Alkoxy radicals, atmosphere

Platz J, OJ Nielsen, J Sehested, TJ Wallington (1995) Atmospheric chemistry of l,Ll-trichloroethane UV absorption spectra and self-reaction kinetics of CCljCHj and CCI3CH2O2 radicals, kinetics of the reactions of the CCljCHjOj radical with NO and NOj, and the fate of alkoxy radical CCI3CH2O. J Phys Chem 99 6570-6579. 

Cox, R.A., Patrick, K.F., and Chant, S.A. Mechanism of atmospheric photooxidation of organic compounds. Reactions of alkoxy radicals in oxidation of n-butane and simple ketones, Environ. Sci Technol, 15(5) 587-592,1981. 

As shown, peroxy radical chemistry plays a substantial role in low-temperature combustion as opposed to the alkoxy radical chemistry of high-temperature combustion. Thus, the peroxy radicals constitute an important class of reactive intermediates with significant implications for low temperature combustion and atmospheric reactions. 

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. ... 

There is evidence that in some cases, the alkoxy radical formed in the R02 + NO reaction contains sufficient excess energy that it can decompose under atmospheric conditions. This is the case, for example, for some of the alkoxy radicals formed in the oxidation of alternate CFCs (see Chapter 13.D.2a). It has also been postulated for the alkoxy radical formed from the NO reaction with H0CH2CH202, formed in the OH + C2H4 reaction (Orlando et al., 1998). In the latter case, about 25% of the excited (H0CH2CH20) decomposes to HCHO + CH2OH, with the remainder being stabilized. The stabilized radicals then decompose to HCHO + CH2OH or react with 02. 

The reactions of R02 with NO and with R02 generate alkoxy radicals (RO). Alkoxy radicals have several possible atmospheric fates, depending on their particular structure. These include reaction with 02, decomposition, and isomerization as we shall see, reactions with NO and N02 are unlikely to be important under most tropospheric conditions. Atkinson et al. (1995b) and Atkinson (1997b) have reviewed reactions of alkoxy radicals and /3-hydroxyalkyl radicals ... 

Atkinson, R., Atmospheric Reactions of Alkoxy and /3-Hydroxy-alkoxy Radicals, Ini. J. Chem. Kinet., 29, 99-111 (1997b). 

Eberhard, J., C. Muller, D. W. Stocker, and J. A. Kerr, Isomerization of Alkoxy Radicals under Atmospheric Conditions, Environ. Sci. Technol., 29, 232-241 (1995). 

Jungkamp, T. P. W J. N. Smith, and J. H. Seinfeld, Atmospheric Oxidation Mechanism of n-Butane The Fate of Alkoxy Radicals, ... 

Sehested, J., and T. J. Wallington, Atmospheric Chemistry of Hydrofluorocarbon 134a. Fate of the Alkoxy Radical CF,0, Environ. Sci. Technol., 27, 146-152 (1993). 

The further decomposition of acetyl nitrate in the atmosphere has not been studied. The oxidation of isoprene by the hydroxyl radical proceeds via repeated steps of OH addition across the double bond, followed by addition of 02 to form a peroxy radical. The peroxy radical then either oxidizes NO to N02 or adds NO to form an organic nitrate. The alkoxy radical produced in the former step underwent decomposition to form both stable and reactive products. A number of possible pathways exist for forming presumably stable organic nitrates (bold in reactions 7 through 16). 

With current estimates of the rates of alkoxy radical reactions [27], isomerization is likely to be the more important of these latter two processes. Clearly, a better understanding of these reactions is required before their role in the atmospheric degradation of aromatic hydrocarbons can be assessed. 

The alkoxy radicals formed in pathway Eq. 26a have very interesting atmospheric chemistry. The atmospheric fate of alkoxy radicals differs with the nature of the R group. Some alkoxy radicals (e.g., CH3O) are lost solely via reaction with 02, others undergo rapid decomposition via C-C bond scission. Long chain alkoxy radicals can undergo isomerization via intramolecular H-atom abstraction ... 

The complexity associated with unraveling the precise degradation mechanism of any given alkane can be appreciated by considering the case of n-hexane. Initial OH radical attack leads to the formation of three different alkoxy radicals, each of which can either react with O2, decompose, isomerize (via several possible pathways), or undergo a combination of these possible loss processes. Our understanding of the atmospheric chemistry of alkoxy radicals is rather crude at present and this is an area of active research [49]. 

More than 140 different alkenes have been identified in the atmosphere [27]. The sources of alkenes are similar to those for the alkanes with combustion of fossil fuel being a major source. The presence of unsaturated bonds makes these compounds much more reactive than the alkanes. The most persistent member of this class of compounds (ethene) has an atmospheric lifetime of the order of a day, while more typically the lifetimes for alkenes are measured in hours. As a result of their short lifetimes the atmospheric concentrations of alkenes are highly variable and decrease dramatically away from their source locations. The mechanisms of atmospheric oxidation of alkenes have recently been reviewed [55]. As with the alkanes the reaction of OH radicals is an important loss mechanism. This reaction proceeds mainly via addition to the unsaturated bond as illustrated for ethene in Fig. 4. In one atmosphere of air at 298 K the dominant atmospheric fate of the alkoxy radical HOCH2CH2O is decomposition via C - C bond scission, while reaction with O2 makes a 20% contribution [56]. The fate of alkoxy radicals resulting from addition of OH to alkenes is generally decomposition via C - C bond scission [8]. Thus, the OH radical initiated oxidation of propene gives acetaldehyde and HCHO, oxida-... 

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]. 

Before moving on to consider the fate of the carbonyl products, it is appropriate to discuss the atmospheric fate of CF30 radicals. The usual modes of alkoxy radical loss are not possible for CF30 radicals. Reaction with O2 and decomposition via F atom elimination are both thermodynamically impossible under atmospheric conditions. Instead, CF3O radicals react with NO and hydrocarbons. 

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