Peroxy radical reaction with nitrate radical

There are a number of competing reactions, however, even at high NO, which promote conversion of RO2 into RO, such as the formation of nitrates in reaction of peroxy radicals with NO. Acetone is also produced by this route requiring an acetone yield higher than that found experimentally. 

It is well established that the reaction of peroxy radicals with NO2 proceeds by a simple association mechanism in which an alkyl peroxy nitrate is formed. The exothermicity associated with formation of the RO2 - NO2 bond resides initially in the RO2NO2 molecule. Collisional deactivation is required to remove this excess energy. 

The organic peracids and peroxides are produced by the same chemistry which forms the PANs and the alkyl nitrates the alkyl nitrates and PANs are formed from peroxyacyl radicals (RCO3) and peroxy radicals (RO2) under high-NOx conditions, whereas the peracids and peroxides are formed under low-NO conditions through reactions with hydroperoxy radical ... 

The titanium-mediated photocatalytic oxidation of a pyridine solution was conducted by Low et al. (1991). They proposed that the reaction of OH radicals with pyridine was initiated by the addition of a OH radical forming the 3-hydro-3-hydroxypyridine radical followed by rapid addition of oxygen forming 2,3-dihydro-2-peroxy-3-hydroxypyridine radical. This was followed by the opening of the ring to give At-(formylimino)-2-butenal which decomposes to a dialdehyde and formamide. The dialdehyde is oxidized by OH radicals yielding carbon dioxide and water. Formamide is unstable in water and decomposes to ammonia and formic acid. Final products also included ammonium, carbonate, and nitrate ions. 

As shown, NO3 radical leads to different chemistry than does HO radical the peroxy radical can decompose to yield several products, including acetaldehyde, formaldehyde, 1,2-propanediol dinitrate (PDDN), nitroxyperoxypropyl nitrate (NPPN), and a-nitrooxyacetone. The reactions of the peroxy radicals with NO , species can lead to highly functionalized (and oxidized) organic compounds. 

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... 

Measurements of these relatively minor species will not only complete the budget of NO, but will also indicate if our understanding of the hydrocarbon oxidation schemes in the atmosphere is complete. The organic nitrates that completed the NO, budget in the example in Figure 9 arose primarily from the oxidation of the naturally emitted hydrocarbon, isoprene (2-methylbutadiene). To demonstrate the oxidation mechanisms believed to be involved in the production of multifunctional organic nitrates, a partial OH oxidation sequence for isoprene is discussed. The reaction pathways described are modeled closely to those described in reference 52 for propene. The first step in this oxidation is addition of the hydroxyl radical across a double bond. Subsequent addition of 02 results in the formation of a peroxy radical. With the two double bonds present in isoprene, there are four possible isomers, as shown in reactions 2-5 ... 

The branching ratios shown are estimates based on reference 65. The further oxidation of each of the isomers is similar, so only the first isomer will be examined in detail. In continental areas with at least tens of pptrv s of NO, the peroxy radical produced in the first step will react with NO. This reaction can result in either oxidation of NO to N02 or production of an organic nitrate (bold type indicates organic nitrates that are assumed to be stable) ... 

Peroxy radicals are also formed in the troposphere through the photolysis of aldehydes (10, 11) and through nitrate radical (N03) reactions (12-14). The hydrogen atom and formyl radical that are formed then react with molecular oxygen (02) (reactions 11 and 12) under tropospheric conditions. 

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]. 

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

Peroxy radicals are stable with respect to reaction with the major constituents of the atmosphere. Their chemistry is dominated by reactions with peroxy radicals, HO2 and RO2, and with the nitrogen oxides, NO, NO2, and NO3. Two recent articles review the spectroscopy, kinetics, and reaction mechanisms of organic peroxy radicals [11,12], Three of the more important reaction channels are illustrated in Figure 1. The reaction with NO2 proceeds exclusively by addition to form the corresponding alkyl-peroxy nitrate. This reaction is generally not important in determining the end products of the degradation process due to the usually rapid dissociation rate of the peroxy nitrate however, it can be relevant to dynamical issues, because it sequesters N02-... 

Pathway (b) is always the minor channel. For small hydrocarbon peroxy radicals the branching ratio kjk increases from <0.014 for R = C2H5 to approximately 0.2 for n-hexane. As the hydrocarbon grows further, a branching ratio of kf,/k = 0.4 is approached (e.g., for n-octane) [12]. With respect to the HCFC-based peroxy radical plus NO reactions, there have been no reports of either a pressure dependence to the reaction or of observations of alkyl nitrate formation. Most of the studies base the conclusion that channel (b) is insignificant on the observation that the NO2 produced approximately equals the initial peroxy radical concentration. 

Reaction pathway a is spectilatlve, but is consistent with one of the proposed routes for formation of alkyl nitrates from alkyl peroxy radicals (49). Hence benzaldehyde and benzylnltrate are products resulting from H-atom abstraction from the CH3-substi-tuted group on toluene. Benzaldehyde will react further with the OH radical. In a manner analogous to the aliphatic aldehydes, to form the C6H5CO radical, which will. In the presence of NO2,... 

Reactions of the nitrate and hydroxyl radical are similar. Hydrogen abstra-tion produces an alkyl radical that would react with oxygen to give the peroxy radical ... 

The methylperoxy radical (and probably other peroxy radicals) reacts very rapidly with NO (4.41) relative to hydrogen abstraction (4.40), so only in very clean environments will the latter reaction occur to a significant extent (McFarland et al., 1979 Hanst and Gay, 1983). In polluted atmospheres, nitrogen oxides interfere with several other of the above reactions, leading to the formation of nitrate and peroxy-nitrate esters (see Section 4.A.4). 

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