📚 A-Level Chemistry Unit 4 Jan20: Mastering Reaction Mechanisms | A-Level 化学 Unit 4 2020年1月：精通反应机理
Reaction mechanisms are the step-by-step pathways by which chemical reactions occur. In A-Level Chemistry Unit 4 (January 2020 paper), understanding mechanisms is essential for explaining organic transformations, predicting products, and linking kinetics to molecular events. This article revises the key mechanisms tested in the exam and provides a bilingual revision guide.
反应机理是化学反应发生的逐步过程。在 A-Level 化学 Unit 4（2020年1月试卷）中，理解机理对于解释有机转化、预测产物以及将动力学与分子事件联系起来至关重要。本文回顾考试中涉及的关键机理，并提供双语复习指南。
1. What is a Reaction Mechanism? | 什么是反应机理？
A reaction mechanism describes the detailed sequence of bond making and bond breaking steps that convert reactants into products. Each step is called an elementary step, and the overall mechanism must be consistent with the rate law and any observed intermediates.
反应机理描述了将反应物转化为产物的一系列成键和断键的详细步骤。每一步称为基元步骤，整个机理必须与速率方程和观察到的任何中间体一致。
Mechanisms are commonly represented using curly arrows, which show the movement of electron pairs from a nucleophilic (electron-rich) site to an electrophilic (electron-deficient) site. Being able to draw and interpret these arrows is a core skill in Unit 4.
机理通常使用弯箭头表示，弯箭头显示电子对从亲核（富电子）位点移向亲电（缺电子）位点的过程。在 Unit 4 中，绘制和解读这些箭头是核心技能。
2. Curly Arrows and Electron Movement | 弯箭头与电子移动
Curly arrows are a universal notation in organic chemistry for tracking electron pairs. A full curly arrow (double-barbed) indicates the donation of a pair of electrons, while a single-barbed ‘fishhook’ arrow shows the movement of a single electron, which appears only in radical mechanisms.
弯箭头是有机化学中追踪电子对的通用符号。全弯箭头（双钩）表示提供一对电子，而半箭头（单钩鱼钩箭头）表示单个电子的移动，这仅出现在自由基机理中。
In A-Level mechanisms, you are expected to use full curly arrows for polar reactions. The arrow must start from the electron pair (lone pair or bond) and point towards the positive centre or atom that will accept the pair. For example, when a nucleophile attacks a carbonyl carbon, the arrow starts at the nucleophile’s lone pair and ends at the carbonyl carbon, pushing the π-electrons onto oxygen.
在 A-Level 机理中，要求对极性反应使用全弯箭头。箭头必须从电子对（孤对电子或键）开始，指向正电中心或接受电子对的原子。例如，当亲核试剂进攻羰基碳时，箭头从亲核试剂的孤对电子开始，指向羰基碳，并将 π 电子推到氧上。
3. Free Radical Substitution | 自由基取代
Free radical substitution occurs when alkanes react with halogens in the presence of ultraviolet light. The mechanism involves three stages: initiation, propagation, and termination.
自由基取代发生在烷烃在紫外光下与卤素反应时。该机理包括三个阶段：引发、传递和终止。
Initiation: The halogen molecule undergoes homolytic fission, forming two halogen radicals. For chlorine, Cl₂ → 2 Cl•. The UV light provides the energy to break the covalent bond symmetrically.
引发：卤素分子发生均裂，生成两个卤素自由基。对于氯气，Cl₂ → 2 Cl•。紫外光提供能量使共价键对称断裂。
Propagation: A chlorine radical abstracts a hydrogen atom from methane to produce hydrogen chloride and a methyl radical (CH₄ + Cl• → •CH₃ + HCl). The methyl radical then reacts with another chlorine molecule to form chloromethane and regenerate the chlorine radical (•CH₃ + Cl₂ → CH₃Cl + Cl•). This chain reaction continues and leads to multiple substitution products.
传递：氯自由基从甲烷中夺取一个氢原子，生成氯化氢和甲基自由基（CH₄ + Cl• → •CH₃ + HCl）。然后甲基自由基与另一个氯分子反应，生成氯甲烷并再生氯自由基（•CH₃ + Cl₂ → CH₃Cl + Cl•）。该链反应持续进行，并可导致多取代产物。
Termination: Two radicals combine to form a stable molecule, ending the chain sequence. Possible terminations include Cl• + Cl• → Cl₂, •CH₃ + Cl• → CH₃Cl, and •CH₃ + •CH₃ → CH₃CH₃.
终止：两个自由基结合形成稳定分子，结束链式序列。可能的终止步骤包括 Cl• + Cl• → Cl₂，•CH₃ + Cl• → CH₃Cl 和 •CH₃ + •CH₃ → CH₃CH₃。
4. Electrophilic Addition to Alkenes | 烯烃的亲电加成
Alkenes undergo electrophilic addition because the electron-rich π bond can attack an electrophile. A typical example is the addition of hydrogen bromide to propene. The mechanism begins with the π electrons of the double bond moving to form a bond with the electrophilic hydrogen of HBr, generating a carbocation intermediate and a bromide ion.
