Methane reaction + radicals

The groups at the termini of the 1,4-pentadiene system also affect the efficiency and direction of the the di-7c-methane reaction. The general trend is that cyclization oceurs at the diene terminus that best stabilizes radical character. Thus, a terminus substituted with aryl groups will cyclize in preference to an unsubstituted or alkyl-substituted terminus ... 

Direct conversion of methane to ethane and ethylene (C2 hydrocarbons) has a large implication towards the utilization of natural gas in the gas-based petrochemical and liquid fuels industries [ 1 ]. CO2 OCM process provides an alternative route to produce useful chemicals and materials where the process utilizes CO2 as the feedstock in an environmentally-benefiting chemical process. Carbon dioxide rather than oxygen seems to be an alternative oxidant as methyl radicals are induced in the presence of oxygen. Basicity, reducibility, and ability of catalyst to form oxygen vacancies are some of the physico-chemical criteria that are essential in designing a suitable catalyst for the CO2 OCM process [2]. The synergism between catalyst reducibility and basicity was reported to play an important role in the activation of the carbon dioxide and methane reaction [2]. 

These results provide strong evidence in favor of the involvement of radical-anions in electron-donor-sensitized l-aza-di-rr-methane rearrangements of aza-dienes 101, 117, 118, and 119 and also in the di-rr-methane reaction of diene 124. These observations open new lines of research in an area in which, due to the large number of smdies carried out for more than 30 years, apparendy there were very few things that remained to be uncovered. Further studies are in progress to determine the scope and synthetic applications of these reactions. 

One important prerequisite to the application of this reaction in hydrocarbon synthesis is the selective monochlorination of methane. Usual radical chlorination of methane is not selective, and high CH4 CI2 ratios are needed to minimize formation of higher chlorinated methanes (see Section 10.2.5). In contrast with radical halogenation, electrophilic halogenation of methane was shown to be a highly selective process.412... 

A final possibility for a type of reaction resulting in acetylene formation which must be considered is that of combination of radicals. This is not likely to be significant in most systems, but in the case of methane reactions it becomes of particular interest. The production of ethylene, and acetylene in the thermal decomposition of methane requires some sort of combination reaction (40, 65). Kassel suggested a mechanism involving methylene radicals ... 

Within the area of SET-promoted di-7t-methane reactions, recent studies have shown that irradiation of 1-aza-1,4-dienes, such as 32, and the 1,4-diene 34, using AW-dimethylaniline (DMA) as electron-donor sensitizer, leads to production of the corresponding cyclopropane derivatives 33 and 35 resulting from 1-ADPM and DPM rearrangements, respectively, in reactions that take place via radical-anion intermediates (Sch. 11) [25]. 

Choi, S.-Y., Eaton, P. E., Kopp, D. A., Lippard, S. J., Newcomb, M., and Shen, R., 1999, Cationic species can be produced in soluble methane monooxygenase-catalyzed hydroxylation reactions radical intermediates are not formed, J. Am. Chem. Soc., in press. 

The slow rate of the methane reaction could then be ascribed to the exceptionally high ionization potential of the methyl radical (10.0 ev.) In turn, the high rate of the Si-H reaction might be explained by the... 

Kebarle, Hiraoka, and coworkcrs applied pulsed electron-beam MS to study the existence and structure of CH5 (CH4) cluster ions in the gas phase. These CHs iCIF), clusters were previously observed by MS by Field and Beggs. Enthalpy and free energy changes they determined are compatible with the Cj symmetrical structure. Recently, Jung and coworkers utilizing electron ionization MS explored ion-molecule reactions within ionized methane clusters including CI I, (CH4), which was found to be the most abundant. This cluster is believed to be the product of the intracluster ion-molecule reaction depicted in Equation (5.6), which includes the methane dimer radical ion 8 ... 

Ni-C-H bend at 770 cm 1 and the C-H stretch at 3000 cm-1. Above 700 K, the CH species dissolve Into the bulk nickel. The importance of the very large stabilities of C2H2 and CH fragments is that these species, not methyl radicals, are present in major concentrations at the temperatures at which steam reforming and methanation reactions are carried out, at least in the absence of lateral interactions. 

Essentially all the simple geminal di- or poly halides studied have been derivatives of methane. Reaction of diiodomethane (25,103) with triethyl phosphite proceeds normally, to furnish tetraethyl methanedi-phosphonate and iodomethanephosphonate. However, the reaction between carbon tetrachloride and trialkyl phosphite, first investigated by Kamai and Egorova (156,181), is catalyzed by peroxides or ultraviolet light, and accordingly it has been formulated by Kamai and Kharrasova (157) as a radical-chain process. 

Carbon monoxide may be hydrogenated to produce either alcohols or hydrocarbons, depending on the catalysts used and the reaction conditions. Temperatures ranging from 100-400°C and pressures as high as 1,000 atm have been studied. Different catalysts yield radically different types of products. Important processes for uch reactions consist of the methanol synthesis, the higher-alcohol synthesis (or the variation termed the iso synthesis), the Fischer-Tropsch reaction (or the version called hydrocarbon synthesis), and the methanation reaction. These syntheses were discovered in the period 1920-1925, at which time the methanol and higher-alcohol syntheses developed rapidly. A brief summary of processes and conditions used for the hydrogenation of carbon monoxide is presented in Table 10-17. 

Note that chlorine radicals are regenerated in reaction (14.5) and that these then react with more methane (reaction (14.4)). In this way the reaction gets faster and faster (a chain reaction) which may cause the reaction mixture to explode. But the reaction cannot go on indefinitely and eventually termination reactions, such as CH3 (g) + CH3 (g) CsHelg) or... 

The stereochemistry of the di-7r-methane reaction has been investigated. Reactions via the singlet state are stereospecific with respect to the stereochemistry about the participating double bonds." This stereospecificity means that any diradical species formed by 2,4-bridging must not live long enough for stereochemical equilibration at the radical center. Stereospedficity is also exhibited with respect to... 

Figure 7.19 Simultaneous LIF excitation spectra of the reaction radicals CHjO, CN, and CFI in a methane-air burner flame. Wavelength scales coi is the fundamental wave of dye laser 2coi is the second harmonic wave coi + W2 is the sum frequency wave of dye laser plus fundamental wave of Nd YAG laser (all in units of nanometres). The PLIF images reveal the predominance of various reaction radicals in different parts of the flame. Data adapted from Bombach and B. Kappeli AppL Phys. B, 1999, 68 251, with permission of Springer Science and Business Media...

Developing the concepts of a radical mechanism of the methane conversion in electric discharge [366], the dominant role in this reaction has been attributed to atomic hydrogen, uggesting that at low current densities, acetylene is formed mainly as a result of the dehydrogenation action of atomic hydrogen which is converted to methane CHg radicals are... 

An important role is played by the abstraction of H atoms from hydrocarbon molecules and radicals, especially from methane (reaction (9—16)). The side reaction of CH3NO2 formation leads to a depletion of NOx and may even cause a temporary reduction in the hydrocarbon conversion rate. 

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