Chlorination of Methane The Radical Chain Mechanism

We have seen that alkanes undergo chemical transformations when subjected to pyrolysis, and that these processes include the formation of radical intermediates. Do alkanes participate in other reactions In this section, we consider the effect of exposing an alkane (methane) to a halogen (chlorine). A chlorination reaction, in which radicals again play a key role, takes place, producing chloromethane and hydrogen chloride. We shall analyze each step in this transformation to establish the mechanism of the reaction. 

When methane and chlorine gas are mixed in the dark at room temperature, no reaction occurs. The mixture must be heated to a temperature above 300°C (denoted by A) or irradiated with ultraviolet light (denoted by hv) before a reaction takes place. One of the two initial products is chloromethane, derived from methane in which a hydrogen atom is removed and replaced by chlorine. The other product of this transformation is hydrogen chloride. Further substitution leads to dichloromethane (methylene chloride), CH2CI2 tri-chloromethane (chloroform), CHCI3 and tetrachloromethane (carbon tetrachloride), CCI4. 

Why should this reaction proceed Consider its A.H°. Note that a C-H bond in methane (DH° = 105 kcal mol ) and a Cl-Cl bond (DH° = 58 kcal mol ) are broken, whereas the C-Cl bond of chloromethane (DH° = 85 kcal mol ) and an H-Cl linkage (DH° = 103 kcal mol ) are formed. The net result is the release of 25 kcal mol in forming stronger bonds The reaction is exothermic (heat releasing). 

does the thermal chlorination of methane not occur at room temperature The fact that a reaction is exothermic does not necessarily mean that it proceeds rapidly and spontaneously. Remember (Section 2-1) that the rate of a chemical transformation depends on its activation energy, which in this case is evidently high. Why is this so What is the function of irradiation when the reaction does proceed at room temperature Answering these questions requires an investigation of the mechanism of the reaction. 

The mechanism explains the experimental conditions required for reaction 

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Chlorination of Methane The Radical Chain Mechanism CHAPTER 3... 

Chemical initiation generates organic radicals, usually by decomposition of a2o (11) or peroxide compounds (12), to form radicals which then react with chlorine to initiate the radical-chain chlorination reaction (see Initiators). Chlorination of methane yields all four possible chlorinated derivatives methyl chloride, methylene chloride, chloroform, and carbon tetrachloride (13). The reaction proceeds by a radical-chain mechanism, as shown in equations 1 through. Chain initiation... 

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. 

In Summary Fluorine, chlorine, and bromine react with methane to give halomethanes. All three reactions follow the radical chain mechanism described for chlorination. In these processes, the first propagation step is always the slower of the two. It becomes more exo-... 

The results of kinetic studies suggest that alkane substitution reactions typically proceed by a radical chain mechanism (Section 13.9). The initiation step in the chlorination of methane is the dissociation of chlorine ... 

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. 

Radical chain mechanism of the chlorination of methane. Test yourself on the concepts in this figure at OrganicChemistryNow. 

Free-radical chain reactions also occur during the chlorination of methane (Chapter 10) and of the methyl group of methylbenzene. Ozone depletion by chlorofluorocarbons (CFCs), acid rain formation and formation of photochemical smog (Chapter 25 on the accompanying website) also involve free-radical reactions. (Free-radical reactions are also operating in unpolluted atmospheres and play an important role in all chemical reactions that occur in the gas phase.) The combustion of hydrocarbons, such as petrol, also proceeds via a free-radical mechanism, which has important consequences for the smooth running and performance of combustion engines. Chain reactions may also have ions as intermediates, as opposed to free radicals. 

The mechanism for the chlorination of methane is an example of a radical chain mechanism. 

This reaction proceeds by a radical chain mechanism analogons to the one observed for methane. As in methane, all of the hydrogen atoms in ethane are indistingnishable from one another. Therefore, we observe only one monochlorination prodnct, chloroethane, regardless of which hydrogen is initially abstracted by chlorine in the first propagation step. 

A chain reaaion is a reaction in which one or more reactive reaction intermediates (frequently radicals) continuously regenerate usually through a repeated cycle of elementary steps (the propagation step ). For example, in the chlorination of methane by a radical mechanism. Cl continuously regenerates in the chain propagation steps ... 

In addition to forming chloromethane, the second propagation step produces another chlorine radical. The chlorine radical can react with another molecule of methane, giving HC1 and a methyl radical, which reacts with Cl2 to give chloromethane and regenerate yet another chlorine radical. In this way, the chain reaction continues until the supply of the reactants is exhausted or some other reaction consumes the radical intermediates. The chain reaction explains why many molecules of methyl chloride and HC1 are formed by each photon of light that is absorbed. We can summarize the reaction mechanism as follows. 

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). 

Any two radicals in the reaction mixture can combine to form a molecule in which all the electrons are paired. The combination of two radicals is called a termination step because it helps bring the reaction to an end by decreasing the number of radicals available to propagate the reaction. The radical chlorination of alkanes other than methane follows the same mechanism. A radical chain reaction, with its characteristic initiation, propagation, and termination steps, was first described in Section 4.10. 

This allylic bromination with NBS is analogous to the methane chlorination reaction discussed in Section 6.3 and occurs hy a similar radical chain reaction mechanism. As in methane halogenation, Br- radical abstracts an allylic hydrogen atom of the alkene, thereby forming an allylic radical plus HBr. This allylic radical then reacts with Br2 to yield the product and a Br-radical, which cycles hack into the first step and carries on the chain. The Br2 results from reaction of NBS with the HBr formed in the first step. 

The first step is the thermal or photochemical breaking of the chlorine-chlorine bond. This initiation reaction is followed by the first propagation step (Rg. 11.40), the abstraction of a hydrogen atom from methane by a chlorine atom to produce a methyl radical and hydrogen chloride. In the second propagation step, the methyl radical abstracts a chlorine atom from a chlorine molecule to give methyl chloride and another chlorine atom that can carry the chain reaction forward. There are many possible termination reactions Figure 11.40, which shows the overall mechanism, includes only one. 

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