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

La halogenación alílica de un alqueno requiere una baja concentración de halógeno y una temperatura alta.
Para llevar a cabo la bromación alílica a temperatura ambiente, se utiliza N-bromosuccinimida, o NBS, como reactivo. Mantiene una baja concentración de bromo y actúa como fuente de radicales de bromo.
Por ejemplo, el propeno reacciona con NBS en presencia de luz o peróxido para formar 3-bromopropeno.
El mecanismo de la bromación alílica implica etapas de iniciación, propagación y terminación.
Durante la etapa de iniciación, NBS se somete a una escisión homolítica inducida por la luz del enlace N-Br para formar un radical de bromo.
En el primer paso de propagación, el radical bromo reacciona con el propeno y abstrae hidrógeno alílico, formando un radical alílico y HBr.
El HBr generado reacciona con NBS para producir la molécula de bromo necesaria para la propagación posterior de la reacción.
En el segundo paso de propagación, la molécula de bromo reacciona con el radical alílico para dar bromuro de alilo y un átomo de bromo.
Finalmente, en el paso de terminación, los diferentes radicales se acoplan entre sí para dar productos no radicales, deteniendo la reacción posterior.

sustitución radical: bromación alílica

Radical Substitution: Allylic Bromination

En la síntesis orgánica, la formación de productos se puede alterar cambiando las condiciones de reacción. Por ejemplo, se forma un producto de adición de dibromo cuando se trata propeno con bromo a temperatura ambiente. Por el contrario, el propeno sufre sustitución alílica en disolventes no polares a altas temperaturas para dar 3-bromopropeno. Para evitar la reacción de adición, la concentración de bromo debe mantenerse lo más baja posible durante toda la reacción. Esto se puede lograr utilizando N-bromosuccinimida (NBS) como reactivo en lugar de bromo molecular.


 El propeno reacciona con NBS en presencia de luz o peróxido mediante sustitución de radicales para formar bromuro de alilo o 3-bromopropeno. Al igual que las reacciones radicalarias, el mecanismo de bromación alílica implica tres pasos: iniciación, propagación y terminación. En el paso de iniciación, NBS sufre una escisión homolítica de enlaces débiles N-Br en presencia de luz o peróxido para formar un radical bromo. Durante el primer paso de propagación, el radical bromo generado extrae el hidrógeno alílico para dar un radical alílico estabilizado por resonancia y HBr. El HBr formado reacciona inmediatamente con NBS en una reacción iónica produciendo Br_2, que participa en el segundo paso de propagación. Finalmente, en el paso de terminación, se combinan diferentes radicales, lo que da como resultado la formación de productos no radicales que conducen a la terminación de la reacción. Durante toda la reacción, las concentraciones de HBr y Br_2 se mantienen al mínimo. En estas condiciones, es decir, en un disolvente apolar con una concentración muy baja de bromo, la adición iónica de Br_2 no compite exitosamente con la bromación radicalaria.


 La bromación radical de alquenos sustituidos alílicos forma una mezcla de productos. Esto se debe a la estabilización por resonancia del radical alílico intermedio formado, que puede extraer halógeno de cualquier sitio (Figura 1).

<img alt="Figure1" src="/files/ftp_upload/12900/PT_12900_Figure%201.svg" style="text-align: center; height: 150px;">

Sustitución radical: bromación alílica

En la síntesis orgánica, la formación de productos puede alterarse cambiando las condiciones de reacción. Por ejemplo, un producto de adición de dibromo es ...

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JoVE Core

Organic Chemistry

Química Orgánica

Unit 1

Unidad 1

Radical Chemistry

Química de radicales

KEY TERMS AND CONCEPTS

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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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