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

알켄의 알릴 할로겐화는 낮은 농도의 할로겐과 높은 온도를 필요로 합니다.
실온에서 알릴 브롬화를 수행하기 위해 N-브로모숙신이미드 또는 NBS가 시약으로 사용됩니다. 그것은 낮은 브롬 농도를 유지하고 브롬 라디칼의 공급원으로 작용합니다.
예를 들어, 프로펜은 빛이나 과산화물이 있는 상태에서 NBS와 반응하여 3-브로모프로펜을 형성합니다.
알릴 브롬화의 메커니즘은 시작, 전파 및 종료 단계를 포함합니다.
개시 단계에서 NBS는 N-Br 결합의 광 유도 동질분해 절단을 거쳐 브롬 라디칼을 형성합니다.
첫 번째 전파 단계에서 브롬 라디칼은 프로펜과 반응하여 알릴 수소를 추상화하여 알릴 라디칼과 HBr을 형성합니다.
생성된 HBr은 NBS와 반응하여 반응의 추가 증식에 필요한 브롬 분자를 생성합니다.
두 번째 전파 단계에서 브롬 분자는 알릴 라디칼과 반응하여 알릴 브로마이드와 브롬 원자를 생성합니다.
마지막으로, 종결 단계에서 서로 다른 라디칼은 서로 결합하여 비라디칼 산물을 제공하여 추가 반응을 중지합니다.

라디칼 치환: 알릴 브롬화

Radical Substitution: Allylic Bromination

유기 합성에서는 반응 조건을 변경하여 형성되는 생성물을 변경할 수 있습니다. 예를 들어, 프로펜을 실온에서 브롬으로 처리하면 디브로모 부가 생성물이 형성됩니다. 대조적으로, 프로펜은 고온에서 비극성 용매에서 알릴 치환을 거쳐 3-브로모프로펜을 생성합니다. 첨가 반응을 피하기 위해서는 반응 전반에 걸쳐 브롬 농도를 가능한 한 낮게 유지해야 합니다. 이는 분자 브롬 대신 NBS(N-bromosuccinimide)를 시약으로 사용하여 달성할 수 있습니다.

프로펜은 빛이나 과산화물이 있을 때 라디칼 치환을 통해 NBS와 반응하여 알릴 브로마이드 또는 3-브로모프로펜을 형성합니다. 라디칼 반응과 유사하게, 알릴 브롬화 메커니즘은 개시, 전파 및 종결의 세 단계를 포함합니다. 개시 단계에서 NBS는 빛이나 과산화물이 있을 때 약한 N-Br 결합의 균일 절단을 거쳐 브롬 라디칼을 형성합니다. 첫 번째 전파 단계에서 생성된 브롬 라디칼은 알릴 수소를 추출하여 공명 안정화 알릴 라디칼과 HBr을 제공합니다. 형성된 HBr은 이온 반응에서 NBS와 즉시 반응하여 두 번째 전파 단계에 참여하는 Br_2를 생성합니다. 결국 종결 단계에서는 서로 다른 라디칼이 결합하면서 비라디칼 생성물이 형성되어 반응이 종결됩니다. 반응 전반에 걸쳐 HBr 및 Br_2 농도는 최소로 유지됩니다. 이러한 조건, 즉 브롬 농도가 매우 낮은 비극성 용매에서 Br_2의 이온 첨가는 라디칼 브롬화와 제대로 경쟁하지 못합니다.

알릴 치환 알켄의 라디칼 브롬화는 생성물의 혼합물을 형성합니다. 이는 형성된 알릴 라디칼 중간체의 공명 안정화로 인해 두 위치 중 하나에서 할로겐을 추출할 수 있기 때문입니다(그림 1).

<img alt="Figure1" src="/files/ftp_upload/12900/PT_12900_Figure%201.svg" style="text-align: center; height: 150px;">

유기 합성에서 생성물의 형성은 반응 조건을 변경함으로써 변경될 수 있습니다. 예를 들어, dibromo 첨가 제품은 ...

Education

교육

JoVE Core

Organic Chemistry

유기 화학

Unit 1

제1단원

Radical Chemistry

라디컬 화학

KEY TERMS AND CONCEPTS

주요 용어 및 개념

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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