Halogenation

Halogenation is a chemical reaction that involves the addition of one or more halogens to a compound or material. Dehalogenation is the reverse of halogenation and results in the removal of a halogen from a molecule. The pathway and stoichiometry of halogenation depends on the structural features and functional groups of the organic substrate, as well as on the specific halogen. Inorganic compounds such as metals also undergo halogenation.

Several pathways exist for the halogenation of organic compounds, including free radical halogenation, ketone halogenation, electrophilic halogenation, and halogen addition reaction. The structure of the substrate is one factor that determines the pathway.

Saturated hydrocarbons typically do not add halogens but undergo free radical halogenation, involving substitution of hydrogen atoms by halogen. The regiochemistry of the halogenation of alkanes is usually determined by the relative weakness of the available C–H bonds. The preference for reaction at tertiary and secondary positions results from greater stability of the corresponding free radicals and the transition state leading to them. Free radical halogenation is used for the industrial production of chlorinated methanes:

Rearrangement often accompany such free radical reactions.

Unsaturated compounds, especially alkenes and alkynes, add halogens:

The addition of halogens to alkenes proceeds via intermediate halonium ions. In special cases, such intermediates have been isolated.

Aromatic compounds are subject to electrophilic halogenation:

The facility of halogenation is influenced by the halogen. Fluorine and chlorine are more electrophilic and are more aggressive halogenating agents. Bromine is a weaker halogenating agent than both fluorine and chlorine, while iodine is least reactive of them all. The facility of hydrogenolysis follows the reverse trend: iodine is most easily removed from organic compounds and organofluorine compounds are highly stable.

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