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

请记住，反马氏加成仅在 HBr 中观察到。HCl 和 HI 的自由基加成在热力学上是不利的。
但为什么？
让我们分析自由基反应机理中每一步链增长过程的热力学。
要使反应自发进行，吉布斯自由能的变化量 ∆G 必须为负值。
在首次传播步骤中，∆G 的符号由焓项与熵项之间的竞争关系决定。
对于 HBr 和 HCl 而言，在低温下焓项占主导地位，∆G 变为负值，表明该反应在热力学上是有利的。
然而，在HI的情况下，焓变项和熵变项均为正值，这意味着∆G 为正值，与温度无关，表明该反应在热力学上不利。
在第二步链增长反应中，由于反应物和产物分子的数量相等，熵项接近于零。因此，吉布斯自由能变∆G的符号由焓项决定。
显然，HCl 的 ∆G 为正值，使得该反应在热力学上不利。
总体而言，这两种链增长步骤仅在 HBr 的情况下在热力学上是有利的。

烯烃的自由基反马尔可夫尼科夫加成:热力学

Radical Anti-Markovnikov Addition to Alkenes: Thermodynamics

卤化氢与烯烃的反马可夫尼科夫加成反应仅在 HBr 下才在热力学上可行。 与其他卤化氢（如 HCl 和 HI）的自由基加成反应在热力学上是不利的。


 <img alt="Figure1" src="/files/ftp_upload/12904/12904_PT_Figure_1.svg" style="text-align: center; height: 125px;">


 热力学因素


 温度影响反应的自发性，可以根据吉布斯自由能 ΔG 的变化来评估。 如果吉布斯自由能的变化 ΔG 为负，则反应自发发生。 如下所示，ΔG 可以直接根据 ΔH 和 −TΔS 这两项的值来计算。


 <img alt="Figure2" src="/files/ftp_upload/12904/12904_PT_Figure_2.svg" style="text-align: center; height: 14px;">


 HBr 反马尔可夫尼科夫加成的热力学可行性可以从自由基机理中涉及的每个增长步骤（propagation step）来预测。


 在自由基反应的第一个增长步骤中，∆G 符号由焓项和熵项之间的竞争决定。 在较低温度下，HBr 和 HCl 的自由基加成在焓项中占主导地位。 因此，ΔG 为负，表明该反应在热力学上是有利的。 然而，在高温下，该反应是吸热的并且在热力学上是不利的。 相反，对于 HI，无论温度如何，焓和熵项均为正值。 结果，ΔG 变为正值，表明该反应在热力学上是不利的。


 在第二个增长步骤中，由于反应物和产物分子的数量相等，熵项接近于零，这意味着 ∆G 符号取决于焓项。 由于 HBr 和 HI 中的焓项占主导地位，因此该反应是放热反应，在热力学上是有利的。 然而，对于 HCl，∆G 为正，这意味着该反应是吸热反应，在热力学上是不利的。


 总之，只有 HBr 反应中的两个增长步骤在热力学上是有利的。

只有 HBr 与烯烃的反马氏加成在热力学上是可行的。与其他……的自由基加成反应

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

Education

教育

JoVE Core

Organic Chemistry

有机化学

Unit 1

第一单元

Radical Chemistry

自由基化学

KEY TERMS AND CONCEPTS

关键术语与概念

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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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 有机化学（Organic Chemistry）使用简明易懂的动画视频课程解释立体化学、反应和机制的基本概念。 此外，科学家在行动视频展示了相关概念在当今世界各地实验室进行的经典和原创研究实验中的应用。

HBr Selectivity in Alkene Addition Thermodynamics

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