Radicals, and reaction

Clearly, whether or not ozone is formed depends also on the rate at which, for example, unsaturated hydrocarbons react with it. Rates of reactions of ozone with alkanes are, as noted above, much slower than for reaction with OH radicals, and reactions with ozone are of the greatest significance with unsaturated aliphatic compounds. The pathways plausibly follow those involved in chemical ozonization (Hudlicky 1990). 

The 3-position of coumarin is the most susceptible to attack by radicals and reaction with 1,3-diphenyltriazene gives 3-phenylcoumarin (79JHC97). The reaction of sulfuryl... 

Asycough et al. (8) studied the competitive photochlorination of mixtures of cts-1,2-dichloroethylene and either vinyl chloride or trichloroethylene. An expression for the ratio of the rates of formation of the photochlorination products from the two olefins competing for Cl atoms was derived. This was based on the mechanism which included reactions of the hot radicals and reactions (1), (2 ), and (3)... 

Gas- and particle-phase organic compounds can undergo chemical change by a number of routes (Atkinson, 1995, 1996). For gas-phase chemicals, these involve photolysis, reaction with ozone (03), reaction with the hydroxyl (OFI) radical, and reaction with the nitrate (N03) radical (Atkinson, 1995). (For a discussion of photolysis, see Chapter 15). 

For the majority of gaseous organic compounds, the potential loss or transformation processes in the troposphere are wet and dry deposition, photolysis, reaction with OH radicals, reaction with N03 radicals, and reaction with 03. The overall lifetime, xoverall, of a chemical present in the gas phase is then given by ... 

II/III/IV Organometallic radicals and reactions, excluding transition elements and radical ions literature survey 1970-1971 111... 

A positive charge perturbs the electron distribution and thus reduces the aromaticity of a six-membered cationic ring. As expected, reaction with free radicals and reactions via cyclic transition states (both intra- and intermolecular) are facilitated. The uptake of an electron to form a neutral radical is especially easy. 

In addition to OH radicals, unsaturated bonds are reactive towards O3 and NO3 radicals and reaction with these species is an important atmospheric degradation mechanism for unsaturated compounds. Table 4 lists rate constants for the reactions of 03 and NO3 radicals with selected alkenes and acetylene. To place such rate constants into perspective we need to consider the typical ambient atmospheric concentrations of O3 and NO3 radicals. Typical ozone concentrations in pristine environments are 20-40 ppb while concentrations in the range 100-200 ppb are experienced in polluted air. The ambient concentration of NO3 is limited by the availability of NO sources. In remote marine environments the NO levels are extremely low (a few ppt) and NO3 radicals do not play an important role in atmospheric chemistry. In continental and urban areas the NO levels are much higher (up to several hundred ppb in polluted urban areas) and NO3 radicals can build up to 5-100 ppt at night (N03 radicals are photolyzed rapidly and are not present in appreciable amounts during the day). For the purposes of the present discussion we have calculated the atmospheric lifetimes of selected unsaturated compounds in Table 4 in the presence of 100 ppb (2.5 x 1012 cm 3) of O3 and 10 ppt (2.5 x 108 cnr3) of NO3. Lifetimes in other environments can be evaluated by appropriate scaling of the data in Table 4. As seen from Table 4, the more reactive unsaturated compounds have lifetimes with respect to reaction with O3 and NO3 radicals of only a few minutes ... 

Trifunac, A. D. EPR and NMR detection of transient radicals and reaction products. In The Study of Fast Processes and Transient Species by Electron Pulse Radiolysis, Baxendale, J. H. Busi, E Eds. Proceedings of the ASI (NATO), Capri Italy, 1982. 

Although this paper is primarily concerned with inorganic free radicals and reactions of inorganic free radicals, it also discusses some unstable nitrogen compounds, and metastable atoms and molecules. The production, detection, and identification of free radicals, and the determination of bond-dissociation energies for the radicals, are considered in some detail. Radicals produced by heterogeneous reactions, such as the catalytic reactions on heated filaments in the mass spectrometer ion source, are not considered. 

In principle, heavy radicals could undergo also H-abstraction, addition on unsaturated bonds and recombination reactions. It is quite easy to demonstrate how little relevance these reactions have compared with the isomerization and decomposition ones. This helps drastically reduce the total number of radicals and reactions to be considered. All of the intermediate alkyl radicals, higher than C4, are supposed to be instantaneously transformed into their final products. With reference to the primary products of Table III, the heavy radicals from pentyl up to octyl undergo direct isomerization and decomposition reactions to form smaller radicals and alkenes. Therefore, large sections of the kinetic scheme can be reduced to a few equivalent or lumped reactions whilst still maintaining a high level of accuracy. The complete kinetic scheme shown in Fig. 2 can be then simply reduced to this single, equivalent or lumped reaction ... 

Decomposition of the H0CH2CH20- radical and reaction with 02 are competitive processes with about 72% of the reaction proceeding by the decomposition path. For alkenes with carbon numbers greater than 3, reaction of the /3-hydroxyalkoxy radical with 02 is negligible compared with decomposition. 

Aldehydes are emitted by combustion processes and also are formed in the atmosphere from the photochemical degradation of other organic compounds. Aldehydes undergo photolysis, reaction with OH radicals, and reaction with N03 radicals in the troposphere. Reaction with N03 radicals is of relatively minor importance as a loss process for these compounds, but can be a minor contributor to the H02 (from formaldehyde) and peroxyacetyl nitrate (PAN) formation during nighttime hours (Stockwell and Calvert, 1983 Cantrell et al., 1985). Thus, the major loss processes involve photolysis and reaction with OH radicals. 

The benzophenone-sensitized photo-oxygenation of 2-propanol (53) may be considered as a typical example of a Type I process, in which the electronically excited benzophenone initiates a free radical oxidation by abstracting a hydrogen atom from 2-propanol. The initiation is then followed by O2 addition to the 2-hydroxyisopropyl radical and reaction... 

In most cases, free radical oxidations of hydrocarbons can be described by the reactions of the peroxy radical (ROp with a hydrocarbon (RH), leading to the formation of an alkyl hydroperoxide (ROOH) (Eqs. (4.4) and (4.5)) [11-14]. The chain oxidation is initiated by usually very slow initiation reaction (Eq. (4.3)) which may occur via different mechanisms and is only important in the absence of ROOH at the very begiiming of oxidation. Once trace amounts of ROOH are formed in reaction (4.5), its decomposition becomes the main source of radicals and reaction (4.3) becomes irrelevant. 

New evidence suggests that rearrangements in P-ketoalkyl radicals and reactions of 0( P) atoms with butadiene involve little, if any, participation of cyclopropoxy radicals. ... 

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