Potential energy profile
ReactantsInt.Products
Ready
Electrophile generated
Cl⁺ is generated from Cl₂ + FeCl₃. It approaches the electron-rich benzene ring.
Step 1 — slow
Cl⁺ attacks ring
π electrons from the ring attack Cl⁺. One C becomes sp³, losing aromaticity. Arenium ion forms.
Intermediate
Arenium ion
The + charge is delocalised over the 5 remaining carbons. Aromaticity is broken. Simulation paused.
Step 2 — fast
[FeCl₄]⁻ abstracts H⁺
[FeCl₄]⁻ removes H⁺ from the sp³ carbon. C–H electrons restore aromaticity.
Product
Chlorobenzene
Chlorobenzene formed. Aromaticity fully restored. FeCl₃ regenerated as catalyst.
Speed:
1.0×
Ready
Generating the electrophile
FeCl₃ acts as a Lewis acid catalyst. It reacts with Cl₂ to form Cl⁺ and [FeCl₄]⁻. Without FeCl₃, Cl₂ is not electrophilic enough to attack benzene.
Step 1 — electrophilic attack
The π electrons of benzene attack Cl⁺. One ring carbon becomes sp³, disrupting aromaticity and forming the arenium ion with delocalised positive charge.
Step 2 — restoring aromaticity
[FeCl₄]⁻ abstracts H⁺ from the sp³ carbon. The C–H electrons restore the π system, giving back full aromaticity and regenerating FeCl₃.
Why substitution, not addition?
Addition would permanently destroy aromaticity. The strong driving force to restore the stable aromatic system means H⁺ is eliminated rather than a nucleophile adding.