Peroxy radical reaction with

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

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. 

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]. 

Reaction 36 may occur through a peroxy radical complex with the metal ion (2,25,182). In any event, reaction 34 followed by reaction 36 is the equivalent of a metal ion-cataly2ed hydrogen abstraction by a peroxy radical. 

In addition, another computational study in the frame of DFT, using the hybrid functional MPW1K,53 had suggested that o-QM may be an intermediate in the reaction of the peroxy radical (HO2") with the benzyl radical at the ortho-position (Scheme 2.16),54 which should be significant in atmospheric processes and low-temperature combustion systems (T < 1500 K). 

In the presence of propane (C3H8), the reaction mechanism is initiated by hydrogen abstraction from C3H8 by OH radicals, producing alkyl radicals, which then rapidly react with 02 to form peroxy radicals [88], The peroxy radicals react with NO and oxidize it to N02 ... 

Since the reactions occur under oxygen saturation, the principal stabilizing steps are the interactions of the HALS derivatives with the alkoxy and peroxy radicals and with the hydroperoxides. 

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 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) ... 

For example, methylsulfinyl peroxynitrate, CH3S(0)00N02, has been observed by FTIR (e.g., see Barnes et al., 1987 Hatakeyama, 1989 and Jensen et al., 1992), presumably from reaction (59) followed by reaction of the peroxy radical formed with NOz. 

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). 

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. 

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... 

In the atmosphere peroxy radicals react with NO, NO2, HO2 radicals and other peroxy radicals (R 02). The importance of these reactions is dictated by the abundances of NO, NO2, and HO2 radicals and by the rates of the reactions of RO2 radicals with these species. In the troposphere the concentrations of NO, NO2, and HO2 vary widely, however, for the present purposes reasonable average concentrations are approximately (2.5—10) x 10s cm-3. Under atmospheric conditions, typical rate constants for the reactions of RO2 radicals with NO, NO2, and HO2 radicals lie in the ranges (8-20)xlO-12, (5-10) xlO 12, and (5-15)xl0 12 cm3molecule 1 s, respectively [4]. Hence, on average these reactions are of comparable importance in the atmospheric fate of RO2 radicals. On a local scale one reaction may dominate because of variation in the concentrations of NO (NO and NO2) and HO2 radicals. Thus, in remote marine locations with low NO levels, reaction of RO2 radicals with HO2 will be much more important than in urban air masses with high NO concentrations. 

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 ... 

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]. 

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) ... 

Reactions (5)-(8) and (10) lead to the formation of stable molecules (hydrocarbons and aldehydes). Subsequent reactions of peroxy- (CnH2n+i02) and oxy-radicals (CnH2n+iO) formed in reactions (9) and (11) lead to the formation of oxygenates (alcohols, aldehydes, etc.), carbon oxides, and/or olefins. The fractions of radicals transformed into different final products depend on the reaction conditions (temperature, oxygen pressure) and on the number of carbon atoms in the alkane molecule. For example, the stability of peroxy radicals decreases with increase of the number of carbon atoms in the alkyl fragment, that is why the probability of total oxidation via their subsequent transformations decreases from methane to... 

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 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. 

Both chlorine peroxy radical (ClOO) and chlorine dioxide molecule (OCIO) play a pivotal role in the destruction of O3 and in the combustion of AP [20-21]. They can be formed by the reactions of Cl and CIO radical reactions with O2 and S-CIO3. OCIO is also a potential intermediate in the O -I- CIO reaction under high-pressure conditions, although such a possibility has not been assumed in most studies under normal laboratory experimental conditions. 

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

The most important daytime loss process for the biogenics is reaction with OH radicals. Rate constants for the OH reaction with isoprene, a-and /3-pinene are 1.01 X 10-10 cm3 molecule-1 s-1, 5.37 X 10-11 cm3 molecule-1 s-1, and 7.89 X 10 11 cm3 molecule-1 s-1, respectively. Hydroxyl radical reactions with dialkenes and monoterpenes proceed primarily via OH addition across the double bond. Subsequent addition of oxygen to the radical produces a peroxy radical. The reactions of the resulting peroxy radical proceed in a manner similar to those of alkyl peroxy radicals. 

Ketone oxidation chains terminate when two peroxy radicals react with each other. This is the main reaction of chain termination if the ketone contains no inhibitor and the oxygen pressure is sufficiently high for fast conversion of R- to R02-. The values of kt measured by the chemiluminescence technique [81] are shown in Table 10. 

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. 

Alkyl peroxy radicals react with NO2 by combination to yield the peroxynitrates (recall reaction 5.41),... 

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

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