Halogenaatio

Halogenaatio, or halogenation, is a class of chemical reactions in which a halogen atom (fluorine, chlorine, bromine or iodine) is introduced into a molecule. The process can involve substitution on saturated substrates, addition to unsaturated systems, or electrophilic substitution on aromatic rings. Halogenation products are widely used as intermediates in organic synthesis and in the production of pharmaceuticals, agrichemicals, polymers and materials.

For saturated hydrocarbons (alkanes and related compounds), halogenation is typically a substitution reaction that replaces a hydrogen atom with a halogen. This often proceeds via a free-radical chain mechanism initiated by light or heat, using halogen sources such as Cl2 or Br2. Bromination is generally more selective than chlorination, while fluorination tends to be highly reactive and less selective, making controlled synthesis more challenging.

For unsaturated substrates such as alkenes or alkynes, halogenation usually occurs by electrophilic addition across the multiple bond. A halogen molecule forms a halonium ion intermediate (for example a bromonium ion with Br2), and the reaction proceeds with anti-addition to give vicinal dihalides. This method is widely used to convert alkenes into dihalogenated products, with the choice of halogen determining the identity of the substituents.

Arenes and other activated aromatic systems are halogenated primarily by electrophilic aromatic substitution, often using X2 in the presence of a Lewis acid catalyst (for example FeX3) to introduce a halogen onto the ring.

Safety and handling considerations are important in halogenation, due to the toxicity and corrosiveness of halogens and halogenating reagents. Proper ventilation, protective equipment and waste management are essential.