''N''-Bromosuccinimide or NBS is a
chemical reagent used in
radical substitution,
electrophilic addition, and
electrophilic substitution
Electrophilic substitution reactions are chemical reactions in which an electrophile displaces a functional group in a compound, which is typically, but not always, aromatic. Aromatic substitution reactions are characteristic of aromatic compounds ...
reactions
Reaction may refer to a process or to a response to an action, event, or exposure:
Physics and chemistry
*Chemical reaction
*Nuclear reaction
*Reaction (physics), as defined by Newton's third law
*Chain reaction (disambiguation).
Biology and me ...
in
organic chemistry. NBS can be a convenient source of Br
•, the
bromine radical.
Preparation
NBS is commercially available. It can also be synthesized in the laboratory. To do so,
sodium hydroxide
Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions .
Sodium hydroxide is a highly caustic base and alkali ...
and bromine are added to an ice-water solution of
succinimide. The NBS product
precipitates and can be collected by filtration.
Crude NBS gives better yield in the
Wohl-Ziegler reaction. In other cases, impure NBS (slightly yellow in color) may give unreliable results. It can be purified by recrystallization from 90 to 95 °C water (10 g of NBS for 100 mL of water).
Reactions
Addition to alkenes
NBS will react with alkenes 1 in aqueous solvents to give
bromohydrins 2. The preferred conditions are the portionwise addition of NBS to a solution of the alkene in 50% aqueous
DMSO,
DME,
THF, or
''tert''-butanol at 0 °C. Formation of a
bromonium ion and immediate attack by water gives strong
Markovnikov addition and ''anti'' stereochemical selectivities.
:
Side reactions include the formation of α-bromoketones and dibromo compounds. These can be minimized by the use of freshly
recrystallized NBS.
With the addition of
nucleophile
In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are ...
s, instead of
water, various bifunctional alkanes can be synthesized.
:
Allylic and benzylic bromination
Standard conditions for using NBS in allylic and/or benzylic
bromination involves refluxing a solution of NBS in
anhydrous
A substance is anhydrous if it contains no water. Many processes in chemistry can be impeded by the presence of water; therefore, it is important that water-free reagents and techniques are used. In practice, however, it is very difficult to achie ...
CCl4 with a radical initiator—usually azobisisobutyronitrile (
AIBN) or
benzoyl peroxide
Benzoyl peroxide is a chemical compound (specifically, an organic peroxide) with structural formula , often abbreviated as (BzO)2. In terms of its structure, the molecule can be described as two benzoyl (, Bz) groups connected by a peroxide () ...
, irradiation, or both to effect
radical
Radical may refer to:
Politics and ideology Politics
*Radical politics, the political intent of fundamental societal change
*Radicalism (historical), the Radical Movement that began in late 18th century Britain and spread to continental Europe and ...
initiation
Initiation is a rite of passage marking entrance or acceptance into a group or society. It could also be a formal admission to adulthood in a community or one of its formal components. In an extended sense, it can also signify a transformation ...
. The allylic and benzylic radical intermediates formed during this reaction are more stable than other carbon radicals and the major products are allylic and benzylic bromides. This is also called the
Wohl–Ziegler reaction.
:
The
carbon tetrachloride
Carbon tetrachloride, also known by many other names (such as tetrachloromethane, also IUPAC nomenclature of inorganic chemistry, recognised by the IUPAC, carbon tet in the cleaning industry, Halon-104 in firefighting, and Refrigerant-10 in HVAC ...
must be maintained anhydrous throughout the reaction, as the presence of
water may likely
hydrolyze the desired product.
Barium carbonate is often added to maintain anhydrous and acid-free conditions.
In the above reaction, while a mixture of isomeric allylic bromide products are possible, only one is created due to the greater stability of the 4-position radical over the methyl-centered radical.
Bromination of carbonyl derivatives
NBS can α-brominate carbonyl derivatives via either a radical pathway (as above) or via acid-catalysis. For example,
hexanoyl chloride 1 can be brominated in the alpha-position by NBS using acid catalysis.
:
The reaction of
enolates,
enol ethers, or
enol acetate
In organic chemistry, alkenols (shortened to enols) are a type of Functional group, reactive structure or chemical intermediate, intermediate in organic chemistry that is represented as an alkene (olefin) with a hydroxyl group attached to one en ...
s with NBS is the preferred method of α-bromination as it is high-yielding with few side-products.
Bromination of aromatic derivatives
Electron-rich
aromatic compounds, such as
phenols,
anilines, and various aromatic
heterocycles, can be brominated using NBS. Using
DMF as the solvent gives high levels of para-selectivity.
Hofmann rearrangement
NBS, in the presence of a strong base, such as
DBU, reacts with primary
amides to produce a
carbamate
In organic chemistry, a carbamate is a category of organic compounds with the general formula and structure , which are formally derived from carbamic acid (). The term includes organic compounds (e.g., the ester ethyl carbamate), formally o ...
via the
Hofmann rearrangement.
:
Selective oxidation of alcohols
It is uncommon, but possible for NBS to oxidize alcohols.
E. J. Corey
Elias James Corey (born July 12, 1928) is an American organic chemist. In 1990, he won the Nobel Prize in Chemistry "for his development of the theory and methodology of organic synthesis", specifically retrosynthetic analysis. Regarded by many a ...
''et al.'' found that one can selectively
oxidize secondary
alcohol
Alcohol most commonly refers to:
* Alcohol (chemistry), an organic compound in which a hydroxyl group is bound to a carbon atom
* Alcohol (drug), an intoxicant found in alcoholic drinks
Alcohol may also refer to:
Chemicals
* Ethanol, one of sev ...
s in the presence of primary alcohols using NBS in aqueous
dimethoxyethane (DME).
:
Oxidative decarboxylation of α-amino acids
NBS electrophilically brominates the amine, which is followed by decarboxylation and release of an imine. Further hydrolysis will yield an aldehyde and ammonia.
(cf. non-oxidative PLP dependent decarboxylation)
:
Precautions
Although NBS is easier and safer to handle than bromine, precautions should be taken to avoid inhalation. NBS should be stored in a refrigerator. NBS will decompose over time giving off bromine. Pure NBS is white, but it is often found to be off-white or brown colored by bromine.
In general, reactions involving NBS are exothermic. Therefore, extra precautions should be taken when using on a large scale.
See also
*
Halohydrin formation
In organic chemistry a halohydrin (also a haloalcohol or β-halo alcohol) is a functional group in which a halogen and a hydroxyl are bonded to adjacent carbon atoms, which otherwise bear only hydrogen or hydrocarbyl groups (e.g. 2-chloroethanol ...
*
''N''-Chlorosuccinimide
*
''N''-Iodosuccinimide
References
External links
* Usage o
''N''-bromosuccinimide in organic synthesisReactions with NBS
{{DEFAULTSORT:Bromosuccinimide, N-
Organobromides
Reagents for organic chemistry
Succinimides