Halogenation of methane

Ttie second large-scale use of methane is its halogenation [I], where chlorine and fluorine are the most important halogens for methane substitution. Chloro-methanes aie formed by thermal chlorination or catalyzed uxychlorination of methane. Monochlorination is controlled by using a ten-fold excess of methane. 

Tlte catalyst for oxychlorination (6) is a CuCli KCI melt, which is alsu used as a chlorinating agent. 

Patents also report on the use of supported catalysts, such as CuCl(KCI) on Si03,TiOj or a AljOj in the presence of LaClj [6.  

The mechanisms of methane-halogen reactions are chain radical processes. Carbon tetrachloride Is also prepared from carbon disulfide and chlorine. Sulfur is formed in this reaction, but it is recovered and used for the regeneration of CS3. Carbon disulfide is obtained from the elements, but is also formed from methane and sulfur (see Section 1,1.5.4). 

Ftuorochloromethanes are generally prepared from chloron ethanes, where the chlorine is progressively replaced by iluorine, using HF as the fluorine source. 

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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... 

Typical radical reactions are substitution and addition reactions as shown below (Scheme b). A typical substitution reaction is the halogenation of methane with chlorine gas under photolytic conditions, and generally available chlorohydrocarbons are prepared by this method. The chlorination reaction proceeds through a chain pathway via the initiation step, propagation step, and termination step as shown below (Scheme 1.1). 

Up to now, we have limited our discussions to the halogenation of methane. Beginning our study with such a simple compound allowed us to concentrate on the thermodynamics and kinetics of the reaction. Now we consider halogenation of the higher alkanes, meaning those of higher molecular weight. 

As we might expect, halogenation of the higher alkanes is essentially the same as the halogenation of methane. It can be complicated, however, by the formation of mixtures of isomers. 

Halogenation of alkanes proceeds by the same mechanism as halogenation of methane ... 

For any atom Y and for any atom Z, only one substance of formula CH2YZ has ever been found. Halogenation of methane, for example, yields only one compound of formula CH2CI2, only one compound of formula CH2Br2, and only one compound of formula CH2ClBr. 

Bond dissociation energy, 13, 151—153, 155 acetylene, 343 aryl halides, 918 benzene, 918 ethane, 151, 343, 918 ethylene, 171, 343, 918 ethyl halides, 918 and halogenation of methane, 155 2-methylpropane, 151,152, 414 peroxides, 220 propane, 151 propene, 370, 414 table, 151 vinyl hahdes, 918... 

Other Radical Halogenations of Methane Similarities and differences. 

Concerning the electrophilic halogenation of methane it should also be pointed out that the singlet-triplet energy difference of positive halogens (as illustrated for the hypothetical ions) favor the latter Electrophilic halogenations thus may be more complex and can involve radical ions even under conditions where conventional radical chain halogenation is basically absent. 

Combining the halogenation of methane with catalytic, preferentially gas-phase hydrolysis, methyl alcohol (and dimethyl ether) can be obtained in high selectivity (Scheme 18) . The hydrolysis of methyl chloride with caustic was first carried out by Berthelot in the mid 1800 . This first preparation of methyl alcohol, however,... 

Consequently halogenation of methane followed by hydrolysis is a suitable way to obtain methyl alcohol (and dimethyl ether) (equation 63). 

Using data from Table 8.7 and Appendix 3, we can calculate the enthalpy of reaction, ATA, for the halogenation of methane. Energy is required to break CHj—H and Cl—Cl bonds [439 and 247 kj (105 and 59 kcal)/mol, respectively]. Energy is released in making the CHj—Cl and H—Cl bonds [-351 and -431 kJ (-84 and -103 kcal)/mol, respectively]. Based upon these enthalpies, we calculate that chlorination of methane to form chloromethane and hydrogen chloride liberates 96 kJ (23 kcal)/mol. 

Mechanism and electron pushing for the free radical halogenation of methane. 

Bowman, R., Stangland, E., Jones, M., etal. (2010). Worldwide Patent 2010062427A2, Oxidative Mono-Halogenation of Methane (Dow Global Technologies Inc., USA.). 

Table 3-5 Enthalpies of the Propagation Steps in the Halogenation of Methane [kcal mol (kj mol )]...

A hypothetical alternative mechanism for the halogenation of methane has the following propagation steps. 

The data tell us that the total energy change, AH°, in halogenation of methane decreases in the order... 

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