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

항 마르코프니코프 첨가는 HBr에서만 관찰된다는 점을 기억하십시오. HCl 및 HI를 사용한 급진적 첨가는 열역학적으로 바람직하지 않습니다.
하지만 왜?
라디칼 메커니즘과 관련된 각 전파 단계에 대한 열역학을 살펴 보겠습니다.
반응이 자발적이려면 깁스 자유 에너지 ∆G의 변화가 음수여야 합니다.
첫 번째 전파 단계에서 ∆G의 부호는 엔탈피 항과 엔트로피 항 간의 경쟁에 의해 결정됩니다.
HBr 및 HCl의 경우 저온에서 엔탈피 항이 우세하고 ∆G는 음수가 되어 열역학적으로 유리한 반응을 의미합니다.
그러나 HI의 경우 엔탈피 항과 엔트로피 항이 모두 양수이며, 이는 온도에 관계없이 ∆G가 양수임을 의미하며, 이는 열역학적으로 바람직하지 않은 반응을 나타냅니다.
두 번째 전파 단계의 경우, 엔트로피 항은 반응물과 생성물 분자의 수가 같기 때문에 0에 가깝습니다. 따라서 엔탈피 항은 ∆G의 기호를 지시합니다.
명백한 바와 같이, ∆G는 HCl에 대해 양성이므로 반응이 열역학적으로 불리합니다.
전반적으로 두 가지 전파 단계는 HBr의 경우에만 열역학적으로 유리합니다.

알켄에 대한 라디칼 항마르코프니코프 첨가: 열역학

Radical Anti-Markovnikov Addition to Alkenes: Thermodynamics

알켄에 할로겐화수소를 첨가하는 반마코브니코프 반응은 HBr에서만 열역학적으로 가능합니다. HCl 및 HI와 같은 다른 할로겐화수소와의 라디칼 첨가 반응은 열역학적으로 바람직하지 않습니다.

<img alt="Figure1" src="/files/ftp_upload/12904/12904_PT_Figure_1.svg" style="text-align: center; height: 125px;">

열역학적 요인

온도는 반응의 자발성에 영향을 미치며 이는 깁스 자유 에너지 ΔG의 변화를 기반으로 평가할 수 있습니다. 깁스 자유 에너지의 변화 ΔG가 음수이면 반응은 자발적으로 발생합니다. 아래에 표시된 것처럼 ΔG는 ΔH 및 -TΔS라는 두 항의 값에서 직접 평가할 수 있습니다.

<img alt="Figure2" src="/files/ftp_upload/12904/12904_PT_Figure_2.svg" style="text-align: center; height: 14px;">

HBr을 이용한 항마르코프니코프 첨가의 열역학적 타당성은 라디칼 메커니즘과 관련된 각 전파 단계에서 예측할 수 있습니다.

라디칼 반응의 첫 번째 전파 단계에서 ΔG 부호는 엔탈피와 엔트로피 항 간의 경쟁에 의해 결정됩니다. 더 낮은 온도에서는 HBr 및 HCl을 사용한 라디칼 첨가가 엔탈피 항을 지배합니다. 결과적으로 ΔG는 음수이며 이는 반응이 열역학적으로 유리하다는 것을 나타냅니다. 그러나 고온에서 반응은 흡열성이고 열역학적으로 바람직하지 않습니다. 대조적으로, HI에서는 온도에 관계없이 엔탈피와 엔트로피 항이 모두 양수입니다. 결과적으로 ΔG는 양의 값이 되며, 이는 반응이 열역학적으로 불리하다는 것을 의미합니다.

두 번째 전파 단계에서는 반응물과 생성물 분자의 수가 동일하므로 엔트로피 항은 0에 가깝습니다. 즉, ΔG 부호는 엔탈피 항에 따라 달라집니다. 엔탈피 항이 HBr과 HI에 대해 지배적이기 때문에 반응은 발열성이고 열역학적으로 유리합니다. 그러나 HCl의 경우 ΔG는 양수입니다. 이는 반응이 흡열적이고 열역학적으로 바람직하지 않음을 의미합니다.

요약하면, 반응의 두 가지 전파 단계는 HBr에서만 열역학적으로 유리합니다.

알켄에 할로겐화수소를 첨가하는 안티 마르코프니코프(anti-Markovnikov)는 HBr에서만 열역학적으로 실현 가능합니다. 다른 과격한 추가 반응은 ...

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

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JoVE Core

Organic Chemistry

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Unit 1

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Radical Chemistry

라디컬 화학

KEY TERMS AND CONCEPTS

주요 용어 및 개념

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.

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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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