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

A halogenação alílica de um alceno requer uma baixa concentração de halogênio e uma alta temperatura.
Para realizar a bromação alílica à temperatura ambiente, a N-bromosuccinimida, ou NBS, é usada como reagente. Mantém uma baixa concentração de bromo e atua como fonte de radicais de bromo.
Por exemplo, o propeno reage com NBS na presença de luz ou peróxido para formar 3-bromopropeno.
O mecanismo de bromação alílica envolve etapas de iniciação, propagação e terminação.
Durante a etapa de iniciação, o NBS sofre clivagem homolítica induzida por luz da ligação N-Br para formar um radical de bromo.
Na primeira etapa de propagação, o radical bromo reage com o propeno e abstrai o hidrogênio alílico, formando um radical alílico e HBr.
O HBr gerado reage com o NBS para produzir a molécula de bromo necessária para a propagação posterior da reação.
Na segunda etapa de propagação, a molécula de bromo reage com o radical alílico para dar brometo de alila e um átomo de bromo.
Finalmente, na etapa de terminação, os diferentes radicais se acoplam para dar produtos não radicais, interrompendo novas reações.

substituição de radicais: bromação alílica

Radical Substitution: Allylic Bromination

Na síntese orgânica, a formação de produtos pode ser alterada modificando as condições de reação. Por exemplo, um produto de adição de dibromo é formado quando o propeno é tratado com bromo à temperatura ambiente. Em contraste, o propeno sofre substituição alílica em solventes apolares em altas temperaturas para dar 3-bromopropeno. Para evitar a reação de adição, a concentração de bromo deve ser mantida tão baixa quanto possível durante toda a reação. Isso pode ser conseguido usando N-bromosuccinimida (NBS) como reagente em vez de bromo molecular.

O propeno reage com NBS na presença de luz ou peróxido por meio de substituição radical para formar brometo de alila ou 3-bromopropeno. Semelhante às reações radicais, o mecanismo de bromação alílica envolve três etapas: iniciação, propagação e terminação. Na etapa de iniciação, o NBS sofre clivagem homolítica de ligações N – Br fracas na presença de luz ou peróxido para formar o radical bromo. Durante a primeira etapa de propagação, o radical bromo gerado abstrai o hidrogênio alílico para fornecer radical alílico estabilizado por ressonância e HBr. O HBr formado reage imediatamente com o NBS em uma reação iônica produzindo Br_2, que participa da segunda etapa de propagação. Eventualmente, na etapa de terminação, diferentes radicais se combinam, o que resulta na formação de produtos não radicais que levam ao término da reação. Durante toda a reação, as concentrações de HBr e Br_2 são mantidas no mínimo. Nestas condições, isto é, em um solvente apolar com uma concentração muito baixa de bromo, a adição iónica de Br_2 não compete com sucesso com a bromação radical.

A bromação radical de alcenos substituídos com alílico forma uma mistura de produtos. Isso se deve à estabilização por ressonância do intermediário radical alílico formado, que pode abstrair o halogênio de qualquer um dos locais (Figura 1).

<img alt="Figure1" src="/files/ftp_upload/12900/PT_12900_Figure%201.svg" style="text-align: center; height: 150px;">

Substituição de Radicais: Bromação Alílica

Na síntese orgânica, a formação de produtos pode ser alterada alterando as condições de reação. Por exemplo, um produto de adição de dibromo é ...

Education

JoVE Core

Organic Chemistry

Química Orgânica

Unit 1

Unidade 1

Radical Chemistry

Química de Radicais

KEY TERMS AND CONCEPTS

TERMOS-CHAVE E CONCEITOS

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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