Everything about N-bromosuccinimide totally explained
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N-Bromosuccinimide or
NBS is a
chemical reagent which is used in
radical substitution and
electrophilic addition reactions in
organic chemistry. NBS can be considered a convenient source of molecular
bromine.
Chemistry
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-bromo-ketones and dibromo compounds. These can be minimized by the use of freshly
recrystallized NBS.
With the addition of
nucleophiles, 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 CCl4 with a radical initiator, usually azo-
bis-isobutyronitrile (AIBN),
benzoyl peroxide, irradiation, or both to effect
radical initiation. 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.
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The
carbon tetrachloride 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.
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The reaction of
enolates,
enol ethers, or
enol acetates with NBS is the preferred method of α-bromination as it's 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 via the
Hofmann rearrangement.
»
Selective oxidation of alcohols
It is uncommon, but possible for NBS to oxidize alcohols.
E. J. Corey et al. found that one can selectively
oxidize secondary
alcohols in the presence of primary alcohols using NBS in aqueous
DME.
»
Preparation
NBS is commercially available. It can also be synthesized in the laboratory. To do so,
sodium hydroxide and bromine are added to an ice-water solution of
succinimide. The NBS product
precipitates out and can be collected by filtration.
Crude NBS gives better yield in the
Wohl-Ziegler reaction. In other cases, impure NBS (slightly yellow-colored) may give unreliable results. It can be purified by recrystallization from 90-95 °C water.
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's 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 used on a large scale.
Further Information
Get more info on 'N-bromosuccinimide'.
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