Substituting the sp³ C–H next to a C=C by way of a resonance-stabilized allylic radical.
Quick answer
NBS with light or heat replaces an allylic hydrogen (the sp³ C–H beside a C=C) with bromine through a radical chain. It works because abstracting that hydrogen gives a resonance-stabilized allylic radical, and NBS keeps Br₂ so dilute that substitution beats ionic addition.
⚙ Mechanism · Allylic Bromination (NBS)3 steps
Initiation — homolysis of Br2.
NBS supplies the Br2, by reacting with the HBr the chain itself produces. That is the whole trick: the Br2 concentration stays very low — enough to sustain the chain, far too little to add across the double bond.
Propagation 1 — a bromine radical takes the allylic hydrogen.
The allylic C–H is the weakest in the molecule because the radical left behind is delocalised across three carbons. Draw the second resonance form and you will see why this reaction gives two products: bromine can end up at either end.
Propagation 2 — the allyl radical takes a bromine from Br2.
NBS matters because of what it does not do: it keeps the Br2 concentration extremely low. Enough for this step, far too little for bromine to add across the C=C — which is the competing reaction you are avoiding.
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.
allyl radical
resonance form
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.
4. Delocalization lets bromine land at either end of the allyl system.
allyl bromide
3-bromocyclohexene
Because the radical is spread out, rearranged products and mixtures are common.
5. Benzylic C–H reacts the same way for the same reason.
allyl radical
benzyl radical
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.
How each reagent works — the arrow pushing
Electron flow only. Follow the arrows; the structures do the talking.
NBS — allylic (benzylic) bromination1 step
Why it works · NBS quietly maintains a very low Br2 concentration — too low for addition across the C=C, but plenty for a radical chain. A bromine radical abstracts the weak allylic C–H (the radical is resonance-stabilized over the π system), and Br then caps it. Net: swap an allylic (or benzylic) H for Br, leaving the double bond intact.
Quiz yourself
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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.