📚 AS Chemistry Unit 2: Reaction Mechanisms | AS化学第二单元：反应机理
Understanding reaction mechanisms is central to organic chemistry at AS Level. It reveals how bonds are broken and formed, and how electrons move to transform reactants into products. This article revisits the key mechanisms featured in the AS Chemistry Unit 2 January 2020 question paper insert, covering free radical substitution, electrophilic addition, and nucleophilic substitution. Mastering these mechanisms will strengthen your ability to predict products and explain organic reactions.
理解反应机理是AS级有机化学的核心。它揭示了化学键如何断裂和形成，以及电子如何移动使反应物转变为产物。本文回顾2020年1月AS化学第二单元试卷插页中涉及的关键机理，涵盖自由基取代、亲电加成和亲核取代。掌握这些机理将增强你预测产物和解释有机反应的能力。
1. What Are Reaction Mechanisms? | 什么是反应机理？
A reaction mechanism is a step-by-step description of how a chemical reaction occurs at the molecular level. It shows which bonds break, in what order, and how electrons are redistributed. Curly arrows are used to depict electron movement. In AS Chemistry, mechanisms are categorised according to the type of bond fission and the nature of the attacking species.
反应机理是逐步描述化学反应如何在分子水平发生的过程。它展示了哪些键断裂、以何种顺序以及电子如何重新分配。弯曲箭头用于表示电子移动。在AS化学中，机理根据键断裂的类型和进攻物种的性质进行分类。
2. Types of Bond Fission | 键断裂的类型
Bonds can break by homolytic fission or heterolytic fission. In homolytic fission, each atom takes one electron from the shared pair, producing two free radicals – each with an unpaired electron. This occurs when the bond is non-polar or under UV light. Heterolytic fission results in both electrons going to one atom, forming a cation and an anion.
键可以通过均裂或异裂断裂。在均裂中，每个原子从共用电子对中各获得一个电子，产生两个自由基——每个带有一个未成对电子。这通常发生在非极性键或紫外光照射下。异裂导致两个电子都归一个原子，形成一个阳离子和一个阴离子。
| Fission Type | Homolytic | Heterolytic |
|---|---|---|
| Electron distribution | One electron each | Both electrons to one atom |
| Products | Two radicals (e.g. Cl•) | Ions (e.g. H⁺ and Br⁻) |
| Typical conditions | UV light, high temp | Polar solvents, polarised bonds |
在均裂中，电子平均分配生成自由基；在异裂中，电子对完全转移生成离子。理解断键类型是区分三种主要机理的基础。
3. Free Radical Substitution of Alkanes | 烷烃的自由基取代
Alkanes react with halogens (e.g. Cl₂ or Br₂) in the presence of UV light to form halogenoalkanes. The mechanism proceeds via three stages: initiation, propagation, and termination.
烷烃与卤素（如Cl₂或Br₂）在紫外光下反应生成卤代烷。机理通过三个阶段进行：引发、增长、终止。
Initiation: UV light breaks the halogen bond homolytically to give two halogen radicals. Example: Cl–Cl → 2Cl•.
引发：紫外光使卤键均裂产生两个卤素自由基。例如：Cl–Cl → 2Cl•。
Propagation involves two steps that repeat in a chain reaction. First, a halogen radical abstracts a hydrogen atom from the alkane, generating an alkyl radical and HX. Then the alkyl radical reacts with a halogen molecule, forming the halogenoalkane and regenerating the halogen radical.
增长包含两步链式反应。首先，卤素自由基从烷烃中夺取一个氢原子，产生烷基自由基和HX。然后烷基自由基与卤素分子反应，生成卤代烷并再生卤素自由基。
CH₄ + Cl• → CH₃• + HCl
CH₃• + Cl₂ → CH₃Cl + Cl•
Termination occurs when two radicals combine, consuming radicals and ending the chain. Possible combinations: Cl• + Cl• → Cl₂, CH₃• + Cl• → CH₃Cl, CH₃• + CH₃• → C₂H₆.
终止发生在两个自由基结合时，消耗自由基，结束链反应。可能的结合：Cl• + Cl• → Cl₂, CH₃• + Cl• → CH₃Cl, CH₃• + CH₃• → C₂H₆。
Further substitution leads to a mixture of mono-, di- and polysubstituted products, which is a limitation of this reaction.
进一步的取代会导致单取代、二取代和多取代产物的混合物，这是该反应的一个局限。
4. Electrophilic Addition of Alkenes | 烯烃的亲电加成
Alkenes undergo electrophilic addition because the C=C double bond is electron-rich. An electrophile (electron-deficient species) attacks the π bond, leading to addition across the double bond. Common reagents include HBr, Br₂, H₂SO₄, and H₂O (with an acid catalyst).
烯烃发生亲电加成，因为C=C双键电子云密度高。亲电试剂（缺电子物种）进攻π键，导致双键上的加成。常见试剂包括HBr、Br₂、H₂SO₄和H₂O（酸催化）。
The mechanism always starts with the π electrons forming a bond to the electrophile, generating a carbocation intermediate.
该机理总是从π电子与亲电试剂成键开始，生成一个碳正离子中间体。
5. Mechanism with Hydrogen Halide (HBr) | 与卤化氢（HBr）的加成机理
The HBr molecule is polarised as Hᵟ⁺–Brᵟ⁻. The electrophilic H⁺ is attracted to the π electrons. Step 1: the π bond attacks H⁺ (or the H–Br bond breaks heterolytically), forming a carbocation intermediate and releasing Br⁻. Step 2: the bromide ion acts as a nucleophile and attacks the carbocation, forming the final haloalkane.
HBr分子极化，呈现Hᵟ⁺–Brᵟ⁻。亲电的H⁺被π电子吸引。第一步：π键进攻H⁺（或H–Br键异裂），形成碳正离子中间体并释放Br⁻。第二步：溴离子作为亲核试剂进攻碳正离子，生成最终卤代烷。
Curly arrows are drawn from the C=C bond to the H atom, and from the H–Br bond to the Br, showing the electron pair movements.
弯曲箭头从C=C键画向
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