Potential energy profile
ReactantsInt.Products
Ready
Br₂ approaches
Br₂ is polarised by the electron-rich C=C π bond. The nearer Br becomes δ⁺ and the farther Br becomes δ⁻.
Step 1 — slow
π bond attacks δ⁺Br
The π electrons attack δ⁺Br. The Br–Br bond breaks heterolytically. A carbocation and Br⁻ ion form.
Intermediate
Carbocation
One carbon is sp³ with a positive charge. Br⁻ ion is nearby. Simulation paused.
Step 2 — fast
Br⁻ attacks carbocation
Br⁻ uses its lone pair to attack the carbocation. A new C–Br bond forms.
Product
1,2-dibromoethane
Both carbons are now sp³ with Br attached. Addition is complete.
Speed:
1.0×
Ready
Polarisation of Br₂
As Br₂ approaches the electron-rich C=C, the π cloud induces a dipole in the Br–Br bond. The nearer Br becomes δ⁺ (electrophile) and the farther Br becomes δ⁻.
Step 1 — heterolytic fission
π electrons attack δ⁺Br. The Br–Br bond breaks heterolytically — both electrons go to the δ⁻Br, forming Br⁻. A carbocation intermediate forms (sp³ carbon, empty p orbital).
Step 2 — nucleophilic attack
Br⁻ acts as a nucleophile. It uses a lone pair to attack the carbocation carbon, forming a new C–Br bond and giving the product 1,2-dibromoethane.
Why addition, not substitution?
Unlike arenes, alkenes have no aromatic stabilisation to preserve. The C=C π bond is weaker than a C–C σ bond, so addition is energetically favoured.