Benzylic bromination: practice and mechanism
NBS and light brominate radically at the benzylic position (the carbon next to the ring), not on the ring.
Example
N-bromosuccinimide in light supplies small amounts of Br₂ and bromine radicals. Substitution happens radically at the benzylic position, because the benzyl radical is stabilised by the ring. Without a Lewis acid the ring itself is not brominated.
Mechanism step by step
- 1
Light splits the few Br₂ molecules supplied by NBS into Br· radicals.
- 2
Br· abstracts a benzylic H; the benzyl radical is resonance-stabilised by the ring.
- 3
The benzyl radical reacts with Br₂: benzyl bromide and a new Br· (chain).
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
Often confused with: Aromatic bromination (SEAr) · Side-chain oxidation
Frequently asked questions
Why NBS instead of Br₂?
NBS keeps the Br₂ concentration very low, so there is no addition to double bonds and no ring substitution.
How is this different from ring bromination?
Light/radical starter (side chain) vs. Lewis acid/cold (ring). The German mnemonic: SSS (sun, boiling heat, side chain) vs. KKK (cold, catalyst, core).
Why is only the benzylic position brominated?
The benzylic radical is resonance-stabilised by the ring, so the benzylic C–H bond is much weaker than other C–H bonds.
Why NBS instead of bromine?
NBS keeps the Br₂ concentration very low, so radical substitution happens without addition or ring bromination.