Name: hbr selectivity in alkene addition thermodynamics
Uploaded: 2025-05-22T09:05:43.000Z
Duration: 1 min 32 s
Channel: JoVE

تذكر أن إضافة مناهضة ماركوفنيكوف لا تلاحظ إلا مع HBr. الإضافة الجذرية مع حمض الهيدروكلوريك و HI غير مواتية من الناحية الديناميكية الحرارية.
لكن لماذا؟
دعونا نفحص الديناميكا الحرارية لكل خطوة انتشار متضمنة في الآلية الجذرية.
لكي يكون رد الفعل عفويا ، يجب أن يكون التغيير في طاقة جيبس الحرة ، ∆G ، سلبيا.
بالنسبة لخطوة الانتشار الأولى ، تملي علامة ∆G من خلال المنافسة بين المحتوى الحراري ومصطلحات الانتروبيا.
في حالة HBr و HCl ، يهيمن مصطلح المحتوى الحراري في درجات الحرارة المنخفضة ، ويصبح ∆G سالبا ، مما يعني تفاعلا مناسبا من الناحية الديناميكية الحرارية.
ومع ذلك ، في حالة HI ، يكون كل من مصطلحي المحتوى الحراري والانتروبيا موجبين ، مما يعني أن ∆G موجب ، بغض النظر عن درجة الحرارة ، مما يشير إلى تفاعل غير موات من الناحية الديناميكية الحرارية.
بالنسبة لخطوة الانتشار الثانية ، يكون مصطلح الانتروبيا قريبا من الصفر لأن عدد المواد المتفاعلة وجزيئات المنتج متساوية. لذا ، فإن مصطلح المحتوى الحراري يملي علامة ∆G.
كما هو واضح ، فإن ∆G إيجابي بالنسبة لحمض الهيدروكلوريك ، مما يجعل التفاعل غير موات من الناحية الديناميكية الحرارية.
بشكل عام ، تكون خطوتا الانتشار مواتيتين من الناحية الديناميكية الحرارية فقط في حالة HBr.

الإضافة الجذرية المضادة لقاعدة ماركوفنيكوف إلى الألكينات: الديناميكا الحرارية

Radical Anti-Markovnikov Addition to Alkenes: Thermodynamics

<div style="direction:rtl">إن إضافة هاليدات الهيدروجين المضادة لقاعدة ماركوفنيكوف إلى الألكين ممكنة من الناحية الديناميكية الحرارية فقط مع HBr. يعتبر تفاعل الإضافة الجذري مع هاليدات الهيدروجين الأخرى مثل HCl وHI غير مناسب من الناحية الديناميكية الحرارية. <img alt="Figure1" src="/files/ftp_upload/12904/12904_PT_Figure_1.svg" style="text-align: center; height: 125px;"> العوامل الديناميكية الحرارية تؤثر درجة الحرارة على عفوية التفاعل، والتي يمكن تقييمها على أساس التغير في طاقة جيبس الحرة، ΔG. إذا كان التغير في طاقة جيبس الحرة, ΔG, سالبًا، فإن التفاعل يحدث تلقائيًا. كما هو موضح أدناه، يمكن تقييم ΔG مباشرةً من قيمتي الحدين، ΔH و−TΔS. <img alt="Figure2" src="/files/ftp_upload/12904/12904_PT_Figure_2.svg" style="text-align: center; height: 14px;"> يمكن التنبؤ بالجدوى الديناميكية الحرارية للإضافة المضادة لقاعدة ماركوفنيكوف مع HBr من كل خطوة انتشار تشارك في الآلية الجذرية. في خطوة الانتشار الأولى للتفاعل الجذري، يتم تحديد علامة ∆G من خلال التنافس بين مصطلحي الإنثالبي والإنتروبيا. في درجات الحرارة المنخفضة، يهيمن الإضافة الجذرية مع HBr وHCl على مصطلح المحتوى الحراري. وبالتالي، تكون ∆G سالبة، مما يشير إلى أن التفاعل مناسب من الناحية الديناميكية الحرارية. ومع ذلك، عند درجات الحرارة المرتفعة، يكون التفاعل ماصًا للحرارة وغير مناسب من الناحية الديناميكية الحرارية. على النقيض من ذلك، مع HI، بغض النظر عن درجة الحرارة، فإن كلا من شروط المحتوى الحراري والإنتروبيا إيجابية. ونتيجة لذلك، تصبح ∆G موجبة، مما يدل على أن التفاعل غير مناسب من الناحية الديناميكية الحرارية. في خطوة الانتشار الثانية، يكون حد الإنتروبيا قريبًا من الصفر نظرًا لتساوي عدد المواد المتفاعلة وجزيئات المنتج، مما يعني أن علامة ∆G تعتمد على حد الإنثالبي. وبما أن مصطلح المحتوى الحراري يهيمن على HBr وHI، فإن التفاعل يكون طاردًا للحرارة ومفضلًا من الناحية الديناميكية الحرارية. ومع ذلك، مع حمض الهيدروكلوريك، تكون ∆G موجبة، مما يعني أن التفاعل ماص للحرارة وغير مناسب من الناحية الديناميكية الحرارية. باختصار، خطوتان الانتشار في التفاعل مناسبتان من الناحية الديناميكية الحرارية فقط مع HBr.</div>

