Halogenerte

Halogenerte, or halogenated compounds, are chemical substances in which one or more hydrogen atoms have been replaced by halogen atoms such as fluorine, chlorine, bromine, or iodine. They span simple alkyl and aryl halides to complex, multi-substituted molecules and are common in organic synthesis, medicine, and materials science. Halogenation often changes reactivity, polarity, boiling points, and metabolic properties, making these compounds useful tools but also subjects of safety and regulatory concerns.

Halogenation occurs through several general mechanisms. Free-radical halogenation of alkanes uses Cl2 or Br2 under light or heat and proceeds by initiation, propagation, and termination; bromination is more selective than chlorination. Electrophilic addition to alkenes adds halogens across double bonds to give vicinal dihalides via a halonium ion intermediate with anti-addition. Electrophilic aromatic halogenation substitutes a hydrogen on an aromatic ring, typically with chlorine or bromine and a Lewis acid catalyst such as FeCl3 or FeBr3, yielding mono-halogenated products with regioselectivity governed by substituents. Allylic and benzylic halogenation can be achieved with N-halosuccinimides for selective installation at reactive positions. Nucleophilic substitution on suitable substrates can exchange or introduce halogens where a leaving group is present.

Halogenated compounds have wide applications in pharmaceuticals, agrochemicals, solvents, and polymers, such as polyvinyl chloride. They also occur as environmental pollutants and intermediates in chemical synthesis. Because many halogenated organics are persistent and toxic, their production and disposal are subject to strict safety, handling, and regulatory regimes in order to minimize health and ecological risks.