Allylic Bromination Lecture Notes

Introduction to Allylic Bromination

Allylic bromination is a free radical halogenation of alkanes containing double bonds.
General free radical halogenation reactions often yield a mixture of products, making them less useful in synthesis.

Pentane Chlorination: Results in a mixture of products:
- 1-chloro-pentane (minor, primary radical)
- 2-chloro-pentane (major, secondary radical)
- 3-chloro-pentane (major, secondary radical)
Cyclohexane Halogenation: Produces a single product due to molecular symmetry and only having one type of hydrogen.
2-Methyl Propane Bromination: Due to bromination’s selectivity, it yields a single product by forming a stable tertiary radical.

Allylic Compounds:
- Allylic position is adjacent to a C=C double bond.
- Radicals at allylic positions are resonance stabilized, contributing to stability.

Initial Step: Abstraction of a hydrogen atom.
- This can occur at allylic or neighboring positions.
Resonance in Allylic Radicals:
- The allylic radical is resonance stabilized.
- Draw resonance contributors to understand stabilization.
Propagation Step: Bromine adds to the allylic radical, forming the same product regardless of the resonance structure.
Secondary Radical Comparison:
- Less stable than resonance-stabilized radicals, hence less product formation.

Highly selective, useful in organic synthesis.
Side reactions occur if bromine concentration is too high, leading to unwanted products.

Used to maintain low bromine concentration and avoid side reactions in allylic bromination.
NBS produces bromine in the presence of HBr.
Used in elimination reactions as a substitute for Br2.
Requires light or heat to activate the radical reaction.

Allylic bromination is a controlled and selective reaction, effective when using NBS.
Understanding resonance and stabilization helps predict and control product formation.