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.

radicale substitutie: allylbromering

Radical Substitution: Allylic Bromination

Bij organische synthese kan de vorming van producten worden gewijzigd door de reactieomstandigheden te veranderen. Er wordt bijvoorbeeld een dibroomadditieproduct gevormd wanneer propeen bij kamertemperatuur wordt behandeld met broom. Propeen ondergaat daarentegen allylische substitutie in niet-polaire oplosmiddelen bij hoge temperaturen, waardoor 3-broompropeen ontstaat. Om de additiereactie te vermijden, moet de broomconcentratie gedurende de hele reactie zo laag mogelijk worden gehouden. Dit kan worden bereikt door N-broomsuccinimide (NBS) als reagens te gebruiken in plaats van moleculair broom.


 Propeen reageert met NBS in aanwezigheid van licht of peroxide via radicaalsubstitutie om allylbromide of 3-broompropeen te vormen. Net als bij radicaalreacties omvat het allylische bromeringsmechanisme drie stappen: initiatie, vermenigvuldiging en beëindiging. In de initiatiestap ondergaat NBS homolytische splitsing van zwakke N-Br-bindingen in aanwezigheid van licht of peroxide om een broomradicaal te vormen. Tijdens de eerste vermenigvuldigingsstap abstraheert het gegenereerde broomradicaal de allylische waterstof, waardoor een resonantiegestabiliseerd allylisch radicaal en HBr ontstaat. Het gevormde HBr reageert meteen met NBS in een ionische reactie waarbij Br_2 ontstaat, dat deelneemt aan de tweede vermenigvuldigingsstap. Uiteindelijk combineren in de terminatiestap verschillende radicalen, wat resulteert in de vorming van niet-radicale producten die leiden tot de beëindiging van de reactie. Gedurende de gehele reactie worden de HBr- en Br_2-concentraties tot een minimum beperkt. Onder deze omstandigheden, dat wil zeggen in een niet-polair oplosmiddel met een zeer lage broomconcentratie, concurreert de ionische toevoeging van Br_2 niet met succes met radicaalbromering.


 Radicale bromering van allylisch gesubstitueerde alkenen vormt een mengsel van producten. Dit komt door de resonantiestabilisatie van het gevormde allylische radicaaltussenproduct, dat halogeen van beide plaatsen kan onttrekken (Figuur 1).

 <img alt="Figure1" src="/files/ftp_upload/12900/PT_12900_Figure 1.svg" style="text-align: center; height: 150px;" />

Radicale substitutie: allylbromering

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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