双语化学辞典Chem Lexicon
机理 · 有机化学 · 反应机理 Organic chemistry · Reaction mechanisms

自由基取代Free-radical substitution

拼音Pinyin zì yóu jī qǔ dài　别名Also 自由基卤代；光卤代； radical halogenation

定义Definition

自由基取代是经自由基链机理，用其他原子或基团（最常见为卤素）取代烷烃等分子中氢原子的反应，如甲烷在光照下与氯气反应生成氯甲烷。

Free-radical substitution replaces a hydrogen atom (typically in an alkane) by another atom or group — most often a halogen — through a radical chain mechanism, as in the light-induced chlorination of methane.

链机理Chain mechanism

链引发：光或热使 Cl₂ 均裂为 2Cl·。链增长：Cl· + CH₄ → HCl + ·CH₃；·CH₃ + Cl₂ → CH₃Cl + Cl·（循环往复）。链终止：两个自由基结合（2Cl· → Cl₂，Cl· + ·CH₃ → CH₃Cl，2·CH₃ → C₂H₆）。产物通常是一氯到四氯甲烷的混合物。

Initiation: light or heat splits Cl₂ into 2 Cl·. Propagation: Cl· + CH₄ → HCl + ·CH₃, then ·CH₃ + Cl₂ → CH₃Cl + Cl·, repeating. Termination: radicals combine (Cl₂, CH₃Cl, C₂H₆). Products are usually a mixture of mono- to tetrachloromethane.

选择性Selectivity

C–H 键强度：伯 > 仲 > 叔，所以自由基稳定性叔 > 仲 > 伯。氯化活性高、选择性低（室温叔:仲:伯约 5:4:1 每个 H）；溴化吸热、过渡态较晚，选择性高（约 1600:80:1）——哈蒙德假说的典型例子。烯丙位、苄位 C–H 最弱，可用 NBS 选择性溴化。

C–H strength is 1° > 2° > 3°, so radical stability is 3° > 2° > 1°. Chlorination is fast but unselective (~5:4:1 per H at room temperature); bromination is endothermic with a late TS and highly selective (~1600:80:1) — a textbook Hammond example. Allylic and benzylic C–H are weakest and are brominated selectively with NBS.

应用Applications

工业上制氯甲烷类溶剂、氯乙烯前体；沃尔–齐格勒烯丙位溴化；大气中 ·OH 引发的有机物降解也遵循自由基夺氢机理。

Industrial chloromethanes and vinyl chloride precursors; Wohl–Ziegler allylic bromination; atmospheric degradation of organics initiated by ·OH follows the same H-abstraction chemistry.

反应Reactions

R–H + X2 hν 或 △⟶ R–X + HXR–H + X2 hν or △⟶ R–X + HX

光照（紫外）或加热，气相或惰性溶剂

UV light or heat, gas phase or inert solvent

  1. 链引发：X₂ 均裂。

    Initiation: X₂ homolysis.

  2. 链增长：X· 夺 H，R· 夺 X。

    Propagation: X· abstracts H; R· abstracts X.

  3. 链终止：自由基偶合。

    Termination: radical coupling.

CH4 + Cl2 hν⟶ CH3Cl + HCl

甲烷氯代（中学经典实验）。

Chlorination of methane (classic school experiment).

(CH3)3CH + Br2 hν⟶ (CH3)3CBr> 99%

溴化高度选择叔位。

Bromination is highly selective for the tertiary H.

反应机理图Mechanism

红色弯箭头表示一对电子的移动方向：从电子来源（孤对电子或化学键）指向电子去向。Red curly arrows show where an electron pair moves: from its source (lone pair or bond) to its destination.

1

链引发：光照使 Cl–Cl 均裂

Initiation: light splits Cl–Cl homolytically

hν
2

链增长①：Cl· 夺取 H

Propagation ①: Cl· abstracts H

3

链增长②：·CH₃ 夺取 Cl，再生 Cl·

Propagation ②: ·CH₃ abstracts Cl, regenerating Cl·

4

链终止：任意两个自由基结合

Termination: any two radicals combine

年表Timeline

  • 1830s

    杜马研究有机物的氯化取代

    Dumas studies chlorine substitution in organic compounds

  • 1942

    齐格勒发展 NBS 烯丙位溴化

    Ziegler develops NBS allylic bromination