Name: allylic bromination with nbs
Uploaded: 2025-05-22T09:05:43.000Z
Duration: 1 min 27 s
Channel: JoVE

Allylic halogenation of an alkene requires a low concentration of halogen and a high temperature.
To carry out allylic bromination at room temperature, N-bromosuccinimide, or NBS, is used as the reagent. It maintains a low bromine concentration and acts as a source of bromine radicals.
For example, propene reacts with NBS in the presence of light or peroxide to form 3-bromopropene.
The mechanism of allylic bromination involves initiation, propagation, and termination steps.
During the initiation step, NBS undergoes light-induced homolytic cleavage of the N&#8211;Br bond to form a bromine radical.
In the first propagation step, the bromine radical reacts with propene and abstracts allylic hydrogen, forming an allylic radical and HBr.
The generated HBr reacts with NBS to produce the bromine molecule required for further propagation of the reaction.
In the second propagation step, the bromine molecule reacts with the allylic radical to give allyl bromide and a bromine atom.
Finally, in the termination step, the different radicals couple with each other to give nonradical products, stopping further reaction.

radical substitution: allylic bromination

Radical Substitution: Allylic Bromination

In organic synthesis, the formation of products can be altered by changing the reaction conditions. For example, a dibromo addition product is formed when propene is treated with bromine at room temperature. In contrast, propene undergoes allylic substitution in non-polar solvents at high temperatures to give 3-bromopropene. In order to avoid the addition reaction, the bromine concentration must be kept as low as possible throughout the reaction. This can be achieved using N-bromosuccinimide (NBS) as a reagent instead of molecular bromine.
Propene reacts with NBS in the presence of light or peroxide via radical substitution to form allyl bromide or 3-bromopropene. Similar to radical reactions, the allylic bromination mechanism involves three steps: initiation, propagation, and termination. In the initiation step, NBS undergoes homolytic cleavage of weak N&#8211;Br bonds in the presence of light or peroxide to form bromine radical. During the first propagation step, the generated bromine radical abstracts the allylic hydrogen to give resonance stabilized allylic radical and HBr. The formed HBr immediately reacts with NBS in an ionic reaction producing Br2, which participates in the second propagation step. Eventually, in the termination step, different radicals combine, which results in the formation of nonradical products leading to the termination of the reaction. Throughout the reaction, HBr and Br2 concentrations are kept at a minimum. Under these conditions, that is, in a non-polar solvent with a very low concentration of bromine, the ionic addition of Br2 does not successfully compete with radical bromination.
Radical bromination of allylic substituted alkenes forms a mixture of products. This is because of the resonance stabilization of formed allylic radical intermediate, which can abstract halogen from either site (Figure 1).
<img alt="Allylic bromination process, NBS reaction mechanism diagram, radical intermediates, photochemical reaction." src="/files/ftp_upload/12900/PT_12900_Figure%201.svg" style="text-align: center; height: 150px;" data-altupdatedat="1754904545000" data-alt="Free radical bromination mechanism diagram, isomer formation using NBS, allylic substitution." />

In organic synthesis, the formation of products can be altered by changing the reaction conditions. For example, a dibromo addition product is ...

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allylic bromination with nbs

Allylic bromination with NBS changes propene into allyl bromide under radical conditions. In this reaction, the goal is to favor substitution at the allylic position instead of bromine addition across the double bond. Using N-bromosuccinimide, or NBS, helps keep the bromine concentration low so the addition product is less likely to form.

The reaction conditions matter. Propene gives a dibromo addition product with bromine at room temperature. In non-polar solvents at high temperature, propene instead undergoes allylic substitution to form 3-bromopropene. When propene reacts with NBS in the presence of light or peroxide, it forms allyl bromide, which is also called 3-bromopropene.

The mechanism follows three familiar radical steps: initiation, propagation, and termination. During initiation, NBS undergoes homolytic cleavage of its weak N–Br bond when exposed to light or peroxide. This forms a bromine radical. In the first propagation step, that radical removes an allylic hydrogen and leaves behind a resonance-stabilized allylic radical plus HBr.

HBr does not build up. It reacts right away with NBS in an ionic step that produces Br2. The Br2 then takes part in the second propagation step. In the termination step, radicals combine to form nonradical products, which stops the chain reaction.

Keeping both HBr and Br2 at very low levels is important throughout the reaction. Under these conditions, the ionic addition of Br2 does not compete well with radical bromination. For allylic substituted alkenes, the reaction can give a mixture of products because the resonance-stabilized allylic radical can abstract halogen from either site.

Allylic Bromination with NBS

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key terms and concepts

See how NBS helps propene undergo allylic bromination instead of addition. Learn the radical steps, low bromine conditions, and product mix.

Allylic Bromination with NBS | JoVE K12

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Allylic Bromination with NBS

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