Methyl radical methane

As the table indicates C—H bond dissociation energies m alkanes are approxi mately 375 to 435 kJ/mol (90-105 kcal/mol) Homolysis of the H—CH3 bond m methane gives methyl radical and requires 435 kJ/mol (104 kcal/mol) The dissociation energy of the H—CH2CH3 bond m ethane which gives a primary radical is somewhat less (410 kJ/mol or 98 kcal/mol) and is consistent with the notion that ethyl radical (primary) is more stable than methyl... 

Like carbocations most free radicals are exceedingly reactive species—too reac tive to be isolated but capable of being formed as transient intermediates m chemical reactions Methyl radical as we shall see m the following section is an intermediate m the chlorination of methane... 

Chlorine atom Methane Hydrogen chloride Methyl radical... 

Termination steps are m general less likely to occur than the propagation steps Each of the termination steps requires two free radicals to encounter each other m a medium that contains far greater quantities of other materials (methane and chlorine mol ecules) with which they can react Although some chloromethane undoubtedly arises via direct combination of methyl radicals with chlorine atoms most of it is formed by the propagation sequence shown m Figure 4 21... 

Methane oxidations occur only by intermediate and high temperature mechanisms and have been reported not to support cool flames (104,105). However, others have reported that cool flames do occur in methane oxidation, even at temperatures >400 ° C (93,94,106,107). Since methyl radicals caimot participate in reactions 23 or 24, some other mechanism must be operative to achieve the quenching observed in methane cool flames. It has been proposed that the interaction of formaldehyde and its products with radicals decreases their concentrations and inhibits the whole oxidation process (93). 

A typical oxidation is conducted at 700°C (113). Methyl radicals generated on the surface are effectively injected into the vapor space before further reaction occurs (114). Under these conditions, methyl radicals are not very reactive with oxygen and tend to dimerize. Ethane and its oxidation product ethylene can be produced in good efficiencies but maximum yield is limited to ca 20%. This limitation is imposed by the susceptibiUty of the intermediates to further oxidation (see Figs. 2 and 3). A conservative estimate of the lower limit of the oxidation rate constant ratio for ethane and ethylene with respect to methane is one, and the ratio for methanol may be at least 20 (115). 

Chlorine atoms obtained from the dissociation of chlorine molecules by thermal, photochemical, or chemically initiated processes react with a methane molecule to form hydrogen chloride and a methyl-free radical. The methyl radical reacts with an undissociated chlorine molecule to give methyl chloride and a new chlorine radical necessary to continue the reaction. Other more highly chlorinated products are formed in a similar manner. Chain terrnination may proceed by way of several of the examples cited in equations 6, 7, and 8. The initial radical-producing catalytic process is inhibited by oxygen to an extent that only a few ppm of oxygen can drastically decrease the reaction rate. In some commercial processes, small amounts of air are dehberately added to inhibit chlorination beyond the monochloro stage. 

Chlorination of Methane. Methane can be chlorinated thermally, photochemicaHy, or catalyticaHy. Thermal chlorination, the most difficult method, may be carried out in the absence of light or catalysts. It is a free-radical chain reaction limited by the presence of oxygen and other free-radical inhibitors. The first step in the reaction is the thermal dissociation of the chlorine molecules for which the activation energy is about 84 kj/mol (20 kcal/mol), which is 33 kJ (8 kcal) higher than for catalytic chlorination. This dissociation occurs sufficiendy rapidly in the 400 to 500°C temperature range. The chlorine atoms react with methane to form hydrogen chloride and a methyl radical. The methyl radical in turn reacts with a chlorine molecule to form methyl chloride and another chlorine atom that can continue the reaction. The methane raw material may be natural gas, coke oven gas, or gas from petroleum refining. 

In H abstraction, a hydrogen radical reacts with a molecule (primarily a paraffin) and produces a hydrogen molecule and a radical. In the same way, a methyl radical reacts to produce a radical and methane. Similar reactions with other radicals (ethyl and propyl) can also occur. In addition, some radicals like H, CH, etc, are added to olefins to form heavier radicals. 

The reaction shown above for the steam reforming of methatie led to die formation of a mixture of CO and H2, die so-called synthesis gas. The mixture was given this name since it can be used for the preparation of a large number of organic species with the use of an appropriate catalyst. The simplest example of this is the coupling reaction in which medrane is converted to ethane. The process occurs by the dissociative adsorption of methane on the catalyst, followed by the coupling of two methyl radicals to form ethane, which is then desorbed into the gas phase. 

The lithium oxide-promoted barium oxide also functions as a catalyst for the methane coupling reaction, but the mechanism is not clearly understood at the present time. The only comment that might be offered here is that the presence of ions on the surface of this material might etdrance the formation of methyl radicals drrough the formation of hydroxyl groups thus... 

