Alkyl radicals, atmosphere

At room temperature, unsaturated vapours of the above specified polar and nonpolar liquids do not influence considerably the rate of adsorption and chemical activity of not only adsorbed oxygen layers, but also of acceptors of semiconductor electrons of another type, namely, of alkyl radicals [54]. This is seen from the electric conductivity of ZnO films with adsorbed alkyl radicals or oxygen being invariable in the atmosphere of the saturated vapours of the above specified solvents. In the case of oxygen, this can be also seen from the fact that the oxygen concentration features no decrease. 

Radical ion SO4- generated in situ from S2082- in aqueous solution at 100°C reacts with methane and ethane to generate alkyl radicals. These radicals react with another S04- to afford ROSO3-. Under an atmosphere of CO, the corresponding acid, RCOOH, can be obtained.25 Recently, it has been shown that it is possible to couple methane with... 

It has been generally accepted that the thermal decomposition of paraffinic hydrocarbons proceeds via a free radical chain mechanism [2], In order to explain the different product distributions obtained in terms of experimental conditions (temperature, pressure), two mechanisms were proposed. The first one was by Kossiakoff and Rice [3], This R-K model comes from the studies of low molecular weight alkanes at high temperature (> 600 °C) and atmospheric pressure. In these conditions, the unimolecular reactions are favoured. The alkyl radicals undergo successive decomposition by [3-scission, the main primary products are methane, ethane and 1-alkenes [4], The second one was proposed by Fabuss, Smith and Satterfield [5]. It is adapted to low temperature (< 450 °C) but high pressure (> 100 bar). In this case, the bimolecular reactions are favoured (radical addition, hydrogen abstraction). Thus, an equimolar distribution ofn-alkanes and 1-alkenes is obtained. 

Consequently, in an inert atmosphere/= 2(1 + k(lls/krcc) > 2. When phenoxyl radicals react only with peroxyl radicals, /= 2 and there is no regeneration. At low dioxygen pressures, phenoxyl radicals react with both peroxyl and alkyl radicals / ranges between 2 and 2(1 +kdis/krec) and increases with decreasing p02- In addition to this, the product of phenol oxidation, quinone, becomes the efficient alkyl radical acceptor at low dioxygen pressure (see earlier). 

As already seen, the reactions of OH, N03, and Cl with alkanes generate alkyl radicals. We shall see in the subsequent sections of this chapter that the production of alkyl radicals of various types is a general characteristic of organic oxidations. Here we will trace the atmospheric fates of typical alkyl radicals formed in OH, NO-, and Cl atom oxidations of alkanes. However, the principles are general and can be applied to those formed by the reactions of other organics such as alkenes as well. 

Alkenes. The available kinetic and mechanistic data show that under atmospheric conditions the reaction of HO radicals with alkenes proceeds predominantly via addition of the HO radical to the carbon-carbon double bond(s) [30]. The energized HO-adducts which result are rapidly thermalized to yield HO-substituted alkyl radicals which, in turn, undergo subsequent free-radical reactions leading to the formation of molecular products. Thus, the possible reactions in the HO-inidated oxidation are, in many respects, analogous to those of alkyl radicals described in the preceding section. Product studies on these reactions have mainly been made by the FTIR method [109-112]. 

Autoxidation is achemical reaction that usually takes place at ambient temperatures between atmospheric oxygen and a lipid substrate. In the presence of an initiator such as light, heat, or metal ions, unsaturated lipids (LH) form carbon-centered alkyl radicals (L ) ... 

The electron beam irradiation of poly(e-CL) and cross-linked PDXO in argon yielded a secondary alkyl ether radical and a tertiary alkyl radical, respectively. When the irradiation was carried out in an air atmosphere, peroxy radicals were detected in poly(e-CL) but not in PDXO (150). Oxygen permeabilities of poly(e-CL), and its tri-block copolymer poly(e-CL-b-PEG-b-e-CL) were in the range of 10 10 to 10 9 cm3 (STP) cm/cm2sec cm Hg [184]. 

