Peroxy radicals sources

Important peroxy radical sources include the reactions of the hydroxyl radical with various compounds, for example, carbon monoxide ... 

Peroxy radical recombination appears to be the most important source of the electronic excitation energy emitted during hydrocarbon autoxidation. In addition to the above-mentioned energetic considerations, this is clear from the following experimental facts the termination rate for secondary peroxy radicals is 103 times faster than for tertiary peroxy radicals due to their having no a-hydrogen 14> the termination rate constant decreases by 1.9 with a-deuteration 39 40>. 

The temporal profile of peroxy radicals at night follows that of N03 during the day, there are additional sources, of course, through OH and O, reactions. Carslaw et al. (1997) suggest that the reactions of H02, and perhaps CH3SCH202 (from DMS oxidation), with NO, at night may also be important. 

Faust, B. C., and J. M. Allen, Aqueous-Phase Photochemical Sources of Peroxy Radicals and Singlet Molecular Oxygen in Clouds and Fog, J. Geophys. Res., 97, 129I3-I2926 (1992). 

Schultz, M., M. Heitlinger, D. Mihelcic, and A. Volz-Thomas, Calibration Source for Peroxy Radicals with Built-In Actinome-try Using H20 and 02 Photolysis at 185 nm, . /. Geophys. Res., 100, 18811-18816 (1995). 

Tsubokawa et al. (12-14) have introduced radical sources of azo or peroxy groups by another methods, and successively conducted the radical polymerization of vinyl compounds, such as styrene or methyl methacrylate, to give polymer-grafted particles see Reaction (3). The grafting by the radical polymerization of methyl methacrylate, initiated from a peroxy group introduced on silica, takes place at relatively high efficiency, compared with those from azo group-introduced particles. 

Peroxy radicals are the only gas-phase source of peroxide compounds in the troposphere. 

The mechanisms behind lipid oxidation of foods has been the subject of many research projects. One reaction in particular, autoxida-tion, is consistently believed to be the major source of lipid oxidation in foods (Fennema, 1993). Autoxidation involves self-catalytic reactions with molecular oxygen in which free radicals are formed from unsaturated fatty acids (initiation), followed by reaction with oxygen to form peroxy radicals (propagation), and terminated by reactions with other unsaturated molecules to form hydroperoxides (termination O Connor and O Brien, 1994). Additionally, enzymes inherent in the food system can contribute to lipid oxidization. 

Nitrogen oxides (NOx= N02 and nitrogen monoxide NO) sources are mainly emitting NO into the troposphere. Thai, NO may be converted to N02 by reaction with hydrogen peroxy radical (H02) or with higher peroxy radicals (R02), produced from hydrocarbon oxidation. 

The first radical source we shall consider is thermal homolysis. At sufficiently high temperatures, all chemical bonds will break to form radicals, but in the temperature range of ordinary solution chemistry, below 200°C, the bonds that will do so at reasonable rates are limited to a few types, the most common of which are the peroxy bond and the azo linkage.42 Substances that produce radicals easily in a thermal process are designated initiators. Equations 9.10-9.14 illustrate a few typical examples with activation parameters. 

Nan Y, Rabani J, Henglein A (1976) Pulse radiolytic investigations of peroxy radicals produced from 2-propanol and methanol. J Phys Chem 80 1558-1565 Ingold KU, Paul T, Young MJ, Doiron L (1997) Invention of the first azo compound to serve a superoxide thermal source under physiological conditions concept, synthesis, and chemical properties. J Am Chem Soc 117 12364-12365... 

The phenomenon of catalyst-inhibitor conversion1 2,143,356 may be understood and critical concentration of metal can be deduced by reference to Eq. (280). If decomposition of the hydroperoxide is the source of initiation, it must be formed as rapidly as it is consumed to maintain a steady rate. If termination by metal complex predominates, a steady state occurs when the right-hand side of Eq. (280) equals unity. No oxidation will occur when this quantity is less than unity. Hence, catalyst-inhibitor conversion is observed as the metal concentration is increased to the point that the chain length becomes less than unity. If termination occurs by the bimolecular reaction of peroxy radicals, a chain length of less than unity will result in the depletion of the hydroperoxide until the rate of initiation has decreased to the point where the chain length is unity again. No inhibition is expected or observed. 

In the presence of traces of transition-metal ions, lipid hydroperoxides are a continuous source for the formation of new alkoxy and peroxy radicals which initiate new chain reactions and therefore act as amplifiers for the initial free radical event. 

There are two sources of tropospheric ozone. First, transport from the stratosphere in meteorological events known as tropospheric folding in which a layer of stratospheric air is entrained in tropospheric air-flow and mixed into the troposphere. Second, peroxy radical reactions which oxidize NO to N02. For example, in the OH radical initiated oxidation of CO ... 

Under certain conditions the atmospheric oxidation of one CH4 molecule can lead to the formation of four molecules of ozone. On a global scale, transport from the stratosphere accounts for approximately 10% of tropospheric ozone while peroxy radical chemistry is the source of the remaining 90% [9]. 

Another potential dark source of in the atmosphere, more particularly in the boundary layer, is from the reactions between ozone and alkenes. The ozonolysis of alkenes can lead to the direct production of the OH radical at varying yields (between 7 and 100%) depending on the structure of the alkene, normally accompanied by the co-production of an (organic) peroxy radical. As compared to both the reactions of OH and NO3 with alkenes the initial rate of the reaction of ozone with an alkene is relatively slow, this can be olfset under regimes where there are high concentrations of alkenes and/or ozone. For example, under typical rural conditions the atmospheric lifetimes for the reaction of ethene with OH, O3 and NO3 are 20 h, 9.7 days and 5.2 months, respectively in contrast, for the same reactants with 2-methyl-2-butene the atmospheric lifetimes are 2.0 h, 0.9 h and 0.09 h. 

---