NBS (N-Bromosuccinimide): Allylic Bromination Guide | Chemca

Organic Chemistry

NBS (N-Bromosuccinimide): Allylic Bromination

By Chemca Editorial Team • Last Updated: January 2026 • 8 min read

N-Bromosuccinimide (NBS) is a specialized reagent used in organic chemistry primarily for Free Radical Substitution. It specifically brominates allylic and benzylic positions without adding across the double bond (which would happen if simple Bromine, $Br_2$, were used).

1. General Reaction (Wohl-Ziegler Reaction)

Reaction of an alkene with NBS in the presence of light ($h\nu$) or peroxide initiator (ROOR) in a non-polar solvent like $CCl_4$.

$$ R-CH_2-CH=CH_2 + \text{NBS} \xrightarrow{h\nu, \ CCl_4} \underbrace{R-CH(Br)-CH=CH_2}_{\text{Allylic Bromide}} + \text{Succinimide} $$

Key Features:

Reagent: NBS (Solid source of Bromine).
Condition: Light ($h\nu$), Heat ($\Delta$), or Peroxide ($ROOR$).
Mechanism: Free Radical Substitution.
Selectivity: Attacks the Allylic ($\alpha$ to C=C) or Benzylic ($\alpha$ to Benzene) position.

2. Detailed Mechanism

The reaction proceeds via a radical chain mechanism.

Step 1: Initiation

A small amount of $Br_2$ is generated from NBS. Homolysis of $Br_2$ by light produces Bromine radicals ($Br^\bullet$).

Step 2: Propagation (The Crucial Step)

A. Abstraction: The bromine radical abstracts an allylic hydrogen atom to form HBr and a stable Allylic Radical.
(Why Allylic? Because allylic radicals are resonance stabilized).

$$ R-CH_2-CH=CH_2 + Br^\bullet \rightarrow [R-\dot{C}H-CH=CH_2 \leftrightarrow R-CH=CH-\dot{C}H_2] + HBr $$

B. Substitution: The allylic radical reacts with $Br_2$ (generated in situ) to form the product and regenerate $Br^\bullet$.

$$ Allyl^\bullet + Br_2 \rightarrow Allyl-Br + Br^\bullet $$

Step 3: Role of NBS (Maintaining Low Br2 Conc.)

NBS reacts with the HBr produced in step 2A to generate the $Br_2$ required for step 2B.

$$ \text{NBS} + HBr \rightarrow \text{Succinimide} + Br_2 $$

Why doesn't it add to the double bond?

Electrophilic addition of $Br_2$ across the double bond requires a high concentration of $Br_2$. NBS ensures the concentration of $Br_2$ remains very low, favoring substitution (allylic bromination) over addition.

3. Selectivity Rules

When multiple allylic/benzylic positions are available, the radical stability determines the major product.

Radical Stability Order: Benzylic $\approx$ Allylic > $3^\circ$ > $2^\circ$ > $1^\circ$ > Methyl > Vinylic.

Example 1: Toluene

$$ Ph-CH_3 + NBS \rightarrow Ph-CH_2-Br \text{ (Benzyl Bromide)} $$

Example 2: Ethylbenzene

$$ Ph-CH_2-CH_3 + NBS \rightarrow Ph-CH(Br)-CH_3 \text{ (Secondary Benzylic is preferred over Primary)} $$

Example 3: Cyclohexene

$$ \text{Cyclohexene} + NBS \rightarrow \text{3-Bromocyclohexene} $$

4. Other Applications

Oxidation of Alcohols: NBS can oxidize primary alcohols to aldehydes and secondary alcohols to ketones in aqueous medium (less common in exams but possible).
Bromination of Ketones: NBS can brominate the $\alpha$-position of ketones (via enol formation).

5. Summary vs. Other Reagents

Reagent	Substrate	Product Type
$Br_2 / CCl_4$ (Dark)	Alkene	Vicinal Dibromide (Addition)
NBS / $h\nu$	Alkene	Allyl Bromide (Substitution)
$Br_2 / FeBr_3$	Benzene	Bromobenzene (EAS)

NBS Quiz

Test your concepts on Radical Substitution. 10 MCQs with explanations.

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