Peroxy radicals alkylperoxy

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 CBS-QB3 potential energy surface accounts for the various experimentally observed products, including hydroperoxyl radical, propene, HO, propanal, and oxirane (c-CsHgO). The activation barrier for simultaneous 1,4-H transfer and HO2 expulsion, obtained via calculations, compares well to the experimentally observed barrier (26.0kcal/mol) of DeSain et al. This work provides some ramifications for larger alkylperoxy radicals multiple conformers of long alkylperoxy radicals are likely to play a role in the overall oxidation chemistry and dictate consideration for correct treatment of thermochemistry at lower temperatures T< 500 K), unimolecular reactions dictate peroxy radical chemistry. 

We therefore believe that the pic darret always arises from reactions of alkylperoxy radicals rather than of H02 radicals, and indeed that it can act as a peroxy radical tracer occurring when a critical peroxy radical concentration is achieved. Its disappearance at a certain temperature and reappearance at a higher temperature (see 250-mm. Hg isobar in Figure 4) after the region of negative temperature coefficient is in agreement with this view. 

From my estimates on the thermodynamic properties of peroxy and polyoxide molecules and radicals, we can estimate that the bond dissociation energy of the tetroxide is about 5 kcal. Thus, at room temperature, or even at dry ice temperature, the tetroxide is extremely unstable and should redissociate into the more stable (from a thermodynamic point of view) peroxy radicals. The competing step would be a concerted decomposition into an RO and an R03 (Step 14) radical, which would be uphill by 20 kcal., or else a concerted decomposition into 2 RO radicals and 02 (Step 14 ). The latter is almost thermoneutral. If we take the current data at face value, it provides, from the reported activation energy at least, strong evidence that the propagating interaction of two alkylperoxy radicals proceeds in a concerted fashion. 

Bauer G (2000) Reactive oxygen and nitrogen species efficient, selective and interactive signals during intercellular induction of apoptosis. Anticancer Res 20 4115-4140 Beckwith AU, Davies AG, Davison IGE, Maccoll A, Mruzek MH (1989) The mechanisms of the rearrangements of allylic hydroperoxides 5a-hydroperoxy-3p-hydrocholest-6-ene and 7a-hydro-peroxy-3(1-hydroxycholest-5-ene. J Chem Soc Perkin Trans 2 815-824 Behar D, Czapski G, Rabani J, Dorfman LM, Schwarz HA (1970) The acid dissociation constant and decay kinetics of the perhydroxyl radical. J Phys Chem 74 3209-3213 Benjan EV, Font-Sanchis E, Scaiano JC (2001) Lactone-derived carbon-centered radicals formation and reactivity with oxygen. Org Lett 3 4059-4062 Bennett JE, Summers R (1974) Product studies of the mutual termination reactions of sec- alkylper-oxy radicals Evidence for non-cyclic termination. Can J Chem 52 1377-1379 Bennett JE, Brown DM, Mile B (1970) Studies by electron spin resonance of the reactions of alkyl-peroxy radicals, part 2. Equilibrium between alkylperoxy radicals and tetroxide molecules. Trans Faraday Soc 66 397-405... 

Han P, Bartels DM (1994) Encounters of H and D atoms with 02 in water relative diffusion and reaction rates. In Gauduel Y, Rossky P (eds) AIP conference proceedings 298. "Ultrafast reaction dynamics and solvent effects." AIP Press, New York, 72 pp Hasegawa K, Patterson LK (1978) Pulse radiolysis studies in model lipid systems formation and behavior of peroxy radicals in fatty acids. Photochem Photobiol 28 817-823 Herdener M, Heigold S, Saran M, Bauer G (2000) Target cell-derived superoxide anions cause efficiency and selectivity of intercellular induction of apoptosis. Free Rad Biol Med 29 1260-1271 Hildenbrand K, Schulte-Frohlinde D (1997) Time-resolved EPR studies on the reaction rates of peroxyl radicals of polyfacrylic acid) and of calf thymus DNA with glutathione. Re-examination of a rate constant for DNA. Int J Radiat Biol 71 377-385 Howard JA (1978) Self-reactions of alkylperoxy radicals in solution (1). In Pryor WA(ed) Organic free radicals. ACS Symp Ser 69 413-432... 

