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

يتطلب الهالوجين الأليلي للألكين تركيزا منخفضا من الهالوجين ودرجة حرارة عالية.
لإجراء البرومة الأليلية في درجة حرارة الغرفة ، يتم استخدام N-bromosuccinimide ، أو NBS ، ككاشف. يحافظ على تركيز منخفض من البروم ويعمل كمصدر لجذور البروم.
على سبيل المثال ، يتفاعل البروبين مع NBS في وجود الضوء أو البيروكسيد لتكوين 3-بروموبروبين.
تتضمن آلية البرومة الأليلية خطوات البدء والتكاثر والإنهاء.
أثناء خطوة البدء ، يخضع NBS لانقسام متماثل ناتج عن الضوء لرابطة N-Br لتشكيل جذر البروم.
في خطوة الانتشار الأولى ، يتفاعل جذر البروم مع البروبين ويلخص الهيدروجين الأليليك ، مكونا جذريا آليلك و HBr.
يتفاعل HBr المتولد مع NBS لإنتاج جزيء البروم المطلوب لمزيد من انتشار التفاعل.
في خطوة الانتشار الثانية ، يتفاعل جزيء البروم مع الجذر الأليليك لإعطاء بروميد الأليل وذرة البروم.
أخيرا ، في خطوة الإنهاء ، تتزاوج الجذور المختلفة مع بعضها البعض لإعطاء منتجات غير جذرية ، مما يوقف المزيد من التفاعل.

الاستبدال الجذري: البرومة الأليلية

Radical Substitution: Allylic Bromination

<div style="direction:rtl">في الاصطناع العضوي، يمكن تغيير تكوين المنتجات عن طريق تغيير ظروف التفاعل. على سبيل المثال، يتم تكوين منتج إضافة ثنائي برومو عند معالجة البروبين مع البروم في درجة حرارة الغرفة. في المقابل، يخضع البروبين لاستبدال الأليلي في المذيبات غير القطبية عند درجات حرارة عالية ليعطي 3-بروموبروبين. لتجنب تفاعل الإضافة، يجب أن يظل تركيز البروم منخفضًا قدر الإمكان طوال فترة التفاعل. يمكن تحقيق ذلك باستخدام N-بروموسيليلإيميد(NBS) ككاشف بدلاً من البروم الجزيئي. يتفاعل البروبين مع NBS في وجود الضوء أو البيروكسيد عن طريق الاستبدال الجذري لتكوين بروميد الأليل أو 3-بروموبروبين. على غرار التفاعلات الجذرية، تشتمل آلية المعالجة بالبروم الأليلي على ثلاث خطوات: البدء، والانتشار، والإنهاء. في خطوة البدء، يخضع NBS للانقسام المتجانس لروابط N-Br الضعيفة في وجود الضوء أو البيروكسيد لتكوين جذري البروم. أثناء خطوة الانتشار الأولى، يقوم جذر البروم المتولد باستخلاص الهيدروجين الأليلي لإعطاء جذر أليلي ثابت بالرنين وHBr. يتفاعل HBr المتكون فورًا مع NBS في تفاعل أيوني ينتج عنه Br2، والذي يشارك في خطوة الانتشار الثانية. في النهاية، في خطوة الإنهاء، تتحد الجذور المختلفة، مما يؤدي إلى تكوين منتجات غير جذرية تؤدي إلى إنهاء التفاعل. خلال التفاعل، يتم الاحتفاظ بتركيزات HBr وBr2 عند الحد الأدنى. في ظل هذه الظروف، أي في مذيب غير قطبي بتركيز منخفض جدًا من البروم، فإن الإضافة الأيونية لـ Br2 لا تتنافس بنجاح مع المعالجة بالبروم الجذري. تشكل المعالجة الجذرية للألكينات المستبدلة أليليًا خليطًا من المنتجات. ويرجع ذلك إلى استقرار الوسيط الأليلي الجذري المشكل بالرنين، والذي يمكنه استخلاص الهالوجين من أي من الموقعين (الشكل 1). <img alt="Figure1" src="/files/ftp_upload/12900/PT_12900_Figure%201.svg" style="text-align: center; height: 150px;"></div>

في التخليق العضوي ، يمكن تغيير تكوين المنتجات عن طريق تغيير ظروف التفاعل. على سبيل المثال ، منتج إضافة ثنائي البرومو هو ...

Education

JoVE Core

Organic Chemistry

الكيمياء العضوية

Unit 1

الوحدة الأولى

Radical Chemistry

الكيمياء الجذرية

KEY TERMS AND CONCEPTS

المصطلحات والمفاهيم الأساسية

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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