Free radical halogenation

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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. The reaction is used for the industrial synthesis of chloroform(CH₃Cl), dichloromethane(CH₂Cl₂)

General mechanism

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). The reactivities of these halogens in the halogenation process are however in reverse, since Fluorine molecules homolyse less readily than those of Iodine in the presence of UV light.

Control of halogenation

Halogenation often does not stop at monosubstititution, depending on reaction conditions the chlorination of methane yields dichloromethane, chloroform 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. This relationship is often used as a demonstration of the reactivity–selectivity principle and can be explained with the aid of the Hammond postulate. A bromine radical is not very reactive and the transition state for proton abstraction has much radical character and is reached late. the reactive chlorine radical develops a transition state resembling the reactant with little radical character. When the alkyl radical is fully formed in the transition state it can benefit fully from any resonance stabilization present thereby maximizing selectivity.
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

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