with the best we have been able to get... 

This reaction is certainly exothermic and by analogy with similar hydrogen abstraction reactions might be expected to have an activation energy of anywhere from zero to 8 kcal. If it should be proved that this is the terminating interaction or a possible terminating interaction of primary and secondary alkylperoxy radicals, then we must begin to think anew about the entire mechanism for the over-all chain decomposition and oxidation of hydrocarbons. I believe it would also require us to review seriously our interpretation of the spin resonance work on alkylperoxy radicals at low temperatures. 

Furimsky E, Howard JA, Selwyn J (1980) Absolute rate constants for hydrocarbon autoxidation. 28. A low temperature kinetic electron spin resonance study of the self- reactions of isopropylperoxy and related secondary alkylperoxy radicals in solution. Can J Chem 58 677-680 Gebicki JM, Allen AO (1969) Relationship between critical micelle concentration and rate of radiolysis of aqueous sodium linolenate. J Phys Chem 73 2443-2445 Gebicki JM, Bielski BHJ (1981) Comparison of the capacities of the perhydroxyl and the superoxide radicals to initiate chain oxidation of linoleic acid. J Am Chem Soc 103 7020-7022 Gilbert BC, Holmes RGG, Laue HAH, Norman ROC (1976) Electron spin resonance studies, part L. Reactions of alkoxyl radicals generated from alkylhydroperoxidesand titanium(lll) ion in aqueous solution. J Chem Soc Perkin Trans 2 1047-1052... 

The most common pathway for catalysis of autoxidations by transition metal complexes involves the decomposition of alkyl hydroperoxides. Another route that may be possible for chain initiation involves direct oxygen activation, whereby the complexation of molecular oxygen by a transition metal would lower the energy of activation for direct reaction with the substrate [reaction (9)]. For example, oxygen coordinated to a metal might be expected to possess properties similar to alkylperoxy radicals and undergo hydrogen transfer with a hydrocarbon ... 

Baldwin and Walker [99] have pointed out that, from kinetic considerations, surface reactions of alkylperoxy radicals cannot play a significant role except at very low overall rates of reaction and conclude that it is more likely that surface destruction of relatively stable intermediates such as the alkyl hydroperoxides or hydrogen peroxide are the main cause of surface effects in hydrocarbon oxidation. Luckett and Pollard [68, 134] have provided evidence, which suggests that the surface destruction of tert-butylhydroperoxide is indeed important during the oxidation of isobutane below ca. 320 °C. Since isobutene and acetone are known products of the decomposition of tert-butylhydroperoxide, it is clear that many of the foregoing results can be explained in these terms, but if this is the predominant heterogeneous reaction the yield of acetone would be... 

A free radical flux is maintained in this system. The predominant member of the flux is usually the alkylperoxy radical since it is a relatively weak hydrogen abstractor and, as a result, builds up to the highest concentration [8, 10-12], Each new molecule of hydrocarbon is brought into the reaction process by undergoing hydrogen abstraction. When this reaction involves an alkylperoxy radical, the result is the first propagation step, shown in eq. (2). This produces an alkyl radical and an alkyl hydroperoxide. 

Peroxy radicals are obviously very important in the complex mechanisms of hydrocarbon oxidations. They propagate the reaction chains (eq. (2)), producing hydroperoxides. The selectivities of various alkylperoxy radicals in hydrogen abstractions have been reported to be relatively independent of the alkylperoxy radicals utilized [12, 15]. 

This reaction consumes two alkylperoxy radicals and produces an alcohol and a carbonyl compound. At least one a-hydrogen atom must be present on one of the alkylperoxy radicals. If the peroxy radicals involved retain the carbon skeleton of the starting hydrocarbon, so will the products. 

Laboratory studies of the autooxidation of hydrocarbons (Ingold, 1969 Howard, 1971) have revealed that the self-reactions of alkylperoxy radicals among each other are slow with rate coefficients generally smaller than that for the CH302+CH302 reaction. These reactions need not be considered in the atmosphere. A possible exception may be the reaction of ROO radicals with CH302, which is the most abundant peroxy radical in the atmosphere. 