Examination of Table III reveals that reactivities of peroxy radicals are strongly dependent on their structure. Reactivities are influenced by both steric and polar effects,26,30-32 and, in general, increase as the electron-withdrawing capacity of the a substituent increases. Acylperoxy radicals, which possess a strong electron-withdrawing substituent, are considerably more reactive than other alkylperoxy radicals. For example, the benzoylperoxy radical is 4 X 104 times more reactive than the ferf-butylperoxy radical. 

In recent years much emphasis has been placed on studies of co-oxidations, since they can provide quantitative data about fundamental processes (such as the relative reactivities of peroxy radicals toward various hydrocarbons48-50), which are difficult to obtain by other methods. Co-oxidations are also quite important from a practical viewpoint since it is possible to utilize the alkylperoxy intermediates for additional oxidation processes instead of wasting this active oxygen. That the addition of a second substrate to an autoxidation reaction can produce dramatic effects is illustrated by Russell s observation51 that the presence of 3 mole % of tetralin reduced the rate of cumene oxidation by two-thirds, despite the fact that tetralin itself is oxidized 10 times faster than cumene. The retardation is due to the higher rate of termination of the secondary tetralyl-peroxy radicals compared to the tertiary cumylperoxy radicals (see above). 

The reactions of metal catalysts with alkylperoxy radicals must be considered in liquid phase autoxidations, since peroxy radicals are the most abundant species in solution. The reduction of alkoxy radicals to the corresponding... 

In the early stages of autoxidations, hydroperoxide concentrations are low and chain initiation is inefficient. Under these conditions, Mn(II) and Co(II) can act as inhibitors by scavenging alkylperoxy radicals [reaction (278)]. Competition in the termination step between the usual bimolecular termination of peroxy radicals and their reaction with metal complexes can affect the chain length of the autoxidation. The expression for the chain length in a process involving bimolecular termination of peroxy radicals is... 

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. 

This is not a termination reaction. It is one means of converting alkylperoxy radicals to alkoxy radicals. It is the dominant reaction when neither peroxy radical contains an a-hydrogen, but it even occurs to a significant extent (in one report about 40% of the time [17]) with peroxy radicals that do contain a-hydrogens. Alkoxy radicals are vigorous hydrogen abstractors [12]. This appears to be the main reaction for primary alkoxy radicals the products are primary alcohols. Secondary and tertiary alkoxys, however, tend to undergo a competitive 6-scission reaction to a major extent [18] ... 

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. 

Most oxidation reactions proceed by way of elementary steps involving alkylperoxy and alkoxy radicals therefore quantitative descriptions of oxidation processes require reliable absolute rate coefficients for all important elementary steps. This section provides a compilation of rate coefficients and rate parameters for H-atom transfer (abstraction), addition, ring closures and combinations by peroxy radicals, and for abstraction and cleavage by alkoxy radicals. 

Aromatic nitroxides were found to attack both alkyl and alkylperoxy radicals, but aliphatic nitroxides attacked only alkyl radicals, the rate obeying equation (IX) above. The kinetics of reaction involving nitroxide B were close to predictions made on the basis of equation (VII) and it is interesting to note that the rate of reaction was affected by solvent, possibly via the formation of a loose solvent—nitroxide complex. Hydroxyl-amines A and B reacted only with peroxy radicals, but hydroxylamine C was involved in a complex reaction sequence producing first strong inhibition and then equally strong catalysis of autoxidation ... 

The fate of peroxy radicals at sediment or soil surfaces has been considered by Pohlman and Mill (1983). They examined the ability of common soil constituents to quench the reaction of an alkylperoxy radical with a reactive probe molecule, p-isopropylphenol. They found that reactions with organic matter were dominant over possible reactions with the mineral constituents, except for possibly Cu but natural humic materials appeared to be a rather poor scavenger, presumably because the bulk of their structure consisted of unreactive moieties such as polysaccharides and aliphatic chains. They also concluded that phenols and other reactive xenobiotics might be partially susceptible to removal through reaction with these radicals. 

