Halogenation

Halogenation is a class of chemical reactions that introduces a halogen atom into a molecule. In organic chemistry it most commonly means replacing a hydrogen atom in hydrocarbons with a halogen such as fluorine, chlorine, bromine, or iodine, or adding halogen atoms across unsaturated substrates.

Radical halogenation uses halogen radicals generated by light or initiators (for example Cl• or Br•). A halogen abstracts a hydrogen to form a carbon radical, which then reacts with another halogen molecule to propagate the chain. In alkanes, reactivity follows a stability order of the hydrogen being abstracted: tertiary > secondary > primary. Bromination is more selective than chlorination; chlorine is more reactive but less selective, leading to mixtures. Common laboratory and industrial examples include chlorination or bromination of methane to form chloromethane, bromomethane, and further halogenated products.

Electrophilic halogenation involves halogen molecules acting as electrophiles toward unsaturated substrates. Addition to alkenes yields vicinal dihalides via a halonium ion intermediate and typically occurs with anti addition. In arenes, halogenation proceeds through electrophilic aromatic substitution in the presence of catalysts or Lewis acids, giving haloarenes. N-halosuccinimides such as N-bromosuccinimide enable selective bromination at allylic or benzylic positions via radical mechanisms.

Nucleophilic halogenation refers to substitution reactions in which a halide ion replaces a leaving group on an organic molecule, often by SN2. For example, iodide can displace a chloride on an alkyl halide under suitable solvent conditions, and in some cases organohalides are cross-coupled or otherwise transformed.

Applications include synthesis of organohalides for pharmaceuticals, agrochemicals, and polymers. Halogenation also occurs in biological systems as enzyme-catalyzed or non-enzymatic modifications. Safety concerns include toxicity, corrosivity, and the environmental persistence of certain halogenated compounds.