Radical chlorination of methane

Under the influence of heat or light a molecule of chlorine dissociates each atom takes one of the bonding electrons. 

This step produces two highly reactive chlorine atoms. 

A chlorine atom abstracts a hydrogen atom from a methane molecule. 

This step produces a molecule of hydrogen chloride and a methyl radical. 

A methyl radical abstracts a chlorine atom from a chlorine molecule. 

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Figure 10.1 Mechanism of the radical chlorination of methane. Initiation step Three kinds of steps are required initiation, propagation, and termination. The propagation steps are a repeating cycle, with Cl- a reactant in step 1 and a product in...

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... 

Chlorination of Alkanes. The most direct and economical method for the manufacture of chloromethanes is the thermal free-radical chlorination of methane.176 177 Whereas in the 1940s and 1950s photochlorination was practiced in some plants, thermal chlorination is the principal industrial process today. The product chloromethanes are important solvents and intermediates. Commercial operations perform thermal chlorination at about 400-450°C. Vapor-phase photochemical chlorination of methane may be accomplished at 50-60°C. Fast and effective removal of heat associated with thermally induced free-radical substitution is a crucial point. Inadequate heat control may lead to explosion attributed to the uncontrollable pyrolysis liberating free carbon and much heat ... 

B-6. What are the chain-propagating steps in the free-radical chlorination of methane ... 

Each of the following proposed mechanisms for the free-radical chlorination of methane is wrong. Explain how the experimental evidence disproves each mechanism. 

A reaction of this type is of little use unless you happen to need the four products in the ratios found and can manage to separate them (their boiling points are very similar). But industrially the radical chlorination of methane and ethane are important reactions and the products can be separated... 

Nernst (1918) suggested that free radicals take part in chemical reactions and postulated a radical chain mechanism for the combination of H2 and CI2. Paneth and coworkers (1929) first demonstrated the existence of alkyl free radicals by decomposition of metal alkyls. Norrish (1931) suggested that free radicals could occur as intermediates in the photolysis of carbonyl compounds. Rice and Herzfield (1934) produced radicals from the dissociation of hydrocarbons. Up until relatively recently, radicals were regarded as highly reactive species, whose reactions were unselective and difficult to control (remember the radical chlorination of methane). The last 20 years have seen the field developed to such an extent that it is now recognized that radicals can take part in highly useful and selective reactions. 

Free-Radical Chlorination of Methane THE OVERALL REACTION ... 

Like free-radical chlorination of methane (Section 4.16), the free-radical addition of hydrogen bromide to 1-butene outlined in Mechaiusm 6.8 is characterized by initiation and chain propagation stages. The irfitiation stage, however, involves two steps rather than one and it is Ais extra step that accounts for the role of peroxides. Peroxides are initiators they are not incorporated into the product but act as a source of radicals necessary to get the chain reaction started. 

The production of chloromethane (methyl chloride), dichloromethane (methylene chloride), and chloroform is carried out in one process, the hot radical chlorination of methane [route (a) in Topic 5.3.5 for mechanistic details see Section 2.2]. The process yields a mixture of all the different chloromethanes including the least desired tetrachloromethane. The chlorination reaction is initiated at above 250 °C. This temperature represents the lower temperature limit for the formation of chlorine radicals Cl by thermal decomposition of CI2 in sufficient quantity. During the... 

What type of radiation (in wavelengths, A) would be minimally required to initiate the radical chlorination of methane [Hint The initiation step requires the breaking of the Cl-Cl bond (see Section 3-4).]... 

In the radical chlorination of methane, one propagation step is shown as... 

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