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

L’halogénation allylique d’un alcène nécessite une faible concentration d’halogène et une température élevée.
Pour effectuer la bromation allylique à température ambiante, le N-bromosuccinimide, ou NBS, est utilisé comme réactif. Il maintient une faible concentration de brome et agit comme une source de radicaux bromés.
Par exemple, le propène réagit avec les NBS en présence de lumière ou de peroxyde pour former du 3-bromopropène.
Le mécanisme de la bromation allylique implique des étapes d’initiation, de propagation et de terminaison.
Au cours de l’étape d’initiation, le NBS subit un clivage homolytique induit par la lumière de la liaison N-Br pour former un radical brome.
Dans la première étape de propagation, le radical brome réagit avec le propène et extrait l’hydrogène allylique, formant un radical allylique et HBr.
Le HBr généré réagit avec les NBS pour produire la molécule de brome nécessaire à la propagation ultérieure de la réaction.
Dans la deuxième étape de propagation, la molécule de brome réagit avec le radical allylique pour donner du bromure d’allyle et un atome de brome.
Enfin, dans l’étape de terminaison, les différents radicaux se couplent les uns aux autres pour donner des produits non radicaux, arrêtant ainsi la réaction.

substitution radicalaire : bromation allylique

Radical Substitution: Allylic Bromination

En synthèse organique, la formation de produits peut être modifiée en modifiant les conditions de réaction. Par exemple, un produit d'addition dibromo se forme lorsque le propène est traité avec du brome à température ambiante. En revanche, le propène subit une substitution allylique dans des solvants non polaires à haute température pour donner du 3-bromopropène. Afin d'éviter la réaction d'addition, la concentration en brome doit être maintenue aussi basse que possible tout au long de la réaction. Ceci peut être réalisé en utilisant le N-bromosuccinimide (NBS) comme réactif au lieu du brome moléculaire.

Le propène réagit avec le NBS en présence de lumière ou de peroxyde via une substitution radicalaire pour former du bromure d'allyle ou du 3-bromopropène. Semblable aux réactions radicalaires, le mécanisme de bromation allylique comporte trois étapes : l’initiation, la propagation et la terminaison. Au cours de l’étape d’initiation, le NBS subit un clivage homolytique des liaisons N – Br faibles en présence de lumière ou de peroxyde pour former un radical brome. Au cours de la première étape de propagation, le radical brome généré extrait l’hydrogène allylique pour donner un radical allylique stabilisé par résonance et du HBr. Le HBr formé réagit immédiatement avec le NBS dans une réaction ionique produisant du Br2, qui participe à la deuxième étape de propagation. Finalement, lors de l’étape de terminaison, différents radicaux se combinent, ce qui entraîne la formation de produits non radicalaires conduisant à la terminaison de la réaction. Tout au long de la réaction, les concentrations de HBr et Br2 sont maintenues au minimum. Dans ces conditions, c'est-à-dire dans un solvant apolaire avec une très faible concentration de brome, l'addition ionique de Br2 ne parvient pas à rivaliser avec la bromation radicalaire.

La bromation radicale d'alcènes allyliques substitués forme un mélange de produits. Cela est dû à la stabilisation par résonance du radical intermédiaire allylique formé, qui peut extraire l'halogène de l'un ou l'autre site (Figure 1).

<img alt="Figure1" src="/files/ftp_upload/12900/PT_12900_Figure%201.svg" style="text-align: center; height: 150px;">

Substitution radicalaire : bromation allylique

En synthèse organique, la formation des produits peut être modifiée en modifiant les conditions de réaction. Par exemple, un produit d’addition de dibromo est ...

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Organic Chemistry

Chimie organique

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Radical Chemistry

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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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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...

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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

organic chemistry

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Allylic Bromination with NBS

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