Peroxy radicals ROO are key species in the mechanisms of oxidative and combustion systems. At the same time they have been among the most difficult radicals to study experimentally. There are only limited or no thermochemical information available for unsaturated alkylperoxy and hydroperoxide species. An explanation for this paucity of data could be the fact that these species are unstable and short-lived, and therefore difficult to study and characterise by experimental methods. The difficulty arises in part from the lability of these radicals towards reversible unimolecular dissociation into R + O2 and then to reversible isomerization into hydroperoxy alkyl radicals ROOH, both of these reactions occurring at comparable rates at temperatures below 450 K [2]. 

There is little or no thermochemical property data available for unsaturated alkylperoxy and peroxide species. Peroxides are often impure and/or instable, and therefore difficult to isolate and characterize by experimental methods. There is no experimental data on vinyl, phenyl, ethynyl, or allyl peroxides that we are aware of. Experimental studies on the reaction of vinyl radical and allyl radical with O2 to form the corresponding peroxy radical have been reported by Gutman et al. [11, 12]. The phenyl-peroxy radical was reported by Lin s group as a major product in the phenyl radical reaction with O2 at ambient temperatures [17]. 

Only few data are available for self-reactions of secondary alkylperoxy radicals. One of the principal contributions of the project has been the study of the cyclo-alkylperoxy radicals C-C5H9O2 and c-CeUi 02 radicals. The corresponding rate constants are 40 times smaller than the only linear secondary peroxy radical studied so far, /-C3H7O2, and exhibit a slight positive temperature dependence. More data would be necessary for a better description of such reactions but, due to their relatively low rate constants (< 5 x lO" " cm molecule s ), their contribution to atmospheric chemistry is negligible. 

The primary step in the OH initiated atmospheric oxidation of saturated hydrocarbons is abstraction of an H atom to form an alkyl radical and water. The predominant fate of the alkyl radical is addition of oxygen to form a peroxy radical. However, it is known for ethyl radicals that a pathway exists which leads to formation of ethene and HO2 radicals. Two mechanisms have been discussed in the literature, a) a direct abstraction of an H atom by O2 and b) a Lindemann mechanism involving an excited alkylperoxy radical ... 

The reaction of OH radicals with alkanes results in the abstraction of a hydrogen atom followed by addition of oxygen whereby an alkylperoxy radical is formed In the reaction with alkenes OH is added to the double bond followed by addition of oxygen, generally at the neighbouring carbon atom. The peroxy radicals then enter into a reaction sequence of the type... 

Percent product distribution propanal 50.6 1.2, 2-hydroxy-butanal 7.0 0.3, l-hydroxy-butan-2-one 24.2 0.7, 1,2 dihydroxybutane 18.2 0.7. Computer assisted analysis of the product distribution showed that addition of the OH radical occurs to 26 % at the inner and to 74% at the outer position of the double bond. These reactions produced the corresponding primary and secondary hydroxy-alkylperoxy radicals. The branching ratio for the radical propagating channel of the self-reaction of the secondary peroxy radicals was determined to be issa/ iss = 0.75 0.02 28 % of the hydroxy-alkoxyl radical thus formed reacted with oxygen to produce hydroxyketone. If it is assumed that the rate coefficient for the reaction of the hydroxy-alkoxyl radical with oxygen is 8 x 10 cm molecule s the rate coefficient for the decomposition of this radical to produce propanal is 1 x 10 s V... 

The reaction with NO leads to the formation of CO2 and a methyl radical that is oxidized to formaldehyde by reactions (16)-(20). In addition, the oxidation of CH3 regenerates HO c so that the oxidation cycle continues. Association with NO2 produces peroxyacetyl nitrate (PAN). Its lifetime is longer than that of alkylperoxy nitrates, but strongly temperature dependent, ranging from 1 hr at 298 K to 140 d at 250 K. Thus, PAN can be transported over a great distance before undergoing thermal decomposition. Under conditions of lowNOj concentrations acetyl peroxy radicals interact also with HO2 radicals... 

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