Allylic Bromination Nbs Light

Alkene Reactions: Allylic Bromination using NBS and Light

Allylic bromination of alkenes with NBS and light

N-bromosuccinimide (NBS) with light (hν) or a radical initiator selectively brominates allylic and benzylic positions by a radical chain. The key is that NBS traps the HBr formed during propagation and feeds it back into Br₂, maintaining only a trace concentration of Br₂ so that radical substitution outcompetes electrophilic addition across the π bond.


Quick Summary

  • Reagents/conditions: NBS, light (hν) or a radical initiator such as BPO/AIBN, typically in dry CCl₄ or CH₂Cl₂.
  • Initiation: Br₂ absorbs light (or reacts with initiator radicals) to form two bromine atoms.
  • Propagation: Br· abstracts an allylic/benzylic hydrogen → allyl radical + HBr; the radical then reacts with Br₂ to give the allylic bromide and regenerate Br·.
  • Role of NBS: HBr + NBS → Br₂ + succinimide. The buffer keeps [Br₂] low so substitution dominates over addition.
  • Outcome: Allylic (and benzylic) bromides; expect mixtures whenever more than one allylic C–H site is available.


Mechanism (3 Steps)

Step 1: Substrate aligned for allylic abstraction
Step 1 — The alkene adopts a geometry that highlights the allylic C–H bonds susceptible to radical abstraction.
Step 2: Br2 photolysis to two bromine radicals
Step 2 — Initiation: hν (or BPO/AIBN) homolyzes Br₂, generating two Br·; NBS converts any HBr produced back into Br₂.
Step 3: Bromine radical abstracts an allylic hydrogen
Step 3 — Propagation: Br· abstracts an allylic H to give an allyl radical plus HBr.
Step 4: Allyl radical reacts with Br2
Step 4 — Propagation: the allyl radical captures Br₂, forming the allylic bromide and regenerating Br· to continue the chain.
Step 5: chain summary
Step 5 — Chain maintenance: NBS scavenges the HBr formed, regenerating Br₂ while succinimide precipitates out.

Real allylic radicals can equilibrate by resonance, so substrates with multiple allylic sites often deliver product mixtures. The panels here highlight the direct bromination pathway at the abstracted carbon.


Mechanistic Checklist

  • Radical chain: initiation (Br₂ → 2 Br·), two propagation steps, and optional termination by radical recombination.
  • Br· abstraction targets the weakest available C–H; allylic/benzylic C–H bonds dominate because of resonance stabilisation.
  • NBS rapidly converts HBr to Br₂ + succinimide, buffering [Br₂] so electrophilic addition across C=C is minimal.
  • Resonance delocalisation of the allyl radical allows bromination at multiple carbons, giving mixtures of allylic bromides.
  • Commercial NBS contains a small amount of Br₂; the reaction only needs a light source or a radical initiator to get started.


Worked Example — 2-Methyl-2-butene

Substrate: 2-methyl-2-butene
Substrate — 2-methyl-2-butene
Reagents: NBS, hν (or BPO/AIBN), CCl4 or CH2Cl2
NBS, hν (or BPO/AIBN), dry CCl₄/CH₂Cl₂
Product isomer 1
Allylic bromide A
Product isomer 2
Allylic bromide B

Two nonequivalent allylic positions exist on this substrate, so bromination furnishes a pair of regioisomeric allylic bromides. Reaction conditions (temperature, solvent, initiator, concentration) influence the ratio, but mixtures are the norm.


Multiple Alkenes & Selectivity

  • Allylic and benzylic C–H bonds react fastest; tertiary C–H bonds elsewhere can still participate if they rival allylic stabilisation.
  • Larger rings or polyenes may furnish several allylic sites. Expect mixtures unless one radical is uniquely stabilized or sterically accessible.
  • Keeping [Br₂] low is crucial. Excess Br₂ (or absence of NBS) leads to electrophilic addition and dibromides.
  • Benzylic bromination follows the same chain logic. Resonance across the aromatic ring can generate multiple positional isomers.


Practical Tips & Pitfalls

  • Solvent & exclusion of water: Run the reaction in dry, non-protic solvents (CCl₄, CH₂Cl₂, C₆H₆). Water steers NBS toward bromohydrin formation.
  • Initiation: Use a UV lamp or add a radical initiator (benzoyl peroxide, AIBN). Thermal initiation alone is slow.
  • Control [Br₂]: Add NBS in portions or use a reflux condenser to prevent Br₂ build-up. Keep the mixture dilute to minimise competing addition.
  • Quench & workup: Destroy radicals (e.g., with oxygen or radical inhibitors) and filter off succinimide. Residual Br₂ can be removed with sodium thiosulfate washes.
  • Safety: NBS is an irritant and decomposes in light/moisture. Handle under subdued light with gloves and dry glassware.


Exam-Style Summary

  • Reagents: NBS + light (hν) or a radical initiator; dry, non-protic solvent.
  • Mechanism: Br₂ initiation → Br· abstraction of allylic H → allyl radical → Br₂ capture → allylic bromide + Br·; NBS + HBr → Br₂ + succinimide.
  • Outcome: Allylic/benzylic bromides; more than one allylic site leads to mixtures, while addition across C=C remains suppressed.
  • Selectivity: Allylic/benzylic > tertiary > secondary > primary (radical stability). Sterics and radical stabilisation govern product ratios.

Related Reading


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FAQ / Exam Notes

Why does NBS favour allylic substitution instead of addition? Because NBS converts the HBr formed in propagation back into Br₂; only trace Br₂ is present at any moment, so radical substitution outruns electrophilic addition across the double bond.

Can other initiators replace light? Yes. Benzoyl peroxide, AIBN, or other peroxides generate radicals that abstract H from HBr to produce Br· and kick off the chain.

Do benzylic C–H bonds react the same way? Absolutely. Benzylic radicals are resonance-stabilized, so NBS + hν cleanly brominates the benzylic position with the same chain mechanism.

What if water is present? NBS + water promotes electrophilic addition to form bromohydrins. Keep the medium dry for allylic bromination.