Allylic Bromination (NBS, hv): Allylic C-H to Allylic Br

N-bromosuccinimide (NBS) and N-chlorosuccinimide (NCS) selectively halogenate allylic and benzylic positions via a radical chain mechanism. These reagents maintain a low concentration of X₂ (Br₂ or Cl₂) in situ, ensuring radical substitution outcompetes electrophilic addition across the π bond.

Quick Summary

Reagent System	Halogen	Conditions	Product
NBS + hν	Bromine	Light or radical initiator, dry CCl₄/CH₂Cl₂	Allylic bromide
NBS + EtOH	Bromine	Alcohol solvent (EtOH, MeOH)	Allylic bromide
NCS + hν	Chlorine	Light or radical initiator, dry CCl₄/CH₂Cl₂	Allylic chloride

• Initiation: X₂ absorbs light (or reacts with initiator radicals) to form two halogen atoms.
• Propagation: X· abstracts an allylic/benzylic hydrogen → allyl radical + HX; the radical then reacts with X₂ to give the allylic halide and regenerate X·.
• Role of NBS/NCS: HX + NXS → X₂ + succinimide. The buffer keeps [X₂] low so substitution dominates over addition.
• Outcome: Allylic (and benzylic) halides; expect mixtures whenever more than one allylic C–H site is available.

Mechanism (3 Steps)

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

NBS + Light (hν) — Allylic Bromination

N-bromosuccinimide (NBS) in the presence of light (hν) or a radical initiator performs allylic bromination, selectively adding bromine to an allylic carbon (a carbon adjacent to a double bond).

This is the classic free radical chain reaction. Light initiates homolytic cleavage of Br₂ (generated in situ from NBS and HBr) to form bromine radicals, which abstract a hydrogen from the allylic position. The resulting allylic radical is resonance-stabilized, making this position highly favored.

Reagents/conditions: NBS, light (hν) or a radical initiator such as BPO/AIBN, typically in dry CCl₄ or CH₂Cl₂.

NBS + Alcohol (EtOH/MeOH) — Allylic Bromination

N-bromosuccinimide (NBS) in the presence of an alcohol solvent like ethanol (EtOH) or methanol (MeOH) also performs allylic bromination, selectively adding bromine to an allylic carbon.

This is a free radical chain reaction. The alcohol solvent serves as a hydrogen atom source and helps moderate the reaction conditions. NBS generates a low concentration of Br₂ in situ, which undergoes homolytic cleavage to form bromine radicals that abstract a hydrogen from the allylic position. The resulting allylic radical is resonance-stabilized, making this position highly favored.

Reagents/conditions: NBS, alcohol solvent (EtOH, MeOH), often at room temperature or with mild heating.

Key difference from NBS + hν: The alcohol solvent provides milder conditions and can participate in subsequent substitution reactions if desired.

NCS + Light (hν) — Allylic Chlorination

N-chlorosuccinimide (NCS) in the presence of light (hν) or a radical initiator performs allylic chlorination, selectively adding chlorine to an allylic carbon (a carbon adjacent to a double bond).

This is a free radical chain reaction analogous to NBS bromination. Light initiates homolytic cleavage of Cl₂ (generated in situ from NCS and HCl) to form chlorine radicals, which abstract a hydrogen from the allylic position. The resulting allylic radical is resonance-stabilized, making this position highly favored.

Reagents/conditions: NCS, light (hν) or a radical initiator such as BPO/AIBN, typically in dry CCl₄ or CH₂Cl₂.

Key differences from NBS:

• Chlorine radicals are more reactive but less selective than bromine radicals
• The C–Cl bond is stronger than C–Br, making allylic chlorides more stable but less reactive in subsequent substitution reactions
• Selectivity for tertiary > secondary > primary allylic positions is slightly reduced compared to bromination

Mechanistic Checklist

• Radical chain: initiation (X₂ → 2 X·), two propagation steps, and optional termination by radical recombination.
• X· abstraction targets the weakest available C–H; allylic/benzylic C–H bonds dominate because of resonance stabilization.
• NBS/NCS rapidly converts HX to X₂ + succinimide, buffering [X₂] so electrophilic addition across C=C is minimal.
• Resonance delocalization of the allyl radical allows halogenation at multiple carbons, giving mixtures of allylic halides.
• Commercial NBS/NCS contains a small amount of X₂; the reaction only needs a light source or a radical initiator to get started.

