Aromatic bromination (SEAr): practice and mechanism
With FeBr₃, Br₂ becomes an electrophile and substitutes a ring hydrogen (no addition); the substituent decides the position.
Example
The Lewis acid FeBr₃ polarises Br₂ into an electrophile. An electrophilic aromatic substitution follows, not an addition: the aromatic system is kept. The existing substituent decides the position (o/p or m directing).
Mechanism step by step
- 1
FeBr₃ (Lewis acid) binds one Br of Br₂ and polarises it: Br^δ+–Br–FeBr₃^δ−.
- 2
The arene attacks the positive Br: σ complex.
- 3
FeBr₄⁻ removes the H, regenerating HBr and FeBr₃: bromoarene (substitution, not addition).
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
- Meta would be right for an electron-withdrawing group. This substituent activates (or is a halogen) and directs ortho/para.
- Para would be right for an activating group. This substituent withdraws electrons and directs meta.
Often confused with: Nitration (SEAr) · Benzylic bromination
Frequently asked questions
Why doesn't phenol need FeBr₃?
The OH group activates the ring so strongly that Br₂ alone reacts, three times, to 2,4,6-tribromophenol.
Why are halogens o/p-directing but deactivating?
Their −I effect lowers the overall reactivity, but their +M effect stabilises the ortho/para σ complex in particular.