The bromination of alkenes is an electrophilic addition reaction where bromine adds across the carbon–carbon double bond. Here’s the mechanism explained simply, without equations:
- Polarization of bromine:
- The double bond in an alkene is rich in electrons.
- When bromine (Br₂) comes near, the electron-rich double bond pushes electrons in the Br–Br bond, making one bromine slightly positive and the other slightly negative.
- Formation of bromonium ion:
- The slightly positive bromine atom is attacked by the double bond.
- This leads to the breaking of the Br–Br bond, and one bromine attaches to both carbons of the double bond, forming a three-membered ring called the bromonium ion.
- Nucleophilic attack:
- The negatively charged bromide ion (the other bromine atom) attacks the bromonium ion from the side opposite to where it is attached.
- This opens the three-membered ring.
- Product formation:
- The final product is a vicinal dibromide, meaning two bromine atoms are attached to adjacent carbon atoms.
Key point:
- The addition is anti-addition, meaning the two bromine atoms attach on opposite sides of the original double bond.