Peroxy acetyl radical

Two examples serve to illustrate these photochemical relationships. One member of the family of atmospheric peroxy radicals is the peroxy acetyl radical, CH3C(0)02. In at least the warm portion of the troposphere, PAN is near thermal equilibrium with the peroxy acetyl radical and NO . The equilibrium constant for this reaction has been measured in laboratory studies. Therefore, if concentrations of both PAN and N02 are measured, the concentration of these radicals can be calculated from the equilibrium constant and the ratio of the two nitrogen species as shown in Figure 2. The... 

Figure 2. Peroxy acetyl radical equilibrium chemistry and concentrations derived for a 4-day period during afield study in rural Pennsylvania. The two curves give model predicted concentrations for two different scenarios. (Adapted with permission from reference 8. Copyright 1991 American Geophysical Union.)...

Aeetie acid is produced photoehemieally mainly from reactions of the peroxy acetyl radical (CH3CO3) wifli oflier peroxy radieals. For example, the reaction of CH3CO3 with HO2 is known to lead to about 20% CH3C(0)OH (Tyndall et al, 2001). Aeetie acid in the gas phase is also produced by reaction of ozone with various olefins like propene, butene or pentene (Atkinson and Arey, 2003). A total photochemical source strength of 120 Tg/year has been reported (Baboukas et al, 2000). The contribution of direct emissions from anthropogenie (biomass eombustion, motor exhaust) and biogenie (bacteria metabolisms, emission from soil and vegetation) sourees is estimated at 48 Tg/year (Chebbi and Carlier, 1996). 

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). 

Step 1. Acetyl radicals react with oxygen to form peroxy acid radicals ... 

Step 2. The peroxy acid radicals abstract hydrogen from acetaldehyde molecules to yield peroxy acetic acid and new acetyl radicals ... 

Alternate loss pathways for the vinyl and acetyl radicals formed in these reactions are the bimolecular association reactions with molecular oxygen. They have similar reaction processes and exothermicity to the energized phenyl-peroxy [PhOO ] adduct in this study. 

These reaction pathways are in parallel with those for alkanes mentioned in Sect, 7.2.2, and the reactions (7.7, 7.8 and 7.9) after CH3 radicals are formed in reaction (7.43) are the same as those in the oxidation processes of methane described in Sect. 7.1. The specific feature of oxidation reactions of aldehydes is the formation of a metastable peroxy acyl nitrates from the reaction of peroxy acyl radicals with NO2 by reaction (7.41). In the case of acetaldehyde, peroxy acetyl nitrate, CH3C(0)00N02, is formed. This compound is called PAN (Peroxy Acetyl Nitrate), and is known to have much stronger toxicity to plants than ozone. A group of peroxy acyl nitrates are collectively called PANs. 

Like most radicals, this newly formed one is highly reactive, and almost immediately forms a bond to the oxygen molecules that surround and pepper it with collisions (Figure 24.4). The outcome is yet another radical. Lurking nearby we see an NO2 molecule, which is itself a radical. We see them collide and snap together to form the compound known as PAN, 4 (peroxy-acetyl nitrate). The chain of reactions ceases. PAN is highly soluble in water and moisture and is one of the principal eye irritants of photochemical smog. 

All evidence points to effectively exclusive abstraction of the aldehydic H by NO3 and Cl, as well as OH. The acetyl radical so formed reacts rapidly with O2 to form the acetyl peroxy radical, which in turn gives peroxyacetylnitrate (PAN) on reaction with NO2 and CH3 (and hence CH3O2 under atmospheric conditions) on reaction with NO ... 

Kaiser et al. (2009) examined the products formed in the presence of O2. Under their conditions the reactions were dominated by peroxy-peroxy radical reactions, but the results confirmed the validity of the general outline of the reactions contained in the master chemical mechanism for high NO c conditions, in which the 3-butanoyl radical reacts to form acetaldehyde and the acetyl radical ... 

The fate of the organic nitrate peroxy radical produced in reaction 14 is probably oxidation of NO to N02 and then decomposition, yielding acetyl nitrate, formaldehyde, and a hydroperoxy radical as shown in reactions 15 and 16. 

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). 

Perhydroxyl radical, 75 thermal generation from PNA of, 75 Peroxy radical generation, 75 Peroxide crystal photoinitiated reactions, 310 acetyl benzoyl peroxide (ABP), 311 radical pairs in, 311, 313 stress generated in, 313 diundecanyl peroxide (UP), 313 derivatives of, 317 EPR reaction scheme for, 313 IR reaction scheme for, 316 zero field splitting of, 313 Peorxyacetyl nitrate (PAN), 71, 96 CH3C(0)00 radical from, 96 ethane oxidation formation of, 96 IR spectroscopy detection of, 71, 96 perhydroxyl radical formation of, 96 synthesis of, 97 Peroxyalkyl nitrates, 83 IR absorption spectra of, 83 preparation of, 85 Peroxymethyl reactions, 82 Photochemical mechanisms in crystals, 283 atomic trajectories in, 283 Beer s law and, 294 bimolecular processes in, 291 concepts of, 283... 

The addition of the peroxyl radical to the double bond is governed by the electron density in the alkene bond and by electrophility of the radical. The rate constants of addition reactions increase with an increase of electron density on the double bond and with the increase of the electrophilic character of a radical (Table 6). The considerably larger electrophility of acyl peroxy radical (CH3CO3, C6H5CC>3) may explain by 5 orders faster addition of acyl peroxyl radicals [69] to a-methyl styrene at 20 °C. Electrophility of radicals leads to the marked reduction of activation energy of addition to alkenes methyl peroxyl radical has 47 kJ/mol, while acetyl peroxyl radical has 19 kJ/mol [70]. 

A study of the oxidation of acyl radicals is complicated by surface effects, the acyl peroxy radical appearing to undergo heterogeneous reaction. Nevertheless, some homogeneous reactions do take place, e.g., in the case of acetyl... 

For recent work on the liquid phase bimolecular acetyl peroxy-radical termination see refs. 81 and 83. 

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. 

Acetaldehyde reacts with the OH radical to form CH3CO, which reacts further with O2 forming the acetyl peroxy radical ... 

This acetyl peroxy radical can then react with NO or NO2 (236) ... 

A chain branching process can result from addition reactions of an alkyl peroxy radical to olefins or acetylenes, where vinyl, or acetyl peroxides are formed. The initial radical formed by the addition of the peroxy radical would undergo an addition reaction with molecular oxygen, then undergo molecular elimination of HO2 to form an unsaturated peroxide. The unsaturated peroxide would then undergo rapid cleavage of the weak peroxide bonds. A mechanism that implements this chain branching results from ROO addition to olefins is illustrated by ... 

Acetyl peroxy radicals resulting from the addition of oxygen to CH3C O react further with NO ... 

The reaction with NO leads to the formation of CO2 and a methyl radical that is oxidized to formaldehyde by reactions (16)-(20). In addition, the oxidation of CH3 regenerates HO c so that the oxidation cycle continues. Association with NO2 produces peroxyacetyl nitrate (PAN). Its lifetime is longer than that of alkylperoxy nitrates, but strongly temperature dependent, ranging from 1 hr at 298 K to 140 d at 250 K. Thus, PAN can be transported over a great distance before undergoing thermal decomposition. Under conditions of lowNOj concentrations acetyl peroxy radicals interact also with HO2 radicals... 

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