Halogeners

Halogeners are reagents or catalysts that introduce a halogen atom into an organic molecule through halogenation reactions. They encompass electrophilic, nucleophilic, and radical halogenating agents and can deliver fluorine, chlorine, bromine, or iodine to substrates under suitable conditions. The term is used across synthetic chemistry to denote the functional class of halogen-transfer reagents rather than a single compound.

Electrophilic halogeners include elemental halogens such as chlorine, bromine, and iodine, as well as reagents that generate positive halogen species, such as N-halosuccinimides. These agents tend to activate substrates toward substitution or addition by delivering a halogen electrophile, often enabling selective halogenation at activated positions such as allylic, benzylic, or activated aromatic rings.

Nucleophilic halogeners supply halide anions to substrates, typically in the presence of appropriate activating groups or catalysts, enabling substitution reactions where the halide acts as the nucleophile. Radical halogeners generate halogen radicals, allowing selective C–H functionalization or radical halogenation under light or thermal initiation, frequently useful for late-stage modification or remote site selectivity.

Hypervalent iodine reagents and related organohalogenating catalysts represent additional halogenation pathways that can offer mild conditions and unique selectivity. Reactions mediated by halogeners are widely used in the synthesis of pharmaceuticals, agrochemicals, polymers, and natural products, as well as in radiolabeling and material science. Safety concerns include exposure to toxic halogens and corrosive byproducts, and care must be taken to minimize environmental impact.