Methyl radical with methane

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... 

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... 

One of the four bonds of the carbon is left free. In methane this free bond is satisfied by another hydrogen atom in methyl chloride or iodide by one of chlorine or iodine, making in each case a saturated cornpound. When, therefore, two molecules of methyl iodide each lose their iodine to zinc or sodium we have left the two methyl radicals with this fourth valence of each carbon unsatisfied. These two free valencies satisfy each otherj and we have the two methyl radicals united just as we believe two free atoms unite to form a molecule. We may write the reaction then ... 

There are one or more chain-propagating steps, each of which consumes a reactive particle and generates another here they are the reaction of chlorine atoms with methane (step 2), and of methyl radicals with chlorine (step 3). 

The reaction of cobalt(II) tetrasulfophthalocyanine, [Co(II)(tspc)] , with CH3, CH2CH2OH, CH(CH3)CH20H, CH(CH3)CH(CH3)0H, and CH2C(CH3)20H free radicals has been studied. Results indicate the initial formation of a [Co(II)(tspc-R)] species where the exact nature of the interaction of R and tspc is not clear. There follows a subsequent internal redox formation of [(tspc)Co(III)-R] via a first-order process. Subsequent decomposition produces methane for R or alkenes for ROH. Interaction of methyl radical with [Co(II)(nta)(H20)2]" [nta = N(CH2C02)r] yields [(nta)(H20)Co(III)-CH3] (Ki = 2.7 0.5 X 10 M and k i = 60 10 This reaction is followed by... 

Reactivity. From experimental studies, it has been suggested that the activation of methane over oxide catalysts to form methyl radicals for methane to methanol conversion, or the generation of higher alkanes, involves surface oxide species in low oxidation states such as 0 or O2 . Early simulations (421) of this process relied on comparisons with gas-phase MO dimers, in which it is known that a similar partially reduced oxygen species is present. By studying the simple gas-phase reaction... 

As displayed in Fig. 5, the specific surface area of the sample also declines in the order A, B, D, C, which is parallel to the sequence of methane conversion. Two aspects concerning the effect of the specific surface area on the catalytic properties have to be considered. (1) The larger the spedfic surface area, the higher the number of active centers by unit weifbf of the cataljwt and, of course, the faster the rate of methyl radical formation. (2) Lai e spedfic siirface areas may lead to an increase of e rate of formation of COx [4], due to the collision of methyl radicals with the surface. 

Scheme 2.1.2 Reactivity of radicals, carbocations, and carbanions exemplified for reactions of relevance in chemical technology reaction of (a) the methyl radical with chlorine key-step in methane chlorination) (b) the isopropylium ion with water (key-step in isopropanol synthesis from propene) ...

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... 

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. 

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