Peroxy radical reactions

Our application of the rotating sector technique to hydrocarbon oxidations has been described (14,15,18). Oxidation rates were measured at the longest convenient chain lengths and corrected for the absorption and evolution of gas in initiation and in peroxy radical-peroxy radical reactions. ,a -Azobiscyclohexylnitrile (ACHN) was used as the photoinitiator at 30°C. and a,a -azobis-l-propanol diacetate as the photoinitiator at 56°C. ,a -Azobisisobutyronitrile (AIBN) was used as a thermal initiator at 30° and 56°C. 

The effect of solvents on the oxidation route as regards the composition of reaction products was investigated for methyl ethyl ketone at 145° to 160°C. at a pressure of 50 atm. (9, 10). The composition of products under these conditions is a function of the competition between two routes of the peroxy radical reactions ... 

Stockwell, W. R., J. B. Milford, D. F. Gao, and Y. J. Yang, The Effect of Acetyl Peroxy-Peroxy Radical Reactions on Peroxy-acetyl Nitrate and Ozone Concentrations, Atmos. Enriron., 29, 1591-1599(1995). 

The fate of the CH3SOO adduct is not known but by analogy to other peroxy radical reactions is expected to include reactions with NO and N02 (Turnipseed et al., 1993) ... 

These peroxide bonds, as shown in Equations 3 and 4, are stable at room temperature, but when the temperature is raised, the bonds undergo homolytic cleavage to form peroxy radicals (Reactions 5 and 6). There-... 

The problem of bringing a large magnet into the field for ambient measurements has been overcome in electron paramagnetic resonance (EPR, also called electron spin resonance, ESR) by Mihelcic, Helten, and coworkers (93-99). They combined EPR with a matrix isolation technique to allow the sampling and radical quantification to occur in separate steps. The matrix isolation is also required in this case because EPR is not sensitive enough to measure peroxy radicals directly in the atmosphere. EPR spectroscopy has also been used in laboratory studies of peroxy radical reactions (100, 101). 

Effect of peroxy radical reactions on the predicted concentrations of ozone, nitrogenous compounds, and radicals. foumal of Geophysical Research-Atmospheres, 101 (D15), 21007-22. 

Many radicals have been shown to react rapidly with oxygen. The reaction is usually formulated as an addition to produce a peroxy radical [reactions (51) and (52)] and this mechanism is probably... 

There are two sources of tropospheric ozone. First, transport from the stratosphere in meteorological events known as tropospheric folding in which a layer of stratospheric air is entrained in tropospheric air-flow and mixed into the troposphere. Second, peroxy radical reactions which oxidize NO to N02. For example, in the OH radical initiated oxidation of CO ... 

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 ... 

Under atmospheric conditions, oxygen is expected to rapidly react with the aromatic-OH adduct, forming either an alcohol (Reaction 2a) or a peroxy radical (Reaction 2b) ... 

Stockwell, W.R., J.B. Milford, D. Gao and Y.J. Yang The effect of acetyl peroxy-peroxy radical reactions on peroxyacetyl nitrate and ozone concentrations, Atmos. Environ. 29 (1995) 1591-1599. 

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. 

Phosphites with substituted phenoxy groups also behave as peroxy and alkoxy radical scavengers forming relatively stable phenoxy radicals, which again eliminate peroxy radicals. Reaction (4.55) ... 

In conditions such that a stationary concentration of radicals is maintained, the rate of the formation of radicals (u,-) equals the rate of their disappearance. Termination takes place solely by the interaction of peroxy radicals (reaction (42)), and... 

In the presence of oxygen, the chemiluminescence intensity (/CL) is significantly enhanced with respect to the emission produced under nitrogen. As the samples are highly oxidized in a diffusion-controlled reaction simultaneous to the emission, reaction (b) in Scheme 3.1 is very fast and the relative concentration of [POO ] will be larger in proportion to that of [P ]. The rate of oxidation (R,) in Equation 3.2 increases under these conditions, the bimolecular termination of peroxy radical, reactions (f) and (g) in Scheme 3.1, is, therefore, predominant. All these parameters can be used to evaluate the degradation in different materials and the effectiveness of antioxidants in the polymer stability. 

For the alkyl peroxy radicals, reaction (45a) can form the corresponding alkoxy (RO) radical together with N02, or the corresponding alkyl nitrate,... 

The cyclohexadienyl peroxy radical has an extremely weak C—02- bond ( 5 kcal mole-1) and X126 has been estimated to be 0.11 mole-1 at 50°C [73]. The removal of an H-atom by 02 (23 kcal mole.-1 exothermic) to form an olefinic bond, reaction (128), is undoubtedly slower than the formation of the peroxy radical, reaction (126), but it is not reversible and therefore it can be the major product-producing step. 

