Methane radical halogenations

Chlorination of methane and halogenation of alkanes generally proceed by way of free radical intermediates Alkyl radicals are neutral and have an unpaired electron on carbon... 

Halogenation of alkanes had long been known, and in 1930 the kinetics of the chlorination of chloroform to carbon tetrachloride were reported by Schwab and Heyde (equation 40), while the kinetics of the chlorination of methane were described by Pease and Walz in 1931. Both of these studies showed the currently accepted mechanism, which was extended to reactions in solution by Hass et al. in 1936. The free radical halogenation mechanism of other alkanes was described by Kharasch and co-workers, ° including side chain halogenation of toluene. 

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... 

We now apply what we know about rates to the reaction of methane with halogens. The rate-limiting step for chlorination is the endothermic reaction of the chlorine atom with methane to form a methyl radical and a molecule of HC1. 

Because a high yield of one particular compound is usually needed, and the separation of product mixtures is often difficult, radical halogenations are rarely used with substrates other than simple hydrocarbons. This is not to say that this reaction type is unimportant the chlorination of methane is a major industrial process. Chloromethane is not the only product obtainable if the ratio of the reactants and the reaction conditions are varied, dichloromethane (CH2CI2), trichloromethane (chloroform, CHCI3) and tetrachloromethane (carbon tetrachloride, CCI4) can all be produced. 

In general, the radical halogenation of alkanes is indiscriminate, with substitution occurring at every carbon-hydrogen bond. This limits the usefulness of this type of reaction to small hydrocarbon molecules. However, some such reactions (for example, the chlorination of methane) are of great industrial importance. 

Other Radical Halogenations of Methane Similarities and differences. 

The reaction of methane with chlorine (in the gas phase) provides a good example for studying the mechanism of radical halogenation. 

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

The initial product, methyl chloride (CH3CI), is even more reactive toward radical halogenation than methane. As methyl chloride is formed, it reacts with chlorine to produce methylene chloride. The process continues until carbon tetrachloride is produced. In order to produce methyl chloride as the major product (monohalogenation), it is necessary to use an excess of methane and a small amount of Cl,. Unless otherwise indicated, the conditions of a halogenation reaction are generally controlled so as to produce monohalogenation. 

What happens in the radical halogenation of other alkanes Will the different types of R-H bonds—namely, primary, secondary, and tertiary— react in the same way as those in methane As we saw in Exercise 3-4, the monochlorination of ethane gives chloroethane as the product. 

We know that bromine is less reactive than chlorine in the rate-determining step of halogenation of methane. We also recall that bromine is more selective than chlorine. Both the rate of reaction and the selectivity of free radical halogenation are related to the first propagation step, so let s look at the relationship between reactivity and selectivity in terms of the structure of the transition states for chlorination and bromination. 

Hubig, S. M., Jung, W. and Kochi, J.K. (1994). J. Org. Chem. 59, 6233. Note the competition between ion-pair and radical-pair collapse in halogenation with iodine monochloride on p. 279 is exactly analogous to that in nitration with tetranitro-methane in eqns 82/83 (vide infra)... 

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