haloarenes

Haloarenes are aryl halides, organic compounds in which one or more halogen atoms (fluorine, chlorine, bromine, or iodine) are attached to an aromatic ring, typically a benzene ring. The halogenated arene class includes halobenzenes such as chlorobenzene, bromobenzene, and iodobenzene, as well as more highly substituted or polyhalogenated arenes.

Physicochemical and structural features vary with the halogen. The carbon–halogen bond is part of the aromatic framework, and the C–X bond polarity and strength influence reactivity. Bond strength decreases from C–F to C–I, making iodo- and bromobenzenes generally more reactive in certain transformations than fluoro- and chlorobenzenes. In electrophilic aromatic substitution, halogens are deactivating overall due to the inductive effect but direct ortho and para positions via resonance donation, so halogen substituents direct further substitutions to the adjacent positions.

Key reactions involve the halide as a leaving group in forming new bonds. Haloarenes are central to cross-coupling reactions such as Suzuki–Miyaura, Heck, and Sonogashira couplings, which enable efficient construction of biaryl and substituted aryl compounds. They can also participate in nucleophilic aromatic substitution under activating conditions (often with electron-withdrawing groups or when fluorides are involved in SNAr-type pathways). Halogen–metal exchange and catalytic dehalogenation are other common transformations.

Synthesis of haloarenes typically proceeds by electrophilic halogenation of arenes using appropriate halogen sources, with conditions chosen to favor the desired regioselectivity. Haloarenes serve as versatile intermediates in the production of pharmaceuticals, agrochemicals, dyes, and polymers. They can pose safety and environmental concerns due to toxicity and persistence, necessitating careful handling and disposal.