NBS (N-Bromosuccinimide) (CAS 128-08-5) — Complete Guide to Properties and Applications

Category: Brominating Reagent | Formula: C4H4BrNO2 | MW: 177.98 g/mol

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What Is NBS?

NBS (CAS 128-08-5), or N-bromosuccinimide, is a brominating reagent with the molecular formula C4H4BrNO2 and molecular weight 177.98 g/mol. ^[001] It is a white crystalline solid that serves as a convenient source of low, steady-state bromine (Br2) for radical allylic/benzylic bromination and as a direct electrophilic brominating agent for activated aromatics and alkenes. ^[004]

Unlike elemental bromine (Br2), which is a corrosive liquid that generates large quantities of HBr, NBS is a stable, easy-to-handle solid that releases bromine in a controlled manner. This makes it the reagent of choice for most laboratory bromination reactions. ^[004]

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Physical and Chemical Properties

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How It Works

Wohl-Ziegler Allylic and Benzylic Bromination (Radical Mechanism)

NBS performs selective allylic and benzylic bromination via a radical chain mechanism initiated by a radical source (AIBN, benzoyl peroxide, or light). ^[004]

Initiation: AIBN decomposes thermally to form cyanoisopropyl radicals, which abstract a bromine atom from NBS to generate a succinimidyl radical and Br*.

Propagation:

1. Br* abstracts an allylic or benzylic hydrogen from the substrate, forming an allylic/benzylic radical and HBr.

2. The HBr reacts with NBS to generate Br2 and succinimide.

3. The allylic/benzylic radical reacts with Br2 to form the brominated product and regenerate Br*.

Why is NBS selective? NBS maintains a very low, steady-state concentration of Br2 in the reaction mixture. ^[004] The key to selectivity is that Br* preferentially abstracts the weakest C-H bond. Allylic and benzylic C-H bonds (BDE ~85-90 kcal/mol) are significantly weaker than alkane C-H bonds (BDE ~98-100 kcal/mol). The low Br2 concentration prevents competing electrophilic addition to alkenes, which would occur at higher Br2 concentrations.

Selectivity order: benzylic > allylic > tertiary > secondary > primary C-H. ^[004]

Electrophilic Bromination of Activated Aromatics

In polar solvents (DMF, MeCN, or on silica gel support), NBS acts as an electrophilic bromine source for the bromination of electron-rich aromatics. ^[004] The mechanism involves: (1) polarization of the N-Br bond by the aromatic pi system; (2) formation of a sigma complex (Wheland intermediate); (3) deprotonation to restore aromaticity and release succinimide as the byproduct.

Regioselectivity follows standard electrophilic aromatic substitution rules: substituents that are ortho/para-directing and activating (NH2, OH, OR, alkyl) direct bromination to the ortho and para positions. ^[004]

Bromohydrin Formation

NBS in aqueous DMSO or THF/H2O generates Br2 in situ, which adds to alkenes via a bromonium ion intermediate. ^[005] Water (or another nucleophile) captures the bromonium ion from the backside, giving anti-bromohydrins with Markovnikov regioselectivity. This is mechanistically distinct from radical bromination — it is an electrophilic addition that requires polar protic conditions.

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Key Applications

1. Wohl-Ziegler Allylic and Benzylic Bromination: NBS is the standard reagent for radical allylic and benzylic bromination. NBS + AIBN or benzoyl peroxide (radical initiator) in CCl4 or CH2Cl2 at reflux. Selectivity: benzylic > allylic > tertiary > secondary > primary C-H. The low, steady-state concentration of Br2 generated in situ prevents competing electrophilic addition to alkenes. ^[004]

2. Electrophilic Bromination of Activated Aromatics: NBS in polar solvents (DMF, MeCN, or silica gel-supported) brominates activated aromatics (anilines, phenols, heterocycles) with high regioselectivity at the most electron-rich position. NBS/SiO2 (solvent-free grinding) provides a greener alternative to Br2 in CCl4. ^[004]

