Radicals from photolysis, reactions

Another important aspect that has been well addressed " is on the radiation-induced degradation of benzene and its derivatives in aqueous solution. The Advanced Oxidation Processes (e.g. Oj-HjOj, HjOj-UV, electron beam) make use of the highly reactive OH radical in the degradation of water pollutants. Radiation chemical methods are superior to other methods (Fenton or photolysis) in the generation of peroxyl radicals from the reaction of OH adducts. 

Conversion of reactive compounds to the corresponding radicals can occasionally be inferred from products of reaction. In most esr studies of dione photolyses these radicals have not been observed, presumably because of short lifetimes, but recently Zeldes and Livingston17 detected the dioxanyl radical from photolysis of biacetyl in dioxane solution. The wide variety of compounds which are converted to the corresponding radicals is of considerable theoretical interest since formation of many different types of radicals can occur under comparable conditions allowing qualitative and quantitative comparisons of their behaviour. 

Self-Reaction Kinetics. Of all peroxy radical reactions, the self-reaction between two identical peroxy radicals is perhaps the most studied. The measurement of peroxy radical UV absorption cross sections, discussed above, often occurs under the assumption that all the chlorine or fluorine atoms produced by photolysis are converted quantitatively into peroxy radicals however, this assumption must be corrected for by the loss of peroxy radicals from self-reaction. Furthermore, studies of RO2 -b NO or RO2 -f HO2 reactions usually take place at sufficiently high RO2 concentrations to require knowledge of the self-reaction rate constant, in order to interpret the results of the kinetics measurements. Both concerns make laboratory studies of peroxy self-reaction kinetics an important issue. In contrast, the steady-state atmospheric concentrations of HCFC-based peroxy radicals are probably too small for their self-reactions to be relevant to atmospheric chemistry. In this context, the most important peroxy-peroxy radical reactions would be between the HCFC-based peroxy radicals and CH3O2, but such reactions have not been studied to date. 

In spite of the numerous studies reported on photooxidation of polyolefins, the detailed mechanism of the complete process remains unresolved. The relative contribution by species involved in photoinitiation, the origins of the oxidative scission reaction, and the role played by morphology in the case of photoreactions in solid state are not completely understood. Primary initiator species in polyethylenes [123] and polypropylenes [124] are believed to be mainly ketones and hydroperoxides. During early oxidation hydroperoxides are the dominant initiator, particularly in polypropylene, and can be photolyzed by wavelengths in solar radiation [125]. Macro-oxy radicals from photolysis of polyethylene hydroperoxides undergo rapid conversion to nonradical oxy products as evidenced by ESR studies [126]. Some of the products formed are ketones susceptible to Norrish I and II reactions leading to chain scission [127,128]. Norrish II reactions predominate under ambient conditions [129]. Concurrent with chain scission, crosslinking, for instance via alkoxy macroradical combination [126], can take place with consequent gel formation [130,131]. 

The kinetics of the various reactions have been explored in detail using large-volume chambers that can be used to simulate reactions in the troposphere. They have frequently used hydroxyl radicals formed by photolysis of methyl (or ethyl) nitrite, with the addition of NO to inhibit photolysis of NO2. This would result in the formation of 0( P) atoms, and subsequent reaction with Oj would produce ozone, and hence NO3 radicals from NOj. Nitrate radicals are produced by the thermal decomposition of NjOj, and in experiments with O3, a scavenger for hydroxyl radicals is added. Details of the different experimental procedures for the measurement of absolute and relative rates have been summarized, and attention drawn to the often considerable spread of values for experiments carried out at room temperature (-298 K) (Atkinson 1986). It should be emphasized that in the real troposphere, both the rates—and possibly the products—of transformation will be determined by seasonal differences both in temperature and the intensity of solar radiation. These are determined both by latitude and altitude. 

We have seen that the observation of mixed radical combination products from mixtures of 2CHCOCH< 2 and CH2COCH2 upon photolysis indicates the decarbonylation to be stepwise rather than concerted, at least for these molecules. Further evidence in support of a stepwise process was reported by Robbins and Eastman in a study of p-methoxybenzyl ketone/64 Three products are isolated from this reaction ... 

Hydrogen halides will easily add to unsaturated compounds under radiolysis or photolysis. The free-radical chain reaction process is initiated by the dissociation of the halide or by the radiolytic production of radicals from the halide or the organic compound. Thus, for the radiolysis of a mixture of HBr and ethene the postulated initiation is... 

