Hydrogen abstraction, methane

The process is believed to initiate with formation of chlorine atom (either thermally or photochemically), which then abstracts a hydrogen from methane. 

The formation of dimethyl sulfide, dimethyl sulfone, and methane (by H-abstraction) observed in these photolyses is thus accounted for. Hydrogen abstraction by the methylsulfinyl radical affords methanesulfenic acid, CH3SOH, a very reactive molecule, which rapidly undergoes a series of secondary reactions to produce the methanesulfonic acid, methyl methanethiolsulfonate (CH3S02SCH3), and dimethyl disulfide which were also observed during these photolyses. 

Bernardi, F., Bottom, 1997, Polar Effect in Hydrogen Abstraction Reactions from Halo-Substituted Methanes by Methyl Radical A Comparison Between Hartree-Fock, Perturbation, and Density Functional Theories , J. Phys. Chem., 101, 1912. 

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. 

Not all C-H activation chemistry is mediated by transition metal catalysts. Many of the research groups involved in transition metal catalysis for C-H activation have opted for alternative means of catalysis. The activation of methane and ethane in water by the hexaoxo-/i-peroxodisulfate(2—) ion (S2O82) was studied and proceeds by hydrogen abstraction via an oxo radical. Methane gave rise to acetic acid in the absence of external carbon monoxide, suggesting a reaction of a methyl radical with CO formed in situ. Moreover, the addition of (external) CO to the reaction mixture led to an increase in yield of the acid product (Equation (ll)).20... 

The hydrogen abstraction from alkoxyamidyl 102 has been modeled at B3LYP/ 6-31G level by the reaction of methyl radical (R = CH3 ), with methoxy-formamidyl 102e giving 103e and methane. Energies are presented in Table 13. 

Hydrogen abstraction from methane by Cl and H occurs via the following bimolecular metathesis reactions ... 

In the presence of an amine, the principal method of forming methane is again by hydrogen abstraction from the amine. 

The simplified mechanism which accounts for this phenomenon is that the very high-energy radiation is able to abstract atoms from molecules, which then become unsaturated and can react further. This phenomenon was first reported in 1924 by Lind and Bardwell (L13), who bombarded simple saturated hydrocarbons up to and including butane with alpha particles. They found that hydrogen and methane in a ratio of 5 to 1 were eliminated to give higher hydrocarbons, up to those heavy enough to be liquids and solids. Low yields of condensates are obtained for these endothermic reactions. 

Cycloalkanes may be pyrolized in a manner similar to that for alicyclic alkanes. Cyclopentane, for instance, yields methane, ethane, propane, ethylene, propylene, cyclopentadiene, and hydrogen at 575°C. Analogous to cracking of alicyclic alkanes, the reaction proceeds by abstraction of a hydrogen atom followed by p scission. The cyclopentyl radical may undergo successive hydrogen abstractions to form cyclopentadiene. 

The above analysis indicates clearly that contrary to views expressed by many workers on methane oxidation, the termolecular reaction of CH3 + 02 must be of major importance at these temperatures. Benson (10) has expressed the view that the bimolecular reaction is unimportant even at high temperature and has suggested that the CH302 radical forms CH3O2H by hydrogen abstraction from H02, H202, or possibly H2. Preliminary analysis suggests that even if this were the case, a second-order... 

In contrast to the acetaldehyde decarbonylation, reactions with ethanol over Rh (111) did not lead to formation of methane but rather to an oxametallocycle via methyl hydrogen abstraction. These data suggest that ethanol formed over supported rhodium catalysts may not be due to hydrogenation of acetaldehyde. This study shows how surface science studies of model catalysts and surfaces can be used to extract information about reaction mechanisms since the nature of surface intermediates can often be identified by methods such as temperature programmed desorption and high resolution electron energy loss spectroscopy. 

Only a small percentage of the chlorine released by photolysis of CFCs is present in the active forms as Cl or CIO, however. Most of it is bound up in reservoir compounds such as hydrogen chloride and chlorine nitrate, formed respectively by hydrogen abstraction (equation 10) from methane and addition (equation 11) to nitrogen dioxide. Slow transport of these reservoir species across the tropopause, followed by dissolution in tropospheric water and subsequent rain-out, provide sink processes for stratospheric chlorine. 

---