Aromatic halogenation
Abstract
Aromatic halogenation is a reaction in which a hydrogen atom of an aromatic ring is replaced by a halogen atom (typically Cl or Br) in the presence of a Lewis acid (AlCl3, FeBr3, respectively), yielding an aryl halide. Unlike alkene halogenation, which proceeds via electrophilic addition, aromatic halogenation proceeds via electrophilic aromatic substitution (EAS), preserving the aromaticity of the ring.
Keywords
Aromatic, Aryl halide, EAS
Category
Aromatic
Mechanism
First, the Lewis acid (here, FeBr3) coordinates to one halogen atom of X2 (where X = Cl, Br), strongly polarizing the X-X bond and generating a species with significant electrophilic character on one of the halogen atoms. Then, the 𝜋 electrons of the aromatic ring attack the electrophilic halogen, forming an arenium ion (also called Wheland intermediate). At this stage, aromaticity is lost and the carbon bearing the halogen becomes sp3 hybridized. Finally, the Lewis acid–halide complex (here, FeBr4-) acts as a base and abstracts the proton from the sp3 carbon, thereby restoring aromaticity and releasing the Lewis acid catalyst along with HX.
