📚 Reaction Mechanisms: Essential Revision for CCEA A-Level Chemistry | 反应机理：CCEA A-Level化学考点精讲
Understanding reaction mechanisms is fundamental to mastering organic chemistry at CCEA A-Level. A mechanism shows the step-by-step pathway of bond breaking and bond making, using curly arrows to illustrate the movement of electron pairs. This guide covers key mechanisms, including free radical substitution, electrophilic addition, nucleophilic substitution, and elimination, alongside essential concepts like carbocation stability and experimental evidence.
理解反应机理是掌握 CCEA A-Level 有机化学的基础。机理展示了键断裂和键形成的逐步过程，用弯箭头表示电子对的移动。本指南涵盖了自由基取代、亲电加成、亲核取代和消去反应等关键机理，以及碳正离子稳定性和实验证据等重要概念。
1. What is a Reaction Mechanism? | 什么是反应机理？
A reaction mechanism is a detailed description of how bonds are broken and formed during a chemical reaction. It shows the movement of electrons using curly arrows (↷), where the arrow tail starts from the electron source (e.g., a lone pair or a bond) and the head points to the electron destination. Mechanisms involve intermediates such as carbocations or free radicals, and transition states.
反应机理详细描述了化学反应中键如何断裂和形成。它使用弯箭头 (↷) 显示电子的移动，箭头尾部始于电子来源（如孤对电子或化学键），箭头指向电子的去向。机理涉及中间体（如碳正离子或自由基）和过渡态。
Curly arrow conventions: a full arrow head (↷) represents movement of a pair of electrons; a ‘fish-hook’ half-arrow is used for single electron movement in radical processes. In CCEA, you must be able to draw mechanisms with correct arrow placement and formal charges.
弯箭头惯例：完整的箭头 (↷) 表示一对电子的移动；“鱼钩”半箭头用于自由基过程中单电子的移动。在 CCEA 考试中，你必须能够正确地画出箭头位置和形式电荷。
2. Free Radical Substitution | 自由基取代
This mechanism occurs with alkanes and halogens in the presence of UV light. For example, the chlorination of methane: CH₄ + Cl₂ → CH₃Cl + HCl. It proceeds via three stages: initiation, propagation, and termination.
该机理发生在烷烃和卤素在紫外光照射下。例如，甲烷的氯化：CH₄ + Cl₂ → CH₃Cl + HCl。它通过三个阶段进行：引发、增长和终止。
Initiation: UV light breaks the Cl–Cl bond homolytically, producing two chlorine radicals. Cl–Cl ↷ 2 Cl• (each chlorine atom has an unpaired electron).
引发：紫外光使 Cl-Cl 键均裂，产生两个氯自由基。Cl-Cl → 2 Cl• (每个氯原子有一个未配对电子)。
Propagation (two steps): (i) Cl• + CH₄ → •CH₃ + HCl; (ii) •CH₃ + Cl₂ → CH₃Cl + Cl•. The Cl• radical is regenerated, sustaining the chain reaction.
增长（两步）：(i) Cl• + CH₄ → •CH₃ + HCl; (ii) •CH₃ + Cl₂ → CH₃Cl + Cl•。氯自由基再次生成，维持链反应。
Termination: any two radicals combine to form a stable molecule, e.g., Cl• + Cl• → Cl₂, •CH₃ + •CH₃ → C₂H₆. Write all possible combinations.
终止：任意两个自由基结合形成稳定分子，例如 Cl• + Cl• → Cl₂, •CH₃ + •CH₃ → C₂H₆。写出所有可能的组合。
3. Electrophilic Addition | 亲电加成
Alkenes undergo electrophilic addition because the electron-rich π-bond attracts electrophiles. Common reagents: Br₂, HBr, H₂SO₄, and H₂O with acid catalyst.
