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

L'alogenazione allilica di un alchene richiede una bassa concentrazione di alogeno e un'alta temperatura.
Per eseguire la bromurazione allilica a temperatura ambiente, come reagente viene utilizzata la N-bromosuccinimide, o NBS. Mantiene una bassa concentrazione di bromo e funge da fonte di radicali bromo.
Ad esempio, il propene reagisce con le NBS in presenza di luce o perossido per formare 3-bromopropene.
Il meccanismo della bromurazione allilica coinvolge le fasi di inizio, propagazione e terminazione.
Durante la fase di iniziazione, NBS subisce la scissione omolitica indotta dalla luce del legame N-Br per formare un radicale bromo.
Nella prima fase di propagazione, il radicale bromo reagisce con il propene ed estrae l'idrogeno allilico, formando un radicale allilico e HBr.
L'HBr generato reagisce con NBS per produrre la molecola di bromo necessaria per l'ulteriore propagazione della reazione.
Nella seconda fase di propagazione, la molecola di bromo reagisce con il radicale allilico per dare bromuro di allile e un atomo di bromo.
Infine, nella fase di terminazione, i diversi radicali si accoppiano tra loro per dare prodotti non radicalici, fermando ulteriori reazioni.

sostituzione radicale: bromurazione allilica

Radical Substitution: Allylic Bromination

Nella sintesi organica, la formazione dei prodotti può essere alterata modificando le condizioni di reazione. Ad esempio, un prodotto di addizione di dibromo si forma quando il propene viene trattato con bromo a temperatura ambiente. Al contrario, il propene subisce una sostituzione allilica in solventi non polari ad alte temperature per dare 3-bromopropene. Per evitare la reazione di addizione, la concentrazione di bromo deve essere mantenuta la più bassa possibile durante tutta la reazione. Ciò può essere ottenuto utilizzando la N-bromosuccinimide (NBS) come reagente al posto del bromo molecolare.

Il propene reagisce con l'NBS in presenza di luce o perossido tramite sostituzione radicalica per formare bromuro di allile o 3-bromopropene. Simile alle reazioni radicaliche, il meccanismo della bromurazione allilica prevede tre fasi: inizio, propagazione e terminazione. Nella fase di inizio, NBS subisce la scissione omolitica dei legami deboli N – Br in presenza di luce o perossido per formare radicali bromo. Durante la prima fase di propagazione, il radicale bromo generato estrae l'idrogeno allilico per dare il radicale allilico stabilizzato per risonanza e HBr. L'HBr formato reagisce immediatamente con l'NBS in una reazione ionica producendo Br_2, che partecipa alla seconda fase di propagazione. Alla fine, nella fase di terminazione, diversi radicali si combinano, dando luogo alla formazione di prodotti non radicalici che portano alla terminazione della reazione. Durante tutta la reazione, le concentrazioni di HBr e Br_2 vengono mantenute al minimo. In queste condizioni, cioè in un solvente non polare con una concentrazione molto bassa di bromo, l'aggiunta ionica di Br_2 non compete con successo con la bromurazione radicale.

La bromurazione radicale degli alcheni sostituiti allilici forma una miscela di prodotti. Ciò è dovuto alla stabilizzazione di risonanza del radicale allilico intermedio formato, che può estrarre l'alogeno da entrambi i siti (Figura 1).

<img alt="Figure1" src="/files/ftp_upload/12900/PT_12900_Figure%201.svg" style="text-align: center; height: 150px;">

Sostituzione radicale: bromurazione allilica

Nella sintesi organica, la formazione dei prodotti può essere alterata modificando le condizioni di reazione. Ad esempio, un prodotto per l'aggiunta di dibromo è ...

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Organic Chemistry

Chimica Organica

Unit 1

Unità 1

Radical Chemistry

Chimica dei radicali

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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

organic chemistry

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Allylic Bromination with NBS

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