Allylic bromination with NBS template

N-Bromosuccinimide keeps a low, steady concentration of Br₂: Br• abstracts an allylic H from cyclohexene, the allylic radical (delocalised over both ends) abstracts Br from Br₂, and HBr + NBS regenerate Br₂, giving 3-bromocyclohexene.

Mechanism step by step

A radical chain: Br• takes an allylic H, and the resonance-stabilised allylic radical takes Br from Br₂

1. Initiation – Light (or a peroxide) breaks the weak Br–Br bond of the trace Br₂
2. H abstraction – Br• takes an allylic H (allylic C–H about 360 kJ mol⁻¹, weaker than the vinylic and alkyl C–H)
3. Allylic resonance – The odd electron is shared by both ends (here the two ends are equivalent)
4. Br₂ from NBS – HBr reacts with NBS (ionic): Br₂ is made only as fast as it is used, so its concentration stays low
5. Br transfer – The allylic radical takes Br from Br₂; the new Br• carries the chain

Key points

• The allylic radical is stabilised by resonance (delocalisation over three p orbitals), so allylic C–H bonds are weak and are attacked selectively.
• Low [Br₂] is the key: at high concentration Br₂ adds across the C=C (bromonium ion) instead of substituting.
• An unsymmetrical allylic radical reacts at either end, so mixtures of allylic bromides (sometimes with the C=C moved) can form.
• Benzylic C–H bonds react the same way: toluene with NBS and light gives benzyl bromide.

Drawn with 93 atoms, 17 arrows, 9 shapes – every one editable in MolDraw.

Sources

• Clayden, Greeves & Warren, Organic Chemistry, 2nd ed., OUP 2012, ch. 37 Textbook
• Wade, Organic Chemistry, 8th ed., Pearson 2013, ch. 6 Textbook
• Smith, March's Advanced Organic Chemistry, 8th ed., Wiley 2020, ch. 14 Textbook