Peroxy radicals reactions with organic compounds

Platt, U., G. Le Bras, G. Poulet, J. P. Burrows, and G. Moortgat, Peroxy Radicals from Nighttime Reaction of NOj with Organic Compounds, Nature, 348, 147-149 (1990). 

Peroxy radicals(R02) react with organic compounds either by H-atom transfer or addition to a double bond. These reactions have rate constants ranging from <0.01 to 300 M 1 s 1 at 25°C (Howard, 1972 Hendry et al., 1974 Neta et al., 1990) and are rarely important under environmental conditions because of the low average concentration of R02 in surface waters (Table 15.5). However, H-atom transfers from phenol OH or aniline NH have large... 

In remote atmospheres, where NOx concentrations are very low, reactions of peroxy radicals with HO2, or with other peroxy radicals, compete with the reaction with NOx and result in the formation of various oxidised compounds such as aldehydes, alcohols, organic acids and hydroperoxides. Several studies performed as part of LACTOZ have provided important data for modelling these processes in the atmosphere. 

Peroxy radicals from night-time reaction of NO3 with organic compounds. 

It has been speculated that aqueous solutions of aromatic amines can be oxidized by organic radicals, but there are no actual data on reaction rates. Based on a study of reaction rate data for compounds with structures similar to 3,3 -dichlorobenzidine, an estimate of the half-life of aromatic amines in water is approximately 100 days, assuming a peroxy radical concentration of 10 mole/L in simlit, oxygenated water (EPA 1975). Based on the oxidation rates of similar compounds, the direct oxidation of 3,3 -dichlorobenzidine by singlet oxygen in solution may be treated as a first-order reaction, to arrive at an estimated reaction constant of <4xlOVmole-hour (Mabey et al. 1982). The oxidation rate constant with... 

The chemistry of the troposphere (the layer of the atmosphere closest to earth s surface) overlaps with low-temperature combustion, as one would expect for an oxidative environment. Consequently, the concerns of atmospheric chemistry overlap with those of combustion chemistry. Monks recently published a tutorial review of radical chemistry in the troposphere. Atkinson and Arey have compiled a thorough database of atmospheric degradation reactions of volatile organic compounds (VOCs), while Atkinson et al. have generated a database of reactions for several reactive species with atmospheric implications. Also, Sandler et al. have contributed to the Jet Propulsion Laboratory s extensive database of chemical kinetic and photochemical data. These reviews address reactions with atmospheric implications in far greater detail than is possible for the scope of this review. For our purposes, we can extend the low-temperature combustion reactions [Equations (4) and (5)], whereby peroxy radicals would have the capacity to react with prevalent atmospheric radicals, such as HO2, NO, NO2, and NO3 (the latter three of which are collectively known as NOy) ... 

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. 

Reactivity ratios for all the combinations of butadiene, styrene, Tetralin, and cumene give consistent sets of reactivities for these hydrocarbons in the approximate ratios 30 14 5.5 1 at 50°C. These ratios are nearly independent of the alkyl-peroxy radical involved. Co-oxidations of Tetralin-Decalin mixtures show that steric effects can affect relative reactivities of hydrocarbons by a factor up to 2. Polar effects of similar magnitude may arise when hydrocarbons are cooxidized with other organic compounds. Many of the previously published reactivity ratios appear to be subject to considerable experimental errors. Large abnormalities in oxidation rates of hydrocarbon mixtures are expected with only a few hydrocarbons in which reaction is confined to tertiary carbon-hydrogen bonds. Several measures of relative reactivities of hydrocarbons in oxidations are compared. 

The technique of spin-trapping radicals has been applied to the measurement of atmospheric hydroxyl by Watanabe et al. (102), although there are no reports of its use for peroxy radicals. The principle involves the reaction of the radical of interest with an organic nitrone immobilized on a filter paper or other substrate. The sample is returned to the laboratory, and the nitrone-radical product is dissolved in a suitable solvent and measured with EPR. The disadvantages of the spin-trapping technique are difficulty in finding suitable organic nitrone compounds and the fact that most of these molecules are photochemically unstable. 

The dominant loss of OH radicals is reaction with CO and organic compounds such as CH4, both reactions produce peroxy radicals. Peroxy radicals play a key role in atmospheric chemistry. They are intimately involved in the formation and destruction of ozone and in the photooxidation of all organic compounds in the atmosphere [4], The lifetime of OH radicals with respect to reactions Eq. 3 and Eq. 8 is of the order of a second and in the day-time a steady state condition is established. The OH radical concentration in the atmosphere varies with location, time of day, season, and meteo-... 

Small carbonyl compounds are formed during the photochemical oxidation of many volatile organic compounds (VOC s), in urban as well as in rural areas. Photolysis and reaction with the OH radical are the most important initiation reactions for the atmospheric degradation of these compounds, and lead to the formation of peroxy radicals in the former case and either stable molecules and/or free radicals in the latter case (Finlayson-Pitts and Pitts, 1999). 

For measuring the steady-state concentration of organic peroxy radicals (ROO ) produced in sunlit natural waters series of antioxidants, such as poly-methylphenols, have been successfully applied as selective probe compounds (Faust and Hoigne, 1987). The rates of transformation have shown that the steady-state concentration of the apparent photooxidants increases with the amount of light absorbed by the DOM. The sink for ROO has not been identified, but kinetic evidence is that DOM, even when the DOC amounts up to 5 mgL"1, does not control the lifetime of the peroxy radicals. The following reaction scheme summarizes the results of the kinetic analysis for peroxy radical formation ... 

---