In chemistry free radical halogenation is a type of halogenation This chemical reaction is typical of alkanes and alkyl-substituted aromatics under application of heat or UV light. In the reaction of methane and chlorine:
The initiation step generates halogen radicals by homolysis:
Cl2 → 2 Cl.
followed by a chain reaction:
CH4 + Cl. → CH3. + HCl
CH3. + Cl2 → CH3Cl + Cl.
The net reaction is:
CH4 + Cl2 → CH3Cl + HCl.
The order of reactivity of halogens is F2 >> Cl2 > Br2 >> (I2).
control of halogenation
- Halogenation often does not stop at monosubstititution, depending on reaction conditions the chlorination of methane yields dichloromethane, trichloromethane and tetrachloromethane.
- In most hydrocarbons more than one possible product exists 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. 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. 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
- oxygen is a halogenation inhibitor
