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

Для аллильного галогенирования алкена требуется низкая концентрация галогена и высокая температура.
Для проведения аллилового бромирования при комнатной температуре в качестве реагента используется N-бромсукцинимид, или NBS. Он поддерживает низкую концентрацию брома и выступает в качестве источника радикалов брома.
Например, пропен реагирует с NBS в присутствии света или перекиси с образованием 3-бромпропена.
Механизм аллилового бромирования включает в себя этапы инициации, размножения и терминации.
На стадии инициации NBS подвергается индуцированному светом гомолитическому расщеплению связи N–Br с образованием радикала брома.
На первой стадии размножения бромный радикал вступает в реакцию с пропеном и абстрагирует аллиловый водород, образуя аллильный радикал и HBr.
Полученный HBr реагирует с NBS с образованием молекулы брома, необходимой для дальнейшего распространения реакции.
На второй стадии размножения молекула брома вступает в реакцию с аллильным радикалом, образуя аллилбромид и атом брома.
Наконец, на стадии терминации различные радикалы соединяются друг с другом, образуя нерадикальные продукты, останавливая дальнейшую реакцию.

Радикальное замещение: аллильное бромирование

Radical Substitution: Allylic Bromination

В органическом синтезе образование продуктов можно изменить, изменив условия реакции. Например, продукт присоединения диброма образуется при обработке пропена бромом при комнатной температуре. Напротив, пропен претерпевает аллильное замещение в неполярных растворителях при высоких температурах с образованием 3-бромпропена. Чтобы избежать реакции присоединения, концентрацию брома на протяжении всей реакции следует поддерживать как можно более низкой. Этого можно достичь, используя в качестве реагента N-бромсукцинимид (NBS) вместо молекулярного брома.

Пропен реагирует с NBS в присутствии света или пероксида путем радикального замещения с образованием аллилбромида или 3-бромпропена. Подобно радикальным реакциям, механизм аллильного бромирования включает три стадии: инициирование, распространение и терминацию. На стадии инициирования NBS подвергается гомолитическому расщеплению слабых связей N–Br в присутствии света или пероксида с образованием радикала брома. В начале распространения образующийся радикал брома отрывает аллильный водород с образованием стабилизированного резонансом аллильного радикала и HBr. Образовавшийся HBr немедленно реагирует с NBS в ионной реакции с образованием Br_2, который участвует во второй стадии распространения. В конечном итоге, на стадии терминации различные радикалы объединяются, что приводит к образованию нерадикальных продуктов, приводящих к остановке реакции. На протяжении всей реакции концентрации HBr и Br_2 поддерживаются на минимальном уровне. В этих условиях, т. е. в неполярном растворителе с очень низкой концентрацией брома, ионное присоединение Br_2 не может успешно конкурировать с радикальным бромированием.

Радикальное бромирование аллилзамещенных алкенов образует смесь продуктов. Это происходит из-за резонансной стабилизации образовавшегося промежуточного аллильного радикала, который может стрывать галоген с любого участка (рис. 1).

<img alt="Figure1" src="/files/ftp_upload/12900/PT_12900_Figure%201.svg" style="text-align: center; height: 150px;">

В органическом синтезе образование продуктов может быть изменено путем изменения условий реакции. Например, продукт с добавкой dibromo - это ...

Education

JoVE Core

Organic Chemistry

Органическая химия

Unit 1

Раздел 1

Radical Chemistry

Радикальная химия

KEY TERMS AND CONCEPTS

ОСНОВНЫЕ ТЕРМИНЫ И ПОНЯТИЯ

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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Homolysis is a bond-breaking process that splits a shared electron pair evenly. When a bond is broken with enough energy, the two electrons can also move to one atom in heterolysis, which forms ions. Homolysis gives one electron to each atom and creates radicals, which are highly reactive particles with an unpaired electron. HCl shows the difference between these bond cleavages. In solution, HCl can break into H+ and Cl− ions, and chlorine takes both bonding electrons, leaving a naked proton.

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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

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Allylic Bromination with NBS

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