1. NBS brominates the allylic C–H, not the double bond.
Cyclohexene → 3-bromocyclohexene: the sp³ carbon next to C=C is halogenated.
2. The reaction runs because the allylic radical is resonance-stabilized.
The unpaired electron is delocalized over both ends of the allyl system.
3. NBS is used instead of Br₂ to keep the bromine concentration very low.
Propene → allyl bromide: trace Br₂ favors radical substitution over ionic addition.
4. Delocalization lets bromine land at either end of the allyl system.
Because the radical is spread out, rearranged products and mixtures are common.
5. Benzylic C–H reacts the same way for the same reason.
Toluene → benzyl bromide; both radicals are stabilized by an adjacent π system.
Summary
NBS + light/heat · brominates allylic C–H · via resonance-stabilized allylic radical · low [Br₂] blocks addition · either end can react · benzylic behaves the same.
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The allylic C–H — the sp³ carbon–hydrogen bond directly next to the C=C double bond.
Removing the allylic H gives a resonance-stabilized allylic radical, which is much more stable than an ordinary radical.
NBS releases Br₂ at a very low steady-state concentration, so radical substitution wins over ionic addition to the double bond.
The allylic radical is delocalized, so bromine can bond at either end of the allyl system (allylic rearrangement).
Draw this on the whiteboard
Open the OChem Board whiteboard — benzene rings, wedge/dash bonds, and a clickable periodic table built in. No account needed.