Halogenation of an aromatic ring is a specific type of Electrophilic Aromatic Substitution (EAS) where a halogen atom (e.g., bromine, chlorine) is introduced into the aromatic ring, replacing a hydrogen atom. This process is widely used to synthesize halogenated aromatic compounds, which are important in pharmaceuticals, agrochemicals, and materials science. Let's focus on bromination as an illustrative example, although the mechanism is similar for other halogens.

Bromination of Benzene: A Step-by-Step Mechanism

1. Formation of the Electrophile: The first step involves the generation of a strong electrophile, which, in the case of bromination, is the bromonium ion (Br+). This is typically achieved by mixing bromine (Br2​) with a Lewis acid catalyst such as iron (III) bromide (FeBr3​). The FeBr3​ reacts with Br2​ to form Br+ and FeBr4−, with Br+ being the species that will attack the aromatic ring.
   

2. Electrophilic Attack and Formation of the Sigma Complex: The bromonium ion ($B{r}^{+}$) attacks the $\pi$ electrons of the benzene ring, temporarily disrupting the aromaticity and forming a high-energy carbocation intermediate known as the sigma complex or arenium ion. This intermediate is stabilized by resonance, as the positive charge can be delocalized across three carbon atoms of the ring.
   
   
   
   

3. Deprotonation and Restoration of Aromaticity: A base, often the bromide ion (Br−) from the FeBr4− generated in the first step, removes a proton (H+) from the carbon adjacent to where the bromine has added. This step restores the aromaticity of the ring by re-establishing the conjugated π electron system.

4. Regeneration of the Catalyst: The removal of H+ by Br− forms HBr, and the Lewis acid catalyst (FeBr3​) is regenerated and can catalyze further reactions.

Halogenation reactions like this are valuable for introducing halogen atoms into aromatic systems, providing a gateway to a wide range of chemical modifications and syntheses.