For autoxidations conducted under 1 atmosphere of air, the ratio of peroxyl to alkyl radicals is very high, so Equation 8a is the only important termination (p. 291 in Reference 35.). 

The conversion of halides to alcohols is a typical SN1 or SN2 reaction in the polar reaction method, and generally the reactions require basic conditions. However, the conversion of halides to alcohols by the radical reaction method can be carried out under neutral conditions. The treatment of alkyl halides with Bu3SnH /AIBN in toluene under aerobic conditions (atmosphere) gives the corresponding alcohols, by means of the reaction of the alkyl radical with molecular oxygen, and the subsequent reduction of alkyl hydrogen peroxide (ROOH) with Bu3SnH (eq. 2.22) [52-57]. When 1802 is used instead of 1602 in... 

In an inert atmosphere, alkyl radicals are converted to alkanes by hydrogen transfer with solvent. Radicals can also undergo electron transfer oxidation by the metal oxidant and afford products (alkene, ester, etc.) ascribable to car-bonium ion intermediates,237 249, 288, 333 namely,... 

The majority of the reaction proceeds via the addition channel (b) to give a methyl hydroxycyclohexadienyl radical. Cyclohexadienyl radicals are resonance stabilized and are relatively unreactive. Typical alkyl radicals add O2 rapidly with rate constants of the order of 10 12, in contrast the reaction of cyclohexadienyl and methyl cyclohexadienyl radicals with O2 proceed with rate constants of (2 - 5) x 10 16 cm3 molecule 1 s 1 [67,68]. It can be calculated that in one atmosphere of air the cyclohexadienyl radicals have a lifetime of... 

A generic scheme for the atmospheric oxidation of a C2 haloalkane is given in Fig. 6. Values in parentheses are order of magnitude lifetime estimates. Reaction with OH radicals gives a halogenated alkyl radical which reacts with O2 to give the corresponding peroxy radical (RO2). As discussed in previous sections, peroxy radicals can react with three important trace species in the atmosphere NO, NO2, and HO2 radicals. 

Small alkyl radicals such as methyl CCH3) do not seem to play an important role in biology. (Except that it might be significant that methyl radicals are readily detected by ESR spectroscopy in various flints (cherts) (Griffiths et al., 1982). One wonders whether, in a methane-rich atmosphere, methyl radicals might be of some biological importance.)... 

A photoinduced oxidative addition of Bu"I and Pr I towards [Rh2(dicp)4] (dicp = 1,3-diisocyanopropane) using low-energy irradiation (553 nm) has been described, and proceeds under a nitrogen atmosphere with quantum yields of 25.2 and 22.6, respectively. Based on the kinetics measured in the presence and absence of quenchers, an electron-transfer chain mechanism has been proposed in which alkyl radicals are involved. " The same authors have also described a new methodology for the experimental determination of redox potentials and have applied this to measure the one-electron oxidation potentials of l-benzyl-l,4-dihydronicotinamide and of [Rh2(dicp)4p. ... 

A theoretical analysis of some selected radical reactions is discussed in the following sections. Of particular interest are the reactions of highly reactive species, which have been recognized as components of the polluted atmosphere,4"6 8 such as 02, NOx, OH, NH2, alkyl radicals, and halogen atoms. The choice of reactions is, of course, subjective and depended on the author s own experience and research interests. 

Reactions of nitric oxide with alkyl radicals are recognized as key reactions in the polluted atmosphere.8 They are also important in combustion processes, where the fate of NO is of great interest in the context of air pollution and possible De-NOx, processes. The reaction of nitric oxide with methyl radicals... 

Atmospheric oxygen could add to the alkyl radical before the formation of epoxide, resulting In a dlalkyl peroxide and a peroxy radical. 

Formaldehyde and other aldehydes and ketones formed during the atmospheric oxidation of more complex organic compounds are photolysed with a loss of a hydrogen atom or an alkyl radical. Photolysis of ketones (6.59) provides one way in which C-C bonds break another is the fragmentation of alkoxyl radicals mentioned earlier (reaction 6.40). Eventually, all organic compounds end up as carbon dioxide and water. 

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