1. Electrophilic Attack:
* Bromine is a polarizable molecule, meaning the electron cloud can be distorted.
* The double bond in cyclohexene is electron-rich, attracting the slightly positive bromine atom.
* This electrophilic attack forms a cyclic bromonium ion intermediate.
2. Nucleophilic Attack:
* The bromide ion (Br⁻) acts as a nucleophile, attacking the carbon atom adjacent to the bromonium ion.
* This opens the ring and forms a vicinal dibromide product (1,2-dibromocyclohexane).
Overall Reaction:
```
Cyclohexene + Br₂ → 1,2-dibromocyclohexane
```
Mechanism:
The reaction proceeds through a two-step mechanism:
1. Formation of the bromonium ion:
```
Br₂ + cyclohexene → [bromonium ion]
```
2. Nucleophilic attack by bromide ion:
```
[bromonium ion] + Br⁻ → 1,2-dibromocyclohexane
```
Key Points:
* The reaction is stereospecific, meaning the product has a specific stereochemical configuration.
* The bromonium ion intermediate is highly reactive and typically short-lived.
* The reaction is generally carried out in a non-polar solvent like dichloromethane.
Applications:
Electrophilic addition reactions with alkenes are widely used in organic synthesis, including:
* Halogenation: Addition of halogens like bromine and chlorine to form dihaloalkanes.
* Hydrohalogenation: Addition of hydrogen halides like HCl and HBr to form haloalkanes.
* Hydration: Addition of water to form alcohols.
