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

Die allylische Halogenierung eines Alkens erfordert eine niedrige Halogenkonzentration und eine hohe Temperatur.
Um die Allylbromierung bei Raumtemperatur durchzuführen, wird N-Bromosuccinimid oder NBS als Reagenz verwendet. Es hält eine niedrige Bromkonzentration aufrecht und wirkt als Quelle für Bromradikale.
Zum Beispiel reagiert Propen in Gegenwart von Licht oder Peroxid mit NBS zu 3-Brompropen.
Der Mechanismus der allylischen Bromierung umfasst Initiierungs-, Vermehrungs- und Beendigungsschritte.
Während des Initiationsschritts durchläuft NBS eine lichtinduzierte homolytische Spaltung der N-Br-Bindung zu einem Bromradikal.
Im ersten Vermehrungsschritt reagiert der Bromrest mit Propen und abstrahiert allylischen Wasserstoff, wobei ein allylischer Rest und HBr gebildet werden.
Das erzeugte HBr reagiert mit NBS, um das Brommolekül zu produzieren, das für die weitere Ausbreitung der Reaktion benötigt wird.
Im zweiten Vermehrungsschritt reagiert das Brommolekül mit dem Allylradikal zu Allylbromid und einem Bromatom.
Schließlich koppeln sich die verschiedenen Radikale im Beendigungsschritt miteinander, um nichtradikale Produkte zu erhalten, die eine weitere Reaktion stoppen.

radikalsubstitution: allylische bromierung

Radical Substitution: Allylic Bromination

In der organischen Synthese kann die Bildung von Produkten durch Änderung der Reaktionsbedingungen beeinflusst werden. So entsteht beispielsweise ein Dibromadditionsprodukt, wenn Propen bei Raumtemperatur mit Brom behandelt wird. Im Gegensatz dazu wird Propen in unpolaren Lösungsmitteln bei hohen Temperaturen allylisch substituiert und es entsteht 3-Brompropen. Um die Additionsreaktion zu vermeiden, muss die Bromkonzentration während der gesamten Reaktion so niedrig wie möglich gehalten werden. Dies kann durch die Verwendung von N-Bromsuccinimid (NBS) als Reagenz anstelle von molekularem Brom erreicht werden.

Propen reagiert mit NBS in Gegenwart von Licht oder Peroxid durch radikalische Substitution zu Allylbromid oder 3-Brompropen. Ähnlich wie bei radikalischen Reaktionen umfasst der Mechanismus der Allylbromierung drei Schritte: Initiierung, Ausbreitung und Beendigung. Im Initiierungsschritt spaltet NBS in Gegenwart von Licht oder Peroxid homolytisch schwache N-Br-Bindungen und bildet ein Bromradikal. Während des ersten Ausbreitungsschritts abstrahiert das gebildete Bromradikal den Allylwasserstoff und bildet ein resonanzstabilisiertes Allylradikal und HBr. Das gebildete HBr reagiert sofort mit NBS in einer ionischen Reaktion unter Bildung von Br_2, das am zweiten Vermehrungsschritt beteiligt ist. In der Endstufe verbinden sich schließlich verschiedene Radikale, was zur Bildung von nichtradikalischen Produkten führt, die die Reaktion beenden. Während der gesamten Reaktion werden die HBr- und Br_2-Konzentrationen auf einem Minimum gehalten. Unter diesen Bedingungen, d. h. in einem unpolaren Lösungsmittel mit einer sehr niedrigen Bromkonzentration, konkurriert die ionische Addition von Br_2 nicht erfolgreich mit der radikalischen Bromierung.

Bei der radikalischen Bromierung von allylsubstituierten Alkenen entsteht ein Gemisch von Produkten. Der Grund dafür ist die Resonanzstabilisierung des gebildeten allylischen Radikal-Zwischenprodukts, das an beiden Stellen Halogen abspalten kann (Abbildung 1).

<img alt="Figure1" src="/files/ftp_upload/12900/PT_12900_Figure%201.svg" style="text-align: center; height: 150px;">

Radikalsubstitution: Allylische Bromierung

In der organischen Synthese kann die Bildung von Produkten durch Änderung der Reaktionsbedingungen verändert werden. Zum Beispiel ist ein Dibromo-Zugabeprodukt ...

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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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