Alkyl peroxy radical

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... 

Reactivity ratios for all the combinations of butadiene, styrene, Tetralin, and cumene give consistent sets of reactivities for these hydrocarbons in the approximate ratios 30 14 5.5 1 at 50°C. These ratios are nearly independent of the alkyl-peroxy radical involved. Co-oxidations of Tetralin-Decalin mixtures show that steric effects can affect relative reactivities of hydrocarbons by a factor up to 2. Polar effects of similar magnitude may arise when hydrocarbons are cooxidized with other organic compounds. Many of the previously published reactivity ratios appear to be subject to considerable experimental errors. Large abnormalities in oxidation rates of hydrocarbon mixtures are expected with only a few hydrocarbons in which reaction is confined to tertiary carbon-hydrogen bonds. Several measures of relative reactivities of hydrocarbons in oxidations are compared. 

Simple alkyl peroxy radicals have been shown to react with N03, e.g., for CH302 ... 

For example, Cantrell and co-workers (1993) estimate the efficiency of conversion of simple alkyl peroxy radicals to vary from 0.93 for CH3CH202 to 0.47 for (CH3)2C02, and it may be even less for larger alkyl peroxy radicals. This may be the reason that in some intercomparison studies, the matrix isolation-ESR technique (vide infra), which measures the sum of ROz, gives some higher concentrations for some individual measurements than the chemical amplifier method (e.g., Zenker et al., 1998). 

In cooxidation of alkenes and aldehydes, and in photosensitized epoxidations acyl peroxy radicals are the epoxidizing agents. The mechanism of the former oxidation, on the basis of kinetic measurements and the nonstereospecificity of the reaction, involves alkyl peroxy radicals formed through the cr-bonded radical 29 as the intermediate ... 

This region of negative temperature coefficient can be quantitatively ascribed to the failure of the system to produce alkyl hydroperoxide as a product at the higher temperatures. Instead, with increasing temperature because of the reversibility of Reaction 1, the equilibrium concentration of alkyl peroxy radicals decreases in favor of alkyl radicals, and the high temperature mechanism supersedes the low temperature mechanism (Reactions 4 and 5). 

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 reaction of the Cu(R2 Dtc)2 complexes with alkyl hydroperoxides is reported to give alkoxy and alkyl peroxy radicals. The kinetics and a mechanism for this reaction have been reported (120). 

Mention has already been made of the relatively small reactivity of allyl peroxy radicals compared with other alkyl peroxy radicals. Jost (88, 96) has reasoned that paraffins react by a small number of long chains, whereas olefins oxidize by a large number of short chains. Olefins are thus attacked more readily than paraffins but form less reactive allyl radicals. In addition, during oxidation chain transfer occurs in which alkyl radicals are replaced by allyl radicals. Shorter chains would then be expected. Comparison of the precombustion products of iso-octane and diisobutylene (154) indicates that marked self-inhibition of reaction chains was occurring in the latter case. 

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... 

Hendry, D.G., Mill, T., Piszkiewicz, L., Howard, J.A., Eigenmann, H. K. (1974) Critical review of hydrogen-atom transfer in the liquid phase. Chlorine atom, alkyltrichloromethyl, alkoxy, and alkyl peroxy radicals. J. Phys. Chem. Ref. Data 3, 937-978. 

The rate constant for the reaction of ethyl radicals and oxygen increases with the increasing concentration of inert helium [57]. This indicates the possible participation of the molecules in the process of stabilization of peroxy radicals formed. The excited alkyl peroxy radicals do not fragmentate only to original reactants but rearrange to products through separate pathways [58]. 

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. 

There exists another pathway of self-reaction of two secondary or primary alkyl peroxy radicals which is even more favorable from the viewpoint of exothermicity than it is Russell s mechanism. It is the reaction in which two molecules of ketone and hydrogen peroxide (or hydrogen and oxygen) are formed as follows ... 

The reaction of alkyl peroxy radicals with NO is very important in the atmospheric degradation mechanism of alkanes. The reaction proceeds via two pathways. 

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 ... 

In the atmosphere the nitrooxy alkyl peroxy radical, > C(0N02) - COO( ) <, behaves like other alkyl peroxy radicals and will react with NO2, HO2, and other peroxy radicals. Reaction of nitrooxy alkyl peroxy radical with NO is unlikely because the conditions necessary for the formation of NO3 radicals (high O3) are incompatible with the presence of significant amounts of NO. For unsymmetrical alkenes the addition of NO3 radicals leads to the formation of two different peroxy radicals, e.g., for propene ... 

Although the stability of the tetroxide molecules is very low at ambient temperatures it seems likely that the irreversible decay of alkyl peroxy-radicals does proceed through the formation of a tetroxide molecule which can then either decompose reversibly into the alkyl peroxy-radicals or decompose irreversibly into non-radical products. 

Its subsequent reactions with O3, CO, and other VOCs, in the presence of nitrogen oxides (NOj ) lead to the formation of HO2, hydrogen peroxide, alkyl peroxy radicals, and ultimately to the production of ozone via the conversion of NO2 to NO [8,21],... 

Condensation Nuclei. Many mechanisms have been proposed (9) involving free radical polymerizations of various radicals which could lead to formation of condensation nuclei. It seems that if condensation nuclei are formed by such reactions, the myriad different radicals in a given system would lead to formation of a highly mixed polymer. Noting this, an oversimplified mechanism by which the system NO2 + a-pinene + hv may form condensation nuclei (Figure 1) is for example, reactions of the alkyl peroxy radical formed in Reaction 9 with a-pinene and molecular oxygen ... 

Alkyl peroxy radicals are important intermediates in low-temperature oxidation of alkanes but there is only limited information on their reactions, particularly at temperatures much above ambient. Their properties have been recently reviewed and the kinetics data for their mutual reactions have been evaluated [70]. 

The lower lines in Fig. 2 plot the conditions when the rates of branching (via RO2) and of termination (via R) are equal for two alkanes of widely differing autoignition propensities. Below the line for a particular fuel low-temperature alkyl peroxy radical chemistry is dominant. Above the line alkyl peroxy radicals are of less significance. 

Examples of free radicals in the atmosphere include the hydroxyl radical (OFF), hydroperoxy radical (HOp), and alkoxy and alkyl peroxy radicals (RO and R02% respectively, where R is an alkyl group). The most important of these free radicals in the oxidation of atmospheric chemicals is the hydroxyl... 

New radical reacts with 02 to form alkyl peroxy radical (R02 ). 

NO reacts with alkyl peroxy radical to form N02, acetaldehyde, and another organic radical. 

Propagation of the radical chain reaction Once an alkyl radical has been formed, this reacts irreversibly with oxygen to form an alkyl peroxy radical. Reaction (4.2), which is extremely fast, = 10 -10 1 moHs and has a very low activation energy k2 is independent of temperature. 

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