Aromatic chlorination (SEAr): practice and mechanism
FeCl₃ activates Cl₂ into an electrophile and a ring H is replaced by Cl. The existing substituent decides the position (o/p- or m-directing).
Example
The Lewis acid FeCl₃ polarises Cl₂ so strongly that a Cl⁺-like electrophile attacks the ring. After losing a proton the ring is aromatic again; the directing rules are the same as for bromination.
Mechanism step by step
- 1
Forming the electrophile: FeCl₃ polarises Cl₂ (Cl–Cl–FeCl₃); the outer Cl becomes the electrophile Cl⁺.
- 2
The ring π electrons attack the electrophile. A carbocation forms (σ-complex, arenium ion) with its charge spread over three carbons; aromaticity is lost for a moment.
- 3
A base removes the H from the attacked carbon. The ring becomes aromatic again and the electrophile has replaced an H.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
- An ortho/para-directing group sends the chlorine ortho or para, not meta.
- Electron-withdrawing groups direct to meta, not para.
Often confused with: Aromatic bromination (SEAr)
Frequently asked questions
Where does the new substituent go?
Electron-donating groups (alkyl, OH, OR, NH₂, NHCOR) and halogens direct ortho/para; electron-withdrawing groups (NO₂, C=O, CN, SO₃H) direct meta. The reason is which σ-complexes are best stabilised.
Why is a catalyst needed?
Benzene is unreactive because of its aromaticity. Without a Lewis acid Cl₂ is not a strong enough electrophile. Only very electron-rich rings (phenol, aniline) react without one.