إن إضافة المضادة لماركونيكوف من هاليدات الهيدروجين إلى الألكين ممكنة ديناميكيا حرارية فقط مع [هبر]. رد فعل الإضافة الجذري مع ...

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Education

JoVE Core

Organic Chemistry

الكيمياء العضوية

Unit 1

الوحدة الأولى

Radical Chemistry

الكيمياء الجذرية

KEY TERMS AND CONCEPTS

المصطلحات والمفاهيم الأساسية

hbr selectivity in alkene addition thermodynamics

Anti-Markovnikov addition of hydrogen halides to alkenes depends on thermodynamics. In this radical reaction, only HBr makes the overall process feasible. With HCl and HI, the same addition is thermodynamically unfavorable.

Temperature affects whether the reaction is spontaneous. Spontaneity is measured with Gibbs free energy, or ΔG. When ΔG is negative, the reaction can occur on its own. ΔG is determined by two terms: ΔH, which is the enthalpy change, and −TΔS, which is the entropy term.

The first propagation step depends on the balance between enthalpy and entropy. At lower temperatures, radical addition with HBr and HCl is driven by the enthalpy term, so ΔG is negative and the step is favorable. At higher temperatures, the reaction becomes endothermic and thermodynamically unfavorable. With HI, both the enthalpy and entropy terms are positive at any temperature, so ΔG stays positive.

The second propagation step is different because the number of reactant and product molecules is equal. That makes the entropy term close to zero, so ΔG depends mainly on enthalpy. For HBr and HI, the enthalpy term makes the step exothermic and favorable. For HCl, ΔG is positive, so the step is endothermic and unfavorable. Taken together, both propagation steps are thermodynamically favorable only with HBr.

HBr Selectivity in Alkene Addition Thermodynamics

hbr-selectivity-in-alkene-addition-thermodynamics

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radical anti-markovnikov addition to alkenes: thermodynamics

Anti-Markovnikov addition of hydrogen halides to alkenes depends on thermodynamics. In this radical reaction, only HBr makes the overall process feasible. With HCl and HI, the same addition is thermodynamically unfavorable. Temperature affects whether the reaction is spontaneous. Spontaneity is measured with Gibbs free energy, or ΔG. When ΔG is negative, the reaction can occur on its own. ΔG is determined by two terms: ΔH, which is the enthalpy change, and −TΔS, which is the entropy term. The...

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key terms and concepts

See why anti-Markovnikov addition to alkenes is thermodynamically favorable only with HBr, and how ΔG, enthalpy, entropy, and temperature affect each step.

HBr Selectivity in Alkene Addition Thermodynamics | JoVE K12

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JoVE Core Organic Chemistry explains basic concepts of stereochemistry, reactions, and mechanisms using concise and easy-to-understand animated video lessons. Additionally, the scientist-in-action videos demonstrate the application of related concepts in classical and original research experiments performed in today's laboratories worldwide.

HBr Selectivity in Alkene Addition Thermodynamics

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