自由基Radical

共价键断裂时，若成键电子对平均分给两个原子，每个原子各得一个电子，就生成两个自由基，这叫均裂。均裂需要能量，常由紫外光或加热提供，例如 Cl₂ 在紫外光下 Cl₂ → 2Cl•。弱键（如 O–O 键）较易均裂，过氧化物因此常用作自由基引发剂。

When a covalent bond breaks so that each atom keeps one of the bonding electrons, two radicals are formed; this is homolytic fission. It needs energy, usually supplied by ultraviolet light or heat, e.g. Cl₂ → 2Cl• in UV light. Weak bonds such as O–O break most easily, which is why peroxides are used as radical initiators.

甲烷与氯气在紫外光下发生自由基取代，分三个阶段。引发：Cl₂ → 2Cl•。增长：CH₄ + Cl• → CH₃• + HCl，CH₃• + Cl₂ → CH₃Cl + Cl•，每一步都再生出一个自由基，使反应持续下去。终止：两个自由基结合，如 CH₃• + Cl• → CH₃Cl、2CH₃• → C₂H₆。生成的乙烷说明反应经过了甲基自由基。产物往往是多种取代物的混合物。

Methane and chlorine undergo free-radical substitution in UV light in three stages. Initiation: Cl₂ → 2Cl•. Propagation: CH₄ + Cl• → CH₃• + HCl, then CH₃• + Cl₂ → CH₃Cl + Cl•; each step regenerates a radical, so the chain continues. Termination: two radicals combine, e.g. CH₃• + Cl• → CH₃Cl or 2CH₃• → C₂H₆. The ethane formed is evidence for methyl radicals. The product is usually a mixture of substitution products.

氯氟烃在平流层受紫外光照射放出 Cl•，Cl• 催化臭氧分解：Cl• + O₃ → ClO• + O₂，ClO• + O → Cl• + O₂，一个氯原子可破坏大量臭氧分子。NO 和 NO₂ 也是自由基。在生物体内，活性氧自由基会损伤 DNA 和膜脂，维生素 E 等抗氧化剂可以捕获它们。

In the stratosphere, UV light releases Cl• from chlorofluorocarbons, and Cl• catalyses ozone breakdown: Cl• + O₃ → ClO• + O₂, then ClO• + O → Cl• + O₂, so one chlorine atom destroys many ozone molecules. NO and NO₂ are also radicals. In living organisms, reactive oxygen radicals damage DNA and membrane lipids; antioxidants such as vitamin E trap them.