N-Bromosuccinimide (NBS) is a source of bromine for radical substitution and electrophilic addition reactions that is easier and safer to handle than bromine. NBS is used for allylic and benzylic brominations, alpha-bromination of carbonyl compounds, and addition reactions to alkenes to form bromohydrins or epoxides. The reactions involve a free radical mechanism initiated by a small amount of bromine radical from NBS.

1. 9. NBS
Dr. Shivendra Singh
UGC-NET-JRF, PhD- IIT Indore
Assistant Professor
shivendrasngh0@gmail.com
Webpage: https://sites.google.com/view/drshivendrasingh/home
Youtube: https://bit.ly/2YM0QG0

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…N-Bromosuccinimide
ü N-Bromosuccinimide (NBS) is a convenient source of bromine for
radical substitution as well as electrophilic addition reaction.
ü It is prepared from succinimide and bromine in the presence of NaOH
solution.
ü The colourless solid obtained is washed with water and recrystallized
from hot water, and stored in a refrigerator and protected from moisture
to avoid decomposition.
ü NBS is easier and safer to handle compared to bromine.

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…N-Bromosuccinimide
Ø Applications
1. Allylic and Benzylic Brominations
2. Alpha-bromination
3. Addition reactions

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1. Allylic and Benzylic Brominations:
Alkenes react with NBS in dry CCl4 under reflux conditions to give allyl
bromide. The reaction is initiated by light or peroxide. Although a number of
reagents are available for bromination of allylic C-H bond of alkenes, NBS is
most commonly used. The reaction is called Wohl-Zigler bromination.

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Mechanism:
ü The reaction involves a free radical process.
ü The reaction is initiated by small amounts of Br radical. The role of NBS
is to afford a constant low concentration of molecular bromine.
Abstraction of an allylic or benzylic hydrogen by Br radical gives a
resonance stabilized allyl or benzyl radical.
ü The selective bromination occurs because the intermediate leading to the
product is stabilized by resonance.

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2. α-Bromination of Carbonyl Derivatives
Ketones having enolizable hydrogen can be brominated at the α-position.
The reaction probably involves an addition of Br2 to the enol form of the
carbonyl derivatives, and elimination of HBr generates the α-bromoketone

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3. Addition Reactions
NBS is often used as the source of electrophilic bromine in polar solvents.
For an example, cyclohexene reacts with NBS in the presence of tetraethyl-
ammonium bromide to give trans-1,2-dibromocyclohexane in good yield.

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A possible mechanism is;

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In the presence of excess of water, alkenes undergo reaction with NBS
to give bromohydrins that could be converted into epoxides in the
presence of base.