Halogenation of benzene is an electrophilic substitution reaction of aromatic hydrocarbons in which a hydrogen atom of the benzene ring is replaced by a halogen atom (chlorine or bromine). This reaction takes place in the presence of a Lewis acid catalyst such as FeCl3 or FeBr3, which helps in the formation of the active electrophile. Due to the stability of the aromatic ring, benzene undergoes substitution reactions rather than addition reactions, thereby preserving its aromaticity.
Reagents
Halogenation of benzene is carried out using molecular halogens such as chlorine (Cl2) or bromine (Br2) in the presence of a Lewis acid catalyst like FeCl3 (ferric chloride) or FeBr3 (ferric bromide).
- The catalyst plays an important role in activating the halogen molecule by polarising it and helping in the formation of the electrophile (Cl⁺ or Br⁺) required for the electrophilic substitution reaction.
- Thus, the reagents ensure the generation of a strong electrophile and facilitate substitution on the benzene ring under mild conditions.
Electrophile Formation
In the halogenation of benzene, the electrophile is formed in the presence of a Lewis acid catalyst such as FeCl3 or FeBr3.
- When chlorine (Cl2) or bromine (Br2) reacts with the catalyst, the halogen molecule becomes polarised.
- One halogen atom gets a partial positive charge (Cl⁺/Br⁺ character), while the catalyst helps in breaking the bond.
- This leads to the formation of a strong electrophile:
Cl2 + FeCl3 → Cl + + FeCl4⁻
- The Cl⁺ / Br⁺ ion is electron-deficient and highly reactive, so it attacks the electron-rich benzene ring in the next step of the reaction.
Mechanism of Halogenation of Benzene
Halogenation of benzene follows the electrophilic substitution mechanism, in which a hydrogen atom of benzene is replaced by a halogen atom (Cl or Br) using a Lewis acid catalyst like FeCl3 / FeBr3.
Step 1: Formation of Electrophile
The catalyst polarises the halogen molecule and generates a strong electrophile:
Cl2 + FeCl3 → Cl + + FeCl4-
- The Cl⁺ (or Br⁺) acts as the electrophile.
Step 2: Attack on Benzene Ring
The electrophile (Cl⁺) attacks the π-electron cloud of the benzene ring, forming an unstable intermediate called the arenium ion or σ-complex.
In this step:
- Aromaticity is lost for a short time
- A positively charged intermediate is formed
Step 3: Removal of Proton (Deprotonation)
The base (FeCl4⁻) removes a hydrogen ion (H⁺) from the intermediate.
This leads to:
- Restoration of aromaticity
- Formation of final product: chlorobenzene (or bromobenzene)
- Regeneration of catalyst (FeCl3)
FeCl_4^- + H^+ \rightarrow HCl + FeCl_3