3. Bromohydrin and Bromoetherification: NBS/H2O converts alkenes to bromohydrins via bromonium ion capture by water. NBS/ROH (alcohol as solvent+nucleophile) produces beta-bromoethers in a single step. These mild conditions enable late-stage functionalization of complex alkenes present in natural products and drug candidates. ^[005]

4. Oxidation and Deprotection Reagent: NBS selectively oxidizes secondary alcohols to ketones, thiols to disulfides, and sulfides to sulfoxides under mild conditions. NBS deprotects dithioacetals and thioglycosides regenerating the parent carbonyl and hemiacetal, respectively. These orthogonal transformations are valuable in multi-step synthesis. ^[005]

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Representative Protocol

Application: Wohl-Ziegler Allylic Bromination

Typical Scale: 5 mmol

Reagents:

- Alkene with allylic C-H: 1.0 eq (5 mmol)

- NBS: 1.1 eq (5.5 mmol, 0.98 g)

- AIBN: 0.05 eq (0.25 mmol, 0.041 g)

- Solvent: CCl4 (25 mL) or CH2Cl2 (25 mL)

Procedure:

1. In a 50 mL round-bottom flask equipped with a stir bar and reflux condenser, combine the alkene (5 mmol), NBS (0.98 g, 5.5 mmol), and AIBN (0.041 g, 0.25 mmol) in CCl4 or CH2Cl2 (25 mL).

2. Degas the solution by bubbling N2 through it for 5 min, or perform three freeze-pump-thaw cycles.

3. Heat to reflux (CCl4: 77 degC; CH2Cl2: 40 degC with higher AIBN loading of 0.1 eq) with stirring.

4. Monitor by TLC (hexanes/EtOAc 9:1). The reaction is complete when the NBS white solid has been consumed (solution becomes clear/colorless) and a new product spot appears.

5. Typical reaction time: 1-3 h.

6. Cool to rt, filter through Celite to remove succinimide byproduct.

7. Wash the filtrate with saturated Na2S2O3 (10 mL) to remove residual bromine, then with brine (10 mL).

8. Dry over Na2SO4, filter, concentrate under reduced pressure.

9. Purify by flash chromatography (SiO2, hexanes -> hexanes/EtOAc 19:1).

Expected Yield: 60-85%.

Troubleshooting:

- Competing alkene dibromination: This indicates too high a Br2 concentration. Ensure rigorous degassing (oxygen inhibits radical chain). Use CCl4 instead of CH2Cl2 for better selectivity. Add NBS portionwise instead of all at once.

- Over-bromination (multiple allylic positions): Use only 1.0 eq NBS. For polybromination, lower temperature (rt with sunlight initiation instead of reflux) can improve mono-selectivity.

- Low conversion: Increase AIBN to 0.1 eq. For CH2Cl2, extend reaction time to 4-6 h or use 2,2'-azobis(2-methylpropionitrile) at 40 degC.

- Succinimide co-elutes with product: Wash organic layer thoroughly with water (succinimide is water-soluble) before chromatography.

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Handling and Storage

- GHS Hazard: H272(38.7%): May intensify fire; oxidizer; H290(38.7%): May be corrosive to metals; H302(55.8%): Harmful if swallowed; H314(35.2%): Causes severe skin burns and eye damage; H315(46.2%): Causes skin irritation; H317(21.6%): May cause an allergic skin reaction; H319(65.3%): Causes serious eye irritation; H335(26.6%): May cause respiratory irritation; H341(15.1%): Suspected of causing genetic defects; H400(43.2%): Very toxic to aquatic life ^[006]

- Signal Word: Danger

- Pictograms: GHS03 (flame over circle), GHS05 (corrosion), GHS07 (exclamation mark), GHS09 (environment)

- Storage: Store at 2-8 degC in a tightly closed container. Keep away from combustible materials, reducing agents, and strong bases. Protect from moisture and light. ^[002]

