halogenations

Halogenations are chemical reactions that introduce one or more halogen atoms (fluorine, chlorine, bromine, or iodine) into a substrate. They can involve substitution of a hydrogen atom by a halogen, or the addition of halogen across a multiple bond, and they are widely used to synthesize organohalogen compounds and to modify reactivity in organic molecules.

Radical halogenation of alkanes proceeds under heat or light with halogen molecules such as Cl2 or Br2, generating halogen radicals that abstract a hydrogen atom and propagate a chain reaction. The general scheme is R-H + X2 -> R-X + HX. Chlorination is fast and less selective, often giving mixtures, whereas bromination is slower but more selective for secondary and tertiary positions. Allylic or benzylic bromination can be achieved with NBS to favor allylic bromides.

Addition of halogens to alkenes converts the double bond into vicinal dihalides. The mechanism involves halonium ion formation followed by anti addition. Cl2, Br2, and I2 give 1,2-dihalides; halohydrin formation can occur in the presence of water, yielding HO- and X- on adjacent carbons.

Aromatic halogenation is an electrophilic aromatic substitution where halogen atoms replace a hydrogen on an aromatic ring, typically catalyzed by Lewis acids such as FeCl3 or AlCl3 for chlorination and bromination. Iodination is less common and often requires oxidative conditions.

Fluorination and other specialized halogenations are achieved with dedicated reagents, and halogenation is a common step in synthesis, polymer chemistry, and medicinal chemistry. Reactions require careful control due to the reactive halogen species and potential hazards.