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Free-radical halogenation
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Free-radical halogenation
In organic chemistry, free-radical halogenation is a type of halogenation. This chemical reaction is typical of alkanes and alkyl-substituted aromatics under application of UV light. The reaction is used for the industrial synthesis of chloroform (CHCl3), dichloromethane (CH2Cl2), and hexachlorobutadiene. It proceeds by a free-radical chain mechanism.
The chain mechanism is as follows, using the chlorination of methane as an example:
The net reaction is:
The steady-state approximation implies that this process has rate law k[CH4][Cl2]1⁄2.
As a radical reaction, the process is halted or severely slowed by radical traps, such as oxygen.
The relative rates at which different halogens react vary considerably:[citation needed]
Radical fluorination with the pure element is difficult to control and highly exothermic; care must be taken to prevent an explosion or a runaway reaction. With chlorine the reaction is moderate to fast; with bromine, slow and requires intense UV irradiation; and with iodine, it is practically nonexistent and thermodynamically unfavored. However, radical iodination can be completed with other iodine sources (see § Variants).
The different rates are often a pedagogical demonstration of the reactivity–selectivity principle and the Hammond postulate. A bromine radical is not very reactive and the transition state for hydrogen 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.[dubious – discuss][citation needed]
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Free-radical halogenation
In organic chemistry, free-radical halogenation is a type of halogenation. This chemical reaction is typical of alkanes and alkyl-substituted aromatics under application of UV light. The reaction is used for the industrial synthesis of chloroform (CHCl3), dichloromethane (CH2Cl2), and hexachlorobutadiene. It proceeds by a free-radical chain mechanism.
The chain mechanism is as follows, using the chlorination of methane as an example:
The net reaction is:
The steady-state approximation implies that this process has rate law k[CH4][Cl2]1⁄2.
As a radical reaction, the process is halted or severely slowed by radical traps, such as oxygen.
The relative rates at which different halogens react vary considerably:[citation needed]
Radical fluorination with the pure element is difficult to control and highly exothermic; care must be taken to prevent an explosion or a runaway reaction. With chlorine the reaction is moderate to fast; with bromine, slow and requires intense UV irradiation; and with iodine, it is practically nonexistent and thermodynamically unfavored. However, radical iodination can be completed with other iodine sources (see § Variants).
The different rates are often a pedagogical demonstration of the reactivity–selectivity principle and the Hammond postulate. A bromine radical is not very reactive and the transition state for hydrogen 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.[dubious – discuss][citation needed]