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This reaction is typical of alkanes and alkyl-substituted aromatics. We use as an example the chlorination of methane, a reaction which has some industrial significance. We first need an initiation step to generate halogen atoms:
Cl2 → 2 Cl.
Once the halogen atoms are formed, the chain reaction can begin:
CH4 + Cl. → CH3. + HCl
CH3. + Cl2 → CH3Cl + Cl.
Notice how each step generates a reactive intermediate (CH3. or Cl.) which is used in the other step. In this way the chain reaction, once begun by the initiation step, is kept going. The net reaction is:
CH4 + Cl2 → CH3Cl + HCl.
In most hydrocarbons there are more than one possible product, depending on which hydrogen is replaced. Butane (CH3-CH2-CH2-CH3), for example, can be chlorinated at the "1" position to give 1-chlorobutane (CH3-CH2-CH2-CH2Cl) or at the "2" position to give 2-chlorobutane (CH3-CH2-CHCl-CH3). The product distribution depends on relative reaction rates: in this case the "2" position of butane reacts faster and 2-chlorobutane is the major product. Free radical halogenation generally proceeds in the following order:
- Fastest
- Carbons with one or more aryl substituents (benzylic positions)
- Carbons with three alkyl substituents (tertiary positions)
- Carbons with two alkyl substituents (secondary positions)
- Carbons with one or zero substituents (primary positions)
- Slowest
Chlorination is generally less selective than bromination. Fluorination is not only even less selective than chlorination, but also highly exothermic and care must be taken to prevent an explosion or a runaway reaction. Free radical iodination is usually not possible because iodine is less reactive than the other halogens.
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