Other evidence for peroxy radical reaction with the carboxylic group is... 

Although inhibition of autoxidation by donation of hydrogen to peroxy radicals, reaction (5), is an important reaction, Boozer and Hammond [7] have suggested that inhibition by complex formation may also be important. Assuming that the major termination step involves reactions (9) and (10), and that the reaction is initiated by azobisisobutyro-nitrile (AIBN), then the rate of initiation is... 

Imagine starting with a given mixture of VOCs and NOx. Because OH reacts about 5.5 times more rapidly with NO2 than with VOCs, NOx tends to be removed from the system faster than VOCs.4 In the absence of fresh NOx emissions, as the system reacts, NOx is depleted more rapidly than VOCs, and the instantaneous VOC N02 ratio will increase with time. Eventually the concentration of NOx becomes sufficiently low as a result of the continual removal of NO by the 0H-N02 reaction that OH reacts preferentially with VOCs to keep the ozone-forming cycle going. At very low NOx concentrations, peroxy radical-peroxy radical reactions begin to become important. 

In general, the most important peroxy radical reaction is with NO. This reaction proceeds via two channels ... 

The alkoxy radicals produced from peroxy radical reactions with NO or with each other have, in general, three atmospheric fates [7] reaction with O2, dissociation, or isomerization. The reaction with oxygen. 

Self-Reaction Kinetics. Of all peroxy radical reactions, the self-reaction between two identical peroxy radicals is perhaps the most studied. The measurement of peroxy radical UV absorption cross sections, discussed above, often occurs under the assumption that all the chlorine or fluorine atoms produced by photolysis are converted quantitatively into peroxy radicals however, this assumption must be corrected for by the loss of peroxy radicals from self-reaction. Furthermore, studies of RO2 -b NO or RO2 -f HO2 reactions usually take place at sufficiently high RO2 concentrations to require knowledge of the self-reaction rate constant, in order to interpret the results of the kinetics measurements. Both concerns make laboratory studies of peroxy self-reaction kinetics an important issue. In contrast, the steady-state atmospheric concentrations of HCFC-based peroxy radicals are probably too small for their self-reactions to be relevant to atmospheric chemistry. In this context, the most important peroxy-peroxy radical reactions would be between the HCFC-based peroxy radicals and CH3O2, but such reactions have not been studied to date. 

Rate constants for FC(0)0, reactions are listed in Table 11. The reaction above has been studied by Wallington and co-workers [83]. The rate coefficient at 296 + 2 K for this reaction is 2.5 + 0.8 x 10" cm s" which is consistent with the measured rate coefficient for other peroxy radical reactions with NO [9]. 

Lipid hydroperoxide caused the oxidation of a-tocopherol to a-tocopherolquinone through an unidentified intermediate (40). Porter et al. (41) proposed a mechanism of oxidation that includes an intermediate from combination of a-tocopherol semiquinone with a peroxy radical (Reaction 0). 

Phenols with /7-substituents often lose them during CIO2 oxidation. For example, p-Cl, O2N-, -COOH, and -CHO groups were lost during the conversion of phenols to quinones (NAS, 1980 Carlson and Lin, 1985 Lin et al., 1984). The details of the mechanisms are not clear, but analogous ipso substitutions have been observed with HOCl and with peroxy radical reactions of phenols (Everly and Traynham, 1979 Larson and Rockwell, 1979 Bickel and Gersman, 1959). Electron-poor phenols such as di- and trinitro derivatives are unreactive toward CIO2 (Paluch, 1964), as are most other aromatic compounds that are not electron-rich. 

In the present work we attempt to gain a better understanding of the destruction mechanism of dibenzofuranyl + O2 system as well as the reaction pathways important in its oxygen-free decomposition. The difficulties encountered now are related to the large size of aromatic species contained in the dibenzofuranyl + O2 system making high level calculations very costly or not possible. The approach we have taken to circumvent the problem is to reduce the system to the smallest representative unit and then to proceed with the computation of the thermochemical properties (Figure 1.1). Based on this approach dibenzofuranyl peroxy radical (A) can be concentrated around the phenyl peroxy radical reaction system (B), which itself can be reduced to vinyl peroxy radical system (C). 

Stabilization is achieved by the fact that Reaction 1.72 competes with peroxy radical reactions in polymer degradation, such as Reaction 1.64, transforming the reactive peroxy radical into a much less reactive phenoxy radical, which, in turn, is capable of reacting with a second peroxy radical according to Reaction 1.73. 

The formation of associations between HALS and hydroperoxides followed by reaction of these associations with peroxy radicals (Reaction 1.101 and Reaction 1.102) represents another possibility of retarding photooxidation. 

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