Using the preceding discussion of alkylperoxy and alkoxy radical reactions as a guide, we present now specific mechanisms for several individual hydrocarbons, starting with alkanes. 

If >>c(oo-)o-h = 75 kcal mole-1, then q = 30 kcal mole-1. Therefore, hyroxyperoxy radicals, in contrast to alkylperoxy radicals, display a dual reactivity. They can take part both in oxidation and in reduction reactions and they would be expected to react not only with radicals but with molecules of the oxidizing agent, with quinones for example. The kinetics of 2-propanol oxidation in the presence of benzoquinone has been studied [80], Quinones are known to terminate chains in hydrocarbon oxidation only by reactions with alkyl radicals [1]. In alcohol oxidation, quinone terminates chains by reaction with hydroxyalkyl as well as with hydroxyperoxy radicals [80]. At 71°C and PQl = 760 torr, 86% of chain termination is due to the reaction >C)0H)00- + quinone. The rate coefficient is M>C(0H)00- + quinone) = 3.2 X 1031 mole-1 s-1 and kQ/kp = 1.0 X 104. Just as in the case of aromatic amines, f> 2 f= 23 for quinone, i.e. quinone is regenerated in the reactions... 

CH3O2 is the most abundant alkylperoxy radical in hydrocarbon-rich atmospheres. It is of interest to assess the role of RO2 + CH3O2 cross-reactions since a series of rate constants has been obtained for such reactions, either resulting from indirect determinations (secondary reactions of RO2 + RO2 reactions) or from direct measurements. Rate constants for this class of reactions are reported in Table 8. 

Our new finding that the kinetic coefficient of alkylperoxy + NO reactions decreases with increasing size (or CHs-substitution) of the alkyl radical is of considerable importance with respect to tropospheric ozone formation from higher hydrocarbons, the more so since the rate constants of the competing alkylperoxy + HO2 reactions [20] increase significantly for larger R. 

Product distributions resulting from the OH radical induced oxidation of the following hydrocarbons have been determined 2-methyl-propane, 2,3-dimethyl-butane, 2-methyl-butane, n-pentane, cyclohexane, methySubstituted 1-butenes, isoprene, toluene. Whenever possible, branching ratios for the self-reactions of alkylperoxy radicals and decomposition rate coefficients for alkoxyl radicals were derived. 

Kinetic results were consistent with a bimolecular termination reaction whereas reaction products and mechanisms were something of a mystery. At that time it was known that the termination rate constant for autoxidation of cumene ( ) is about three orders of magnitude smaller than the termination rate constant for autoxidation of tetralin (7.). It was, however, generally accepted that the tennination rate constants for tertiary ( ) and secondary (9 ) alkylperoxy radicals are insensitive to the structure of the hydrocarbon residue in the radical. 

Other t-alkylperoxys. The self-reactions of a wide variety of other t-alkylperoxy radicals have been examined by hydrocarbon autoxidation and KESR (], 1J, ) and they all exist in... 

The simplest hydrocarbon, methane, has posed a wealth of challenges to experimentalists and theoreticians seeking to discern its combustion mechanism. Methane s reactions have been explored in a wide variety of contexts over the past several decades. We have discussed these briefly the interested reader is referred to the reviews cited in our previous discussion for further details. Due to the scope of this review, we are primarily interested in these reactions insofar as they provide useful benchmarks for the reactions of larger alkylperoxy (RO2 ) and alkoxy (RO ) systems. With respect to the reactive intermediates present in methane combustion and their implications for larger systems, Lightfoot has published a review on the atmospheric role of these species, while Wallington et al. have provided multiple overviews of gas-phase peroxy radical chemistry. Lesclaux has provided multiple reviews of developments in peroxy radical chemistry. Batt published a review of the gas-phase decomposition reactions available to the alkoxy radicals. ... 

The reaction of alkanes with a mixture of SO2 and O2 also occurs more readily than simple autoxidation of the hydrocarbon (reaction 30). Here the radical RSO2OO , formed as shown in Scheme 27, is apparently less prone to termination reactions than a simple alkylperoxy radicaF. ... 

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