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

Напомним, что антимарковниковское присоединение наблюдается только с HBr. Радикальное присоединение с HCl и HI термодинамически неблагоприятно.
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Чтобы реакция была спонтанной, изменение свободной энергии Гиббса, ∆G, должно быть отрицательным.
На первом этапе распространения знак ∆G диктуется конкуренцией между энтальпией и членами энтропии.
В случае HBr и HCl энтальпийный член доминирует при низких температурах, а ∆G становится отрицательным, подразумевая термодинамически благоприятную реакцию.
Однако в случае HI, как энтальпия, так и энтропия являются положительными, подразумевая, что ∆G положительна, независимо от температуры, что указывает на термодинамически неблагоприятную реакцию.
На втором этапе распространения член энтропии близок к нулю, поскольку количество реагентов и молекул продукта равно. Итак, термин энтальпии диктует знак ∆G.
Как видно, ∆G положительно влияет на HCl, что делает реакцию термодинамически неблагоприятной.
В целом, эти две стадии распространения термодинамически благоприятны только в случае HBr.

Радикальное антимарковниковское присоединение к алкенам: термодинамика

Radical Anti-Markovnikov Addition to Alkenes: Thermodynamics

Антимарковниковское присоединение галогеноводородов к алкену термодинамически осуществимо только с HBr. Реакция радикального присоединения с другими галогеноводородами, такими как HCl и HI, термодинамически невыгодна.

<img alt="Figure1" src="/files/ftp_upload/12904/12904_PT_Figure_1.svg" style="text-align: center; height: 125px;">

Термодинамические факторы

Температура влияет на спонтанность реакции, которую можно оценить по изменению свободной энергии Гиббса ΔG. Если изменение свободной энергии Гиббса ΔG отрицательно, реакция происходит самопроизвольно. Как показано ниже, ΔG можно оценить непосредственно по значениям двух членов: ΔH и −TΔS.

<img alt="Figure2" src="/files/ftp_upload/12904/12904_PT_Figure_2.svg" style="text-align: center; height: 14px;">

Термодинамически, возможность антимарковниковского присоединения с HBr можно предсказать на основе каждой стадии распространения, участвующей в радикальном механизме.

На первом этапе распространения радикальной реакции знак ∆G определяется конкуренцией между энтальпийными и энтропийными членами. При более низких температурах радикальное присоединение с HBr и HCl доминирует над энтальпийным членом. Следовательно, ΔG отрицательна, что указывает на термодинамически выгодную реакцию. Однако, при высоких температурах реакция эндотермична и термодинамически невыгодна. Напротив, в случае HI, независимо от температуры, члены как энтальпии, так и энтропии положительны. В результате ∆G становится положительным, что означает, что реакция термодинамически невыгодна.

На втором этапе распространения энтропийный член близок к нулю, поскольку количество реагентов и молекул продукта одинаково, а это означает, что знак ∆G зависит от энтальпийного члена. Поскольку для HBr и HI доминирует энтальпийный член, реакция является экзотермической и термодинамически выгодной. Однако, в случае HCl ∆G положительно, что означает, что реакция эндотермическая и термодинамически невыгодная.

Таким образом, две стадии распространения реакции термодинамически выгодны только для HBr.

Антимарковниковское добавление галогенидов водорода к алкену термодинамически возможно только с HBr. Реакция радикального сложения с другими ...

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

Education

JoVE Core

Organic Chemistry

Органическая химия

Unit 1

Раздел 1

Radical Chemistry

Радикальная химия

KEY TERMS AND CONCEPTS

ОСНОВНЫЕ ТЕРМИНЫ И ПОНЯТИЯ

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

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Peroxide Effect in HBr Addition to Alkenes

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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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HBr Selectivity in Alkene Addition Thermodynamics

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