Alkyl peroxy radicals, atmosphere

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

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 ... 

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... 

As with the methyl radical, the resulting alkyl (R) radical reacts rapidly, and exclusively, with 02 under atmospheric conditions to yield an alkyl peroxy radical (R02j [see the comprehensive reviews of the chemistry of R02 radicals by Lightfoot et al. (1992) and Wallington et al. (1992)] ... 

These alkyl peroxy radicals can be classed as primary, secondary, or tertiary depending on the availability of H atoms RCH200-(primary) RR CHOO- (secondary) RR R"COO-(tertiary). The alkyl radical-02 addition occurs with a room-temperature rate constant of > 10 12 cm3 molecule-1 s 1 at atmospheric pressure. Given the high concentration of 02, the R + 02 reaction can be considered as instantaneous relative to other reactions occurring such as those that form R in the first place. Henceforth, the formation of an alkyl radical can be considered to be equivalent to the formation of an alkyl peroxy radical. 

PROBABLE FATE photolysis photolysis to quinones is rapid, but is greatly hindered by adsorption, atmospheric and aqueous photolytic half-life 1-3 hrs, in the unadsorbed state, will degrade by photolysis from hours to days oxidation oxidation by alkyl peroxy radicals could compete with photolysis dissolved benzo (a) anthracene, photooxidation half-life in water 3.2-160 days photooxidation oxidation half-life in air 0.801-8.01 hrs hydrolysis not an important process volatilization to slow to compete with sorption as a transport process sorption very strong adsorption by suspended solids is the principal transport process, when released to water, will quickly adsorb to sediment or particulate matter biological processes short-term bioaccumulation is accompanied by metabolization, biodegradation is the principal fate, but occurs slowly... 

PROBABLE FATE photolysis dissolved portion may undergo photolysis to quinones, potential for reaction with alkyl peroxy radicals and hydroperoxy radicals which are photo-chemically produced in humic waters, atmospheric and aqueous photolytic half-life 3.8-499 hrs oxidation if chlorine and/or ozone is present in sufficient quantity, rapid oxidation should occur, photooxidation half-life in air 1.1-11 hrs hydrolysis not an important process volatilization probably too slow to compete with adsorption as a transport process sorption dominant transport process, on land, it is strongly adsorbed to soil, remains in the upper soil layers, in water it will adsorb to sediments and particulate matter in the water column biological processes bioaccumulation is short-term accompanied by metabolization, microbial biodegradation is the dominant fate, biodegradation expected to be very slow (half-life 2 yrs with acclimated microorganisms)... 

For longer chain alkanes ( 04) the reaction mechanism becomes more complex due to the IsomerIzatIons of the alkoxy radicals (47, 48), and to the fact that addition of NO to the alkyl-peroxy radicals (49) becomes more Important than NO to NO2 oxidation. For the alkenes and aromatic hydrocarbons the oxidation mechanisms In the atmosphere are more complex, and discussions of these systems, along with a more detailed treatment of the alkanes, are given later. 

In the remote marine boundary layer, the concentrations of NO are typically very low (ca. 5-10 pptv) [47] and reactions of CH3SCH2OO with other species may represent important reaction pathways. By analogy with the oxidation processes of other alkyl peroxy radicals, reactions of CH3SCH2OO with HO2 and organic peroxy radicals deserve consideration as potentially important atmospheric processes. 

Depending on the efficiency of CH3SCH2OO reaction with HO2, the CH3SCH2OO self-reaction and cross-reactions with other alkyl peroxy radicals could be of significance under some atmospheric conditions. Further, it is highly probable that the CH3SCH2OO self-reaction is important at night when the... 

Under relatively low NOx conditions in the atmosphere, a part of alkyl peroxy radicals, and hydroxyalkyl peroxy radicals react with HO2 to give hydroperoxy butane (pathways (f), (k)), and hydroxyhydroperoxy butane (pathway (q)). Thus, in oxidation reactions of alkane in the atmosphere, hydroperoxides, hydoxyhydor-peroxides, and hydroxyalkyl nitrate, could also be produced in addition to the normal aldehydes, ketones and alkyl nitrates. 

Short-chain alkyl hydroperoxides, which were mentioned in Section 16.7, occur at low levels under smoggy conditions, and even in remote atmospheres. It is possible that these species can oxidize DNA bases, causing adverse genetic effects. Alkyl hydroperoxides are formed under smoggy conditions by the reaction of alkyl peroxy radicals with hydroperoxy radical, H02, as shown for the formation of methyl hydroperoxide below ... 

The electron beam irradiation of poly(e-CL) and cross-linked PDXO in argon yielded a secondary alkyl ether radical and a tertiary alkyl radical, respectively. When the irradiation was carried out in an air atmosphere, peroxy radicals were detected in poly(e-CL) but not in PDXO (150). Oxygen permeabilities of poly(e-CL), and its tri-block copolymer poly(e-CL-b-PEG-b-e-CL) were in the range of 10 10 to 10 9 cm3 (STP) cm/cm2sec cm Hg [184]. 

Peroxy nitric adds and organic peroxy nitrates are another precursors of free radicals, which may be introduced into polymers from polluted atmosphere. They produce both peroxy radicals and reactive nitrogen oxides (NO and N02) on decomposition. With alkyl peroxynitrates, decomposition proceeds via OO—N bond fission having activation energy 87 kJ/mol, their half-life being several seconds at 0 °C [12]. 

A generic scheme for the atmospheric oxidation of a C2 haloalkane is given in Fig. 6. Values in parentheses are order of magnitude lifetime estimates. Reaction with OH radicals gives a halogenated alkyl radical which reacts with O2 to give the corresponding peroxy radical (RO2). As discussed in previous sections, peroxy radicals can react with three important trace species in the atmosphere NO, NO2, and HO2 radicals. 

Atmospheric oxygen could add to the alkyl radical before the formation of epoxide, resulting In a dlalkyl peroxide and a peroxy radical. 

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