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

Bir alkenin alilik halojenasyonu, düşük bir halojen konsantrasyonu ve yüksek bir sıcaklık gerektirir.
Oda sıcaklığında alilik bromlama yapmak için, reaktif olarak N-bromosüksinamid veya NBS kullanılır. Düşük brom konsantrasyonunu korur ve brom radikalleri kaynağı olarak işlev görür.
Örneğin, propen, 3-bromopropen oluşturmak için ışık veya peroksit varlığında NBS ile reaksiyona girer.
Alilik brominasyon mekanizması, başlatma, yayılma ve sonlandırma adımlarını içerir.
Başlatma aşaması sırasında, NBS, bir brom radikali oluşturmak için N-Br bağının ışığa bağlı homolitik bölünmesine uğrar.
İlk yayılma adımında, brom radikali propen ile reaksiyona girer ve alilik hidrojeni soyutlayarak bir alilik radikal ve HBr oluşturur.
Üretilen HBr, reaksiyonun daha fazla yayılması için gerekli brom molekülünü üretmek için NBS ile reaksiyona girer.
İkinci yayılma adımında, brom molekülü, alil bromür ve bir brom atomu vermek için alilik radikal ile reaksiyona girer.
Son olarak, sonlandırma adımında, farklı radikaller, radikal olmayan ürünler vermek için birbirleriyle birleşir ve daha fazla reaksiyonu durdurur.

radikal yer değiştirme: alilik brominasyon

Radical Substitution: Allylic Bromination

Organik sentezde reaksiyon koşulları değiştirilerek ürünlerin oluşumu değiştirilebilir. Örneğin, propen oda sıcaklığında brom ile işlendiğinde bir dibromo ekleme ürünü oluşur. Buna karşılık propen, 3-bromopropeni vermek üzere polar olmayan solventlerde yüksek sıcaklıklarda alilik ikameye tabi tutulur. Ekleme reaksiyonunu önlemek için brom konsantrasyonunun reaksiyon boyunca mümkün olduğu kadar düşük tutulması gerekir. Bu, reaktif olarak moleküler bromin yerine N-bromosüksinimid (NBS) kullanılarak elde edilebilir.

Propen, alil bromit veya 3-bromopropeni oluşturmak üzere radikal ikame yoluyla ışık veya peroksit varlığında NBS ile reaksiyona girer. Radikal reaksiyonlara benzer şekilde alilik brominasyon mekanizması üç adımı içerir: başlatma, yayılma ve sonlandırma. Başlatma adımında NBS, brom radikalini oluşturmak üzere ışık veya peroksit varlığında zayıf N-Br bağlarının homolitik bölünmesine uğrar. Birinci yayılma adımı sırasında, üretilen brom radikali, rezonansla stabilize edilmiş alilik radikali ve HBr'yi vermek üzere alilik hidrojeni ayırır. Oluşan HBr, ikinci yayılma aşamasına katılan Br_2'yi üreten bir iyonik reaksiyonda hemen NBS ile reaksiyona girer. Sonunda, sonlandırma adımında, farklı radikaller birleşir ve bu da reaksiyonun sona ermesine yol açan radikal olmayan ürünlerin oluşmasıyla sonuçlanır. Reaksiyon boyunca HBr ve Br_2 konsantrasyonları minimumda tutulur. Bu koşullar altında, yani çok düşük brom konsantrasyonuna sahip polar olmayan bir çözücüde, Br_2'nin iyonik eklenmesi radikal brominasyonla başarılı bir şekilde rekabet edemez.

Alilik ikame edilmiş alkenlerin radikal bromlanması, bir ürün karışımı oluşturur. Bunun nedeni, halojeni her iki bölgeden de çıkarabilen, oluşan alilik radikal ara maddenin rezonans stabilizasyonudur (Şekil 1).

<img alt="Figure1" src="/files/ftp_upload/12900/PT_12900_Figure%201.svg" style="text-align: center; height: 150px;">

Radikal Yer Değiştirme: Alilik Brominasyon

Organik sentezde, reaksiyon koşulları değiştirilerek ürünlerin oluşumu değiştirilebilir. Örneğin, bir dibromo ilavesi ürünü ...

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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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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

JoVE Core Organik Kimya, kısa ve anlaşılması kolay animasyonlu video dersleri kullanarak stereokimya, reaksiyonlar ve mekanizmaların temel kavramlarını açıklar. Buna ek olarak, bilim insanlarının yer aldığı videolar, ilgili kavramların günümüz laboratuvarlarında gerçekleştirilen klasik ve orijinal araştırma deneylerinde nasıl uygulandığını göstermektedir.

Allylic Bromination with NBS

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