- PPE: Nitrile gloves, safety goggles with side shields, lab coat, closed-toe shoes. Work in a well-ventilated fume hood. ^[002]

> Critical Safety Note: ECHA C&L data shows H272(38.7%) — NBS is an oxidizer that can intensify fire. ^[006] It also shows H314(35.2%) (severe skin burns) and H400(43.2%) (very toxic to aquatic life). The original article omitted H272 and H400 entirely, and underestimated the hazard by not listing H314. NBS is not merely a skin sensitizer; it is an oxidizer and corrosive material.

Always consult the Safety Data Sheet (SDS) from your supplier before handling.

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Regulatory & Compliance

Chemical Inventories: NBS (CAS 128-08-5) is listed on major chemical inventories including TSCA (US), EINECS (EU), ENCS (Japan), IECSC (China), KECL (Korea), and AICS (Australia). ^[002]

Export Control: NBS is a standard research chemical. However, as a brominating agent, verify that no specific export control regulations apply to your destination country. ^[002]

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Frequently Asked Questions

Why does NBS selectively brominate allylic positions instead of adding across the double bond?

NBS maintains a very low, steady-state Br2 concentration via the equilibrium: NBS + HBr <-> succinimide + Br2. ^[004] At low Br2 concentrations, the radical chain (Br* abstracting allylic H) outcompetes electrophilic addition across the alkene. If you used elemental Br2 directly at high concentration, alkene addition (giving dibromide) would dominate. This is the key insight of the Wohl-Ziegler reaction.

Can I use NBS without a radical initiator?

For radical bromination, an initiator (AIBN, benzoyl peroxide) or light is required to generate the initial Br* radicals. ^[004] For electrophilic bromination of activated aromatics, no initiator is needed — the electron-rich aromatic directly attacks the polarized N-Br bond. For bromohydrin formation, no initiator is needed either; NBS generates Br2 in situ in aqueous polar solvents.

Why does my NBS reaction give dibromide instead of allylic bromide?

Three common causes: (1) oxygen contamination — O2 quenches radicals and can alter the mechanism; always degas thoroughly; (2) too much NBS — use exactly 1.0-1.1 eq; excess NBS drives up Br2 concentration; (3) wrong solvent — polar protic solvents (MeOH, H2O) favor electrophilic addition; use CCl4 or CH2Cl2 for radical conditions. ^[004]

Is NBS safer than elemental bromine?

Yes, significantly. NBS is a stable crystalline solid with much lower vapor pressure than liquid Br2. ^[005] However, NBS is still hazardous: it is an oxidizer (H272), corrosive (H314), and harmful to aquatic life (H400). ^[006] Handle with appropriate PPE and never dispose of NBS waste down the drain.

What is the advantage of NBS on silica gel (solvent-free bromination)?

NBS adsorbed on silica gel enables solvent-free electrophilic bromination of activated aromatics. ^[004] Grinding the aromatic substrate with NBS/SiO2 at room temperature gives selective monobromination without solvent waste. This is a green chemistry alternative to traditional Br2/CCl4 methods. Yields are typically 70-90% for anilines and phenols.

Can NBS brominate unactivated alkanes?

No. The Wohl-Ziegler reaction requires a weakened C-H bond (allylic or benzylic, BDE ~85-90 kcal/mol). ^[004] Unactivated alkane C-H bonds (BDE ~98-100 kcal/mol) are too strong for selective Br* abstraction. For unactivated C-H bromination, much stronger conditions are required (e.g., radical bromination with Br2/hv at high temperature, which gives mixtures).

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Where to Buy

NBS (CAS 128-08-5) is not currently listed in the CoreyChem catalog. Contact [email protected] for sourcing assistance. ^[003]

Our sourcing team can assist with procurement from qualified manufacturers worldwide. Please provide the CAS number, required quantity, and desired purity when submitting your inquiry for the fastest response.

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