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

נזכיר כי תוספת נגד מרקובניקוב נצפתה רק עם HBr. התוספת הרדיקלית עם HCl ו-HI היא שלילית מבחינה תרמודינמית.
אבל למה?
בואו נבחן את התרמודינמיקה עבור כל שלב התפשטות המעורב במנגנון הרדיקלי.
כדי שתגובה תהיה ספונטנית, השינוי באנרגיה החופשית של גיבס, ∆G, חייב להיות שלילי.
בשלב ההתפשטות הראשון, הסימן ∆G מוכתב על ידי התחרות בין תנאי האנתלפיה והאנטרופיה.
במקרה של HBr ו-HCl, המונח אנתלפיה שולט בטמפרטורות נמוכות, ו∆G הופך לשלילי, מה שמרמז על תגובה חיובית מבחינה תרמודינמית.
עם זאת, במקרה של HI, הן תנאי האנתלפיה והן מונחי האנטרופיה הם חיוביים, מה שמרמז על כך ש∆G הוא חיובי, ללא קשר לטמפרטורה, מה שמצביע על תגובה שלילית מבחינה תרמודינמית.
בשלב ההתפשטות השני, מונח האנטרופיה קרוב לאפס מכיוון שמספר המגיבים ומולקולות המוצר שווים. לכן, המונח אנתלפיה מכתיב את הסימן של ∆G.
כפי שניתן לראות, ∆G הוא חיובי עבור HCl, מה שהופך את התגובה תרמודינמית שלילית.
בסך הכל, שני שלבי ההתפשטות חיוביים מבחינה תרמודינמית רק במקרה של HBr.

﻿סיפוח אנטי-מרקובניקוב רדיקלי לאלקנים: תרמודינמיקה

Radical Anti-Markovnikov Addition to Alkenes: Thermodynamics

<div style="direction:rtl"> סיפוח האנטי-מרקובניקוב של הלידים מימן לאלקן אפשרי מבחינה תרמודינמית רק עם HBr. תגובת הסיפוח הרדיקלי עם הלידי מימן אחרים כמו HCl ו-HI היא לא חיובית מבחינה תרמודינמית. <img alt="Figure1" src="/files/ftp_upload/12904/12904_PT_Figure_1.svg" style="text-align: center; height: 125px;"> גורמים תרמודינמיים הטמפרטורה משפיעה על הספונטניות של תגובה, אותה ניתן להעריך על סמך השינוי באנרגיה החופשית של גיבס, ΔG. אם השינוי באנרגיה החופשית של גיבס, ΔG, הוא שלילי, התגובה מתרחשת באופן ספונטני. כפי שמוצג להלן, ניתן להעריך את ΔG ישירות מהערכים של שני האיברים, ΔH ו-TΔS. <img alt="Figure2" src="/files/ftp_upload/12904/12904_PT_Figure_2.svg" style="text-align: center; height: 14px;"> ניתן לחזות את ההיתכנות התרמודינמית של סיפוח אנטי-מרקובניקוב עם HBr מכל שלב התפשטות המעורב במנגנון הרדיקלי. בשלב ההתפשטות הראשון של התגובה הרדיקלית, הסימן ∆G נקבע על ידי התחרות בין האנתלפיה והאנטרופיה. בטמפרטורות נמוכות יותר, סיפוח רדיקלי עם HBr ו- HCl שולט בערך האנטלפיה. כתוצאה מכך, ΔG שלילי, מה שמצביע על כך שהתגובה חיובית מבחינה תרמודינמית. עם זאת, בטמפרטורות גבוהות, התגובה היא אנדותרמית ולא חיובית מבחינה תרמודינמית. לעומת זאת, עם HI, ללא קשר לטמפרטורה, הן ערכי האנתלפיה והן האנטרופיה חיוביים. כתוצאה מכך, ∆G הופך חיובי, מה שמסמל שהתגובה אינה חיובית מבחינה תרמודינמית. בשלב ההתפשטות השני, ערך האנטרופיה קרוב לאפס מכיוון שמספר המגיבים ומולקולות התוצר שווים, כלומר הסימן ∆G תלוי בערך האנטלפיה. מכיוון שערך האנטלפיה שולט עבור HBr ו-HI, התגובה היא אקסותרמית וטובה מבחינה תרמודינמית. עם זאת, עם HCl, ∆G חיובי, כלומר התגובה היא אנדותרמית ולא חיובית מבחינה תרמודינמית. לסיכום, שני שלבי ההתפשטות בתגובה נוחים מבחינה תרמודינמית רק עם HBr. </div>

תוספת אנטי-מרקובניקוב של מימן הלידים לאלקן אפשרית מבחינה תרמודינמית רק עם HBr. תגובת התוספת הרדיקלית עם ...

radical-anti-markovnikov-addition-to-alkenes-thermodynamics

Education

JoVE Core

Organic Chemistry

כימיה אורגנית

Unit 1

יחידה 1

Radical Chemistry

כימיה רדיקלית

KEY TERMS AND CONCEPTS

מונחים ומושגים מרכזיים

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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EPR Hyperfine Splitting in Organic Radicals

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Radical Addition and Spin Conservation

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Radicals from Elimination Reactions

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Electrophilic Radicals and Alkene Reactions

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Radical Cyclization in One Molecule

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Autoxidation in Oils and Ethers

Autoxidation is the oxidation of an organic compound in the presence of air or oxygen. In this process, compounds such as cumene can react with oxygen to form hydroperoxide. The reaction follows three main stages: initiation, propagation, and termination. Many organic compounds can undergo autoxidation. Ethers are especially vulnerable when oxygen is present, and they can form hydroperoxides. Even though this reaction is slow, old ether bottles may contain small amounts of peroxide. That is...

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Selective MnO2 Oxidation of Alcohols

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How Alkanes Form Alkyl Halides by Halogenation

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Alkane Halogenation: Stability and Reaction Rates

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Radical Halogenation: Chiral Products

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Allylic Chlorination by Radical Chain Reaction

Allylic chlorination is a radical chain reaction in alkenes. It happens when halogen reactions move away from simple addition and toward substitution at the allylic position, which is the carbon next to a double bond. At low temperatures, alkenes usually react with halogens by addition. When the temperature increases, or when the reaction conditions create radicals, allylic substitution becomes favored. This works because allylic hydrogens are very reactive and the intermediate formed is...

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Allylic Bromination with NBS

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Alkyl Halide Reduction with Tributyltin Hydride

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Peroxide Effect in HBr Addition to Alkenes

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Hydrogen Bromide Addition to Alkenes by Radicals

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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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