烯烃发生亲电加成，因为富电子的π键吸引亲电试剂。常见试剂：Br₂、HBr、H₂SO₄ 和酸催化下的水。
Mechanism for addition of HBr to ethene: (i) The π electrons attack the partially positive hydrogen in HBr, forming a carbocation and releasing Br⁻. C₂H₄ + H–Br → CH₃–CH₂⁺ + Br⁻. (ii) Bromide ion attacks the carbocation to form bromoethane. CH₃–CH₂⁺ + Br⁻ → CH₃CH₂Br. Markownikoff’s rule applies for unsymmetrical alkenes: the more stable carbocation intermediate is formed.
溴化氢与乙烯加成的机理：(i) π电子进攻 HBr 中部分带正电的氢，形成碳正离子并释放 Br⁻。C₂H₄ + H-Br → CH₃CH₂⁺ + Br⁻。(ii) 溴离子进攻碳正离子生成溴乙烷。对于不对称烯烃，遵循马氏规则：形成更稳定的碳正离子中间体。
Addition of Br₂: Br₂ is polarised to Brδ⁺–Brδ⁻. The π-bond attacks Brδ⁺, forming a cyclic bromonium ion (three-membered ring) and Br⁻. Then Br⁻ attacks from the opposite side, giving trans addition.
溴的加成：Br₂ 极化为 Brδ⁺–Brδ⁻。π键进攻 Brδ⁺，形成环状溴鎓离子（三元环）和 Br⁻。然后 Br⁻ 从背面进攻，得到反式加成。
4. Nucleophilic Substitution | 亲核取代
Haloalkanes undergo nucleophilic substitution because the polar C–X bond has a δ+ carbon, making it susceptible to nucleophilic attack. CCEA expects knowledge of SN1 and SN2 mechanisms for tertiary and primary haloalkanes, respectively.
卤代烷发生亲核取代，因为极性 C-X 键的碳带 δ+，易受亲核试剂进攻。CCEA 要求掌握叔卤代烷的 SN1 机理和伯卤代烷的 SN2 机理。
SN2 (bimolecular): one step. The nucleophile attacks the carbon from the opposite side of the halogen, forming a transition state with a five-coordinated carbon. Simultaneously, the C–X bond breaks. Example: OH⁻ + CH₃Br → CH₃OH + Br⁻. Rate = k[RX][Nu⁻]. Inversion of configuration (Walden inversion) occurs.
SN2（双分子）：一步。亲核试剂从卤素的反面进攻碳，形成五配位碳的过渡态，同时 C-X 键断裂。例如：OH⁻ + CH₃Br → CH₃OH + Br⁻。速率 = k[RX][Nu⁻]。发生构型翻转（瓦尔登翻转）。
SN1 (unimolecular): two steps. Step 1: slow, heterolytic bond breaking forms a carbocation intermediate. (CH₃)₃C–Br → (CH₃)₃C⁺ + Br⁻. Step 2: fast, nucleophile attacks the carbocation. (CH₃)₃C⁺ + OH⁻ → (CH₃)₃C–OH. Rate depends only on [RX]. Racemisation can occur due to planar carbocation.
SN1（单分子）：两步。第一步：慢，键异裂生成碳正离子中间体。(CH₃)₃C-Br → (CH₃)₃C⁺ + Br⁻。第二步：快，亲核试剂进攻碳正离子。(CH₃)₃C⁺ + OH⁻ → (CH₃)₃C-OH。速率仅取决于 [RX]。因碳正离子平面构型可发生外消旋化。
Common nucleophiles: OH⁻, CN⁻, NH₃ (to form amines). For ammonia, product is a primary amine, but further substitution can occur.
常见亲核试剂：OH⁻、CN⁻、NH₃（生成胺）。对于氨，产物为伯胺，但可发生进一步取代。
5. Elimination Reactions | 消去反应
Haloalkanes can also undergo elimination to form alkenes when treated with hot, ethanolic KOH. This competes with substitution. The mechanism is E2 for primary haloalkanes and E1 for tertiary.
卤代烷在热的乙醇 KOH 溶液中也可发生消去反应生成烯烃，与取代竞争。伯卤代烷主要为 E2 机理，叔卤代烷为 E1。
E2 mechanism: base removes a β
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