Fig. 1. Examples of temperature dependence of the rate constant for the reactions in which the low-temperature rate-constant limit has been observed 1. hydrogen transfer in the excited singlet state of the molecule represented by (6.16) 2. molecular reorientation in methane crystal 3. internal rotation of CHj group in radical (6.25) 4. inversion of radical (6.40) 5. hydrogen transfer in halved molecule (6.16) 6. isomerization of molecule (6.17) in excited triplet state 7. tautomerization in the ground state of 7-azoindole dimer (6.1) 8. polymerization of formaldehyde in reaction (6.44) 9. limiting stage (6.45) of (a) chain hydrobromination, (b) chlorination and (c) bromination of ethylene 10. isomerization of radical (6.18) 11. abstraction of H atom by methyl radical from methanol matrix [reaction (6.19)] 12. radical pair isomerization in dimethylglyoxime crystals [Toriyama et al. 1977].

The simplest of the ethers would be ether that has the simplest hydrocarbon backbones attached those backbones are the radicals of the simplest hydrocarbon, methane. Therefore, the simplest of the ethers is dimethyl ether, whose formula is CH3OCH3. Dimethyl is used because there are two methyl radicals, and di-" is the prefix for two. This compound could also be called methyl methyl ether, or just... 

The competitive method employed for determining relative rates of substitution in homolytic phenylation cannot be applied for methylation because of the high reactivity of the primary reaction products toward free methyl radicals. Szwarc and his co-workers, however, developed a technique for measuring the relative rates of addition of methyl radicals to aromatic and heteroaromatic systems. - In the decomposition of acetyl peroxide in isooctane the most important reaction is the formation of methane by the abstraction of hydrogen atoms from the solvent by methyl radicals. When an aromatic compound is added to this system it competes with the solvent for methyl radicals, Eqs, (28) and (29). Reaction (28) results in a decrease in the amount... 

The Cl atom attacks methane and forms a methyl free radical plus HCI. The methyl radical reacts in a subsequent step with a chlorine molecule, forming methyl chloride and a Cl atom ... 

Alpha-scission is not favored thermodynamically but does occur. Alpha-scission produces a methyl radical, which can extract a hydrogen atom from a neutral hydrocarbon molecule. The hydrogen extraction produces methane and a secondary or tertiary free radical (Equation 4-3). 

For methyl phenyl sulfoxide it was found that 25% of the adduct decomposes to give phenyl radical and methane sulfinic acid, while 75% of the adduct leads to methyl radical and benzene sulfinic acid. For heavier alkyl groups the formation of the benzene sulfinic acid is even more dominant, 96% for R = Et and 100% for R = C2H4C1 or i-Pr. 

Chlorine atoms (each of which has one unpaired electron) are highly reactive they attack methane molecules and extract a hydrogen atom, leaving a methyl radical behind ... 

The reactions of free radicals in ethylene are well known (5, 24), and the applicability of this information to radiation chemistry has been demonstrated (32). Kinetic analysis of radical reactions revealed that the formation of methyl radicals and methane cannot be ascribed to excited precursors alone (32, 37). 

The species participating in propane formation whose yield is not affected by the applied field must therefore by the methyl radical. We conclude that methane and methyl radicals are neutral products of ion-molecule sequences. 

Kinetic examination of the methane yield shows behavior quite similar to that of methyl radical a pressure dependent yield of 0.406 molecule/100 e.v., a pressure independent yield of 0.126 molecule/100 e.v., and a rate constant ratio of kq/kf = 1.5 X 106 mole-1 cc. for the competing steps. 

Scherer, J.J. et al.. Determination of methyl radical concentrations in a methane/air flame by infrared cavity ringdown laser absorption spectroscopy,. Chem. Phys., 107, 6196,1997. 

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]. 

Investigation of direct conversion of methane to transportation fiiels has been an ongoing effort at PETC for over 10 years. One of our current areas of research is the conversion of methane to methanol, under mild conditions, using li t, water, and a semiconductor photocatalyst. Research in our laboratory is directed toward ad ting the chemistry developed for photolysis of water to that of methane conversion. The reaction sequence of interest uses visible light, a doped tungsten oxide photocatalyst and an electron transfer molecule to produce a hydroxyl i cal. Hydroxyl t cal can then react with a methane molecule to produce a methyl radical. In the preferred reaction pathway, the methyl radical then reacts with an additional wata- molecule to produce methanol and hydrogen. 

Appearance energies for CH2 have been measnred from a large nnmber of precursors, including methane, ketene, and even methyl radical. 

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. 

---