Lecture Notes: Bromination at Allylic and Benzylic Positions

Overview

• Radical Halogenation: Previously known technique for halogenating alkanes, specifically useful for predicting bromine's position on an alkane.
• Focus: Bromination at the allylic or benzylic position (positions adjacent to double bonds or benzene rings).

Definitions

• Allylic Position: The carbon atom adjacent to a double bond.
• Benzylic Position: The carbon atom adjacent to a benzene ring.

Bromination at Allylic and Benzylic Positions

• Objective: Place a bromine atom specifically at the allylic or benzylic position.
• Reagent Used: N-bromo succinimide (NBS).

Role of N-bromo Succinimide (NBS)

• Structure: Succinimide with a bromo group on the nitrogen atom.
• Reason for Use:
  • Molecular bromine can cause unwanted dibromination with pi bonds.
  • NBS avoids addition reactions that can occur with molecular bromine.

Mechanism

1. Catalyst: Hydrobromic acid (HBr) helps produce small amounts of molecular bromine.
2. Bromine Radical Formation:
   • Homolysis of bromine occurs, preventing interaction with pi bonds.
   • Bromine radicals are generated in solution.
3. Radical Interaction:
   • Bromine radicals interact with hydrogen at the allylic position.
   • Produces HBr and an allylic radical.
4. Stabilization:
   • Allylic radicals are resonance stabilized by adjacent pi bonds.
5. Propagation:
   • Allylic radical encounters bromine molecule, forming a new carbon-bromine bond.
   • Propagates another bromine radical.
6. Reaction Continuation:
   • Bromination at the allylic position continues until all NBS is consumed.

Key Reaction Considerations

• Minimization of Reactive Species:
  • Keep hydrobromic acid and molecular bromine concentrations low.
  • Ensure sufficient time for bromine radical formation and radical chemistry.

Conclusion

• Importance: Effective bromination at specific molecular sites without unwanted reactions.
• Reagent of Choice: NBS for controlled bromination in the presence of pi bonds.