Nitroxyl radicals produced in the reactions of R02 with aminyl radicals react with peroxyl radicals. The latter reaction is considerably slower than the reaction of peroxyl with aminyl radicals, which can be seen from the following data derived by flash photolysis (the photolysis of bis(l,l-dimethylethyl)peroxide in toluene was performed in the presence of... 

Photolysis of DMDAF in benzene containing methyl alcohol gives the ether expected from the reaction of the singlet carbene. Monitoring this reaction by laser spectroscopy reveals that the detected transient reacts with the alcohol with a bimolecular rate constant very near the diffusion limits. In contrast, the transient reacts with triethylamine at least 100 times more slowly than it does with alcohol (Table 7). This behavior is inconsistent with identification of the transient as the cation or radical and points to its assignment as the singlet carbene. 

Experiments designed to utilize spin trapping to monitor free-radical chemistry in the gas phase were first reported by Janzen and Gerlock (1969). In these, radicals generated by photolysis in a stream of carrier gas were passed over solid PBN. The PBN was then dissolved in benzene, and the solution was found to contain spin adducts of radicals present in the gas stream. Photolysis of t-butyl hypochlorite vapour in this way leads to a nitroxide whose spectrum reveals splitting from two chlorine atoms. This proved to be due to butyl nitroxide (Janzen, 1971 Janzen et al., 1970), and recalls the observation of other nitroxides which apparently result from further reaction of the initial spin adducts. 

Photolytic. Photolysis products include carbon monoxide, ethylene, free radicals, and a polymer (Calvert and Pitts, 1966). Anticipated products from the reaction of acrolein with ozone or OH radicals in the atmosphere are glyoxal, formaldehyde, formic acid, and carbon dioxide (Cupitt,... 

The biperoxy radical produced by the ceric ion oxidation of 2,5-di-methylhexane-2,5-dihydroperoxide decays rapidly with first-order kinetics [k = ioio.e exp( -11,500 1000)/RT sec.1 = 180 sec."1 at 30°C. (30)]. After the first-order decay has run to completion, there is a residual radical concentration (—4% of the initial hydroperoxide concentration) which decays much more slowly by a second-order process. The residual second-order reaction cannot be eliminated or changed even by repeated recrystallization of the dihydroperoxide. This suggests that a small fraction of the biperoxy radicals react intermolecularly rather than by an intramolecular process and thus produce monoperoxy radicals. The bimolecular decay constant for this residual species of peroxy radical is similar to that found for the structurally similar radical from 1,1,3,3-tetra-methylbutyl hydroperoxide. Photolysis of the dihydroperoxide gave radicals with second-order decay kinetics which are presumed to be 2,5-hydroperoxyhexyl-5-peroxy radicals. 

We have demonstrated that intermolecularly, amidyl radicals preferentially abstract an allylic hydrogen rather than add to a TT bond of olefins such as cyclohexene and 1,3-pentadiene (33). This reactivity pattern is completely reversed in intramolecular reactions as shown in the following examples of alkenyl mitro-samide photolysis. In every case, the amidyl radicals generated from photolysis preferentially attack the ir bonds intramolecu-... 

TABLE 10.36 Calculated Atmospheric Lifetimes of Selected PAHs and Nitro-PAHs Due to Gas-Phase Reactions with the OH Radical, the N03 Radical, and Ozone and from Photolysis (from Arey, 1998a)... 

In their original investigations, Ayscough and Steacie42 stressed the simplicity of the reaction scheme and were unable to identify any product which could arise from the reactions of a trifluoroacetyl radical. Further, the ratio of quantum yields of carbon monoxide and hexa-fluoroethane was always close to unity, a fact which has been confirmed by later workers. Recently, Tucker and Whittle43 have shown that when hexafluoroacetone is photolyzed in the presence of excess bromine, trifluoroacetyl bromide is formed, suggesting that the trifluoroacetyl radical must intervene. The absence of hexafluorodiacetyl in the photolysis products of hexafluoroacetone is explained by the assumption that it is not stable. In fact, however, hexafluorodiacetyl may be prepared by the chromium trioxide oxidation of l,l,l,4,4,4-hexafluoro-2,3-di-... 

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