Allylic Bromination with NBS

Overview

This lecture covers how to brominate the allylic position in conjugated systems using NBS, and explains why traditional free radical halogenation does not work in these cases.

Problems with Traditional Free Radical Halogenation

• Standard Br₂ with light and heat works for alkanes but fails with alkenes due to double bond reactivity.
• In conjugated systems, excess Br₂ reacts with the double bond, resulting in anti-addition and not selective allylic bromination.
• The double bond forms a bromonium ion, leading to addition across the double bond instead of allylic substitution.

Allylic Bromination Using NBS

• NBS (N-bromo succinimide) is used instead of Br₂ for selective allylic bromination.
• The reaction is performed with NBS, light, and sometimes heat.
• NBS provides a low concentration of Br₂, avoiding unwanted addition to the double bond.
• Low Br₂ ensures only the allylic position is brominated via a free radical mechanism.

Mechanism with NBS

• Initiation: Low concentration Br₂ homolytically cleaves to produce bromine radicals.
• Propagation 1: Bromine radical abstracts an allylic hydrogen, forming a stable allylic radical and HBr.
• Propagation 2: Allylic radical reacts with Br₂ (from NBS), producing the allylic bromide and another bromine radical.
• HBr formed helps maintain the equilibrium for low Br₂ concentration.
• Overall, NBS allows selective bromination without double bond addition.

Key Terms & Definitions

• Allylic position — carbon atom adjacent to a double bond in an organic molecule.
• Halogenation — the process of introducing a halogen atom (like Br) into a molecule.
• NBS (N-bromo succinimide) — reagent used to generate low concentrations of Br₂ selectively for allylic bromination.
• Free radical mechanism — reaction pathway involving species with unpaired electrons.

Action Items / Next Steps

• Review the differences between allylic bromination with NBS and standard free radical halogenation.
• Practice drawing mechanisms for NBS bromination at the allylic position.
• Be prepared to explain why NBS is necessary for selective allylic bromination in conjugated systems.