Worked Examples

Example 1: NBS + hν — 2-Methylpent-2-ene

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

Example 2: NBS + EtOH — Cyclohexene

Key insight: Cyclohexene is a favorite exam substrate because its symmetry simplifies the analysis. Both allylic positions (C3 and C6) are equivalent, so only one product forms. The bromine ends up on a carbon adjacent to—but not part of—the double bond. The double bond itself remains intact.

Example 3: NCS + hν — 1-Methylcyclohexene

Unlike cyclohexene, 1-methylcyclohexene has two distinct allylic positions: the methyl group attached to C1 and the CH₂ at C6. Both are allylic (adjacent to the C=C), so both can undergo hydrogen abstraction—expect a mixture of products.

Exam tip: When you see NCS instead of NBS, the mechanism is identical—just swap Br for Cl throughout. Chlorine radicals are more reactive but slightly less selective than bromine.

Multiple Alkenes & Selectivity

• Allylic and benzylic C–H bonds react fastest; tertiary C–H bonds elsewhere can still participate if they rival allylic stabilization.
• Larger rings or polyenes may furnish several allylic sites. Expect mixtures unless one radical is uniquely stabilized or sterically accessible.
• Keeping [X₂] low is crucial. Excess X₂ (or absence of NBS/NCS) leads to electrophilic addition and dihalides.
• Benzylic halogenation follows the same chain logic. Resonance across the aromatic ring can generate multiple positional isomers.
• Bromination vs Chlorination selectivity: Bromine radicals are more selective (greater preference for tertiary > secondary > primary), while chlorine radicals are more reactive but less discriminating.

Practical Tips & Pitfalls

• Solvent & exclusion of water: Run the reaction in dry, non-protic solvents (CCl₄, CH₂Cl₂, C₆H₆) for NBS/NCS + hν. Water steers NBS toward bromohydrin formation.
• Alcohol solvents: For NBS + EtOH conditions, the alcohol serves as both solvent and hydrogen source, providing milder reaction conditions.
• Initiation: Use a UV lamp or add a radical initiator (benzoyl peroxide, AIBN). Thermal initiation alone is slow.
• Control [X₂]: Add NBS/NCS in portions or use a reflux condenser to prevent X₂ build-up. Keep the mixture dilute to minimize competing addition.
• Quench & workup: Destroy radicals (e.g., with oxygen or radical inhibitors) and filter off succinimide. Residual X₂ can be removed with sodium thiosulfate washes.
• Safety: NBS and NCS are irritants and decompose in light/moisture. Handle under subdued light with gloves and dry glassware.

Exam-Style Summary

Reaction	Reagents	Mechanism	Product
Allylic Bromination	NBS + hν, dry solvent	Radical chain	Allylic bromide
Allylic Bromination	NBS + EtOH/MeOH	Radical chain	Allylic bromide
Allylic Chlorination	NCS + hν, dry solvent	Radical chain	Allylic chloride

• Mechanism: X₂ initiation → X· abstraction of allylic H → allyl radical → X₂ capture → allylic halide + X·; NBS/NCS + HX → X₂ + succinimide.
• Outcome: Allylic/benzylic halides; 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 stabilization govern product ratios.

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Related Reading

• Alkene anti-Markovnikov bromination (radical HBr)
• Alkene bromination via bromonium
• Alkene epoxidation (mCPBA)

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Interactive Toolbox

• Explore radical chains and allylic halogenation outcomes in the Reaction Solver.
• Visualize each radical step in the Mechanism Solver.
• Practice IUPAC naming of allylic halides with the IUPAC Namer.

FAQ / Exam Notes

Why does NBS/NCS favor allylic substitution instead of addition? Because NBS/NCS converts the HX formed in propagation back into X₂; only trace X₂ 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 HX to produce X· and kick off the chain.

Do benzylic C–H bonds react the same way? Absolutely. Benzylic radicals are resonance-stabilized, so NBS/NCS + hν cleanly halogenates 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 halogenation.

What's the difference between NBS + hν and NBS + EtOH? Both produce allylic bromides via the same radical mechanism. The alcohol solvent in NBS + EtOH provides milder conditions and can participate in subsequent reactions if desired.

Why use NCS instead of NBS? When you need an allylic chloride instead of bromide. Chlorine radicals are more reactive but slightly less selective than bromine radicals.