烯烃因富电子的 π 键能进攻亲电试剂而发生亲电加成。一个典型例子是丙烯与溴化氢的加成。该机理始于双键的 π 电子向 HBr 中亲电的氢移动并成键，生成碳正离子中间体和溴离子。
The most stable carbocation is formed preferentially. For propene, the secondary carbocation (CH₃CH⁺CH₃) is more stable than the primary carbocation (CH₃CH₂CH₂⁺), leading to 2-bromopropane as the major product. This is known as Markovnikov’s rule: the hydrogen adds to the carbon with more hydrogen atoms.
优先生成最稳定的碳正离子。对于丙烯，仲碳正离子（CH₃CH⁺CH₃）比伯碳正离子（CH₃CH₂CH₂⁺）更稳定，因此主要产物是 2-溴丙烷。这称为马氏规则：氢加到含氢较多的碳上。
In the second step, the bromide ion attacks the carbocation, forming the final haloalkane. With symmetrical bromine (Br₂), addition yields a vicinal dibromide via a cyclic bromonium ion intermediate, which accounts for the anti stereochemistry.
在第二步中，溴离子进攻碳正离子，生成最终的卤代烷。当使用对称的溴（Br₂）时，加成通过环状溴鎓离子中间体生成邻二溴化物，解释了反式立体化学。
5. Electrophilic Substitution of Benzene | 苯的亲电取代
Benzene, due to its delocalised π system, prefers substitution over addition to retain aromaticity. In nitration, a mixture of concentrated nitric and sulfuric acids generates the nitronium ion (NO₂⁺) as the electrophile.
苯由于其离域 π 系统，倾向于取代而非加成以保持芳香性。在硝化反应中，浓硝酸和浓硫酸的混合物生成亲电试剂硝鎓离子（NO₂⁺）。
HNO₃ + H₂SO₄ → NO₂⁺ + HSO₄⁻ + H₂O. The nitronium ion is attacked by the benzene ring, forming a delocalised carbocation intermediate called the sigma complex or Wheland intermediate. Loss of a proton then restores aromaticity, giving nitrobenzene (C₆H₅NO₂).
HNO₃ + H₂SO₄ → NO₂⁺ + HSO₄⁻ + H₂O。苯环进攻硝鎓离子，形成离域碳正离子中间体，称为 σ 配合物或威兰德中间体。随后失去一个质子恢复芳香性，得到硝基苯（C₆H₅NO₂）。
The same pattern applies to halogenation (using a halogen carrier like FeBr₃ to generate Br⁺) and Friedel‑Crafts alkylation/acylation. The rate-determining step is the formation of the sigma complex.
同样的模式适用于卤代反应（使用 FeBr₃ 等卤素载体生成 Br⁺）以及傅-克烷基化/酰基化反应。速率决定步骤是 σ 配合物的形成。
6. Nucleophilic Substitution: Sₙ1 and Sₙ2 | 亲核取代：Sₙ1 与 Sₙ2
Nucleophilic substitution reactions involve the replacement of a leaving group (e.g. halide) by a nucleophile. The two principal mechanisms are Sₙ1 (unimolecular) and Sₙ2 (bimolecular), which differ in kinetics, stereochemistry, and substrate preference.
亲核取代反应涉及亲核试剂取代离去基团（如卤离子）。两种主要机理是 Sₙ1（单分子）和 Sₙ2（双分子），它们在动力学、立体化学和底物偏好上有所不同。
The table below compares key features of Sₙ1 and Sₙ2 mechanisms.
下表比较了 Sₙ1 和 Sₙ2 机理的关键特征。
| Feature | Sₙ1 | Sₙ2 |
|---|---|---|
| Rate equation | Rate = k[RX] | Rate = k[RX][Nu] |
| Stereochemistry | Racemisation via planar carbocation | Inversion of configuration |
| Intermediate | Carbocation | Pentacoordinate transition state |
| Substrate reactivity | 3° > 2° > 1° (stabilised carbocation) | 1° > 2° > 3° (steric hindrance) |
In Sₙ1, the leaving group departs first to form a planar carbocation, which is then attacked by the nucleophile from either side, leading to racemisation. In Sₙ2, the nucleophile attacks the carbon from the opposite side of the leaving group in a concerted step, resulting in complete inversion (Walden inversion).
在 Sₙ1 中，离去基团首先离去形成平面碳正离子，然后亲核试剂从任一侧进攻，导致外消旋化。在 Sₙ2 中，亲核试剂在协同步骤中从离去基团的反面进攻碳，导致完全翻转（瓦尔登翻转）。
7. Nucleophilic Addition of Carbonyls | 羰基的亲核加成
Carbonyl compounds (aldehydes and ketones) are susceptible to nucleophilic attack at the electrophilic carbon because of the polar C=O bond. A typical reaction is the addition of cyanide ions to form cyanohydrins.
羰基化合物（醛和酮）由于极性的 C=O 键，容易在亲电碳上发生亲核进攻。典型的反应是氰离子加成生成氰醇。
The mechanism: the nucleophile CN⁻ attacks the carbonyl carbon, pushing the π electrons onto oxygen and forming an alkoxide ion. This intermediate is then protonated by a weak acid (e.g. HCN or water) to give the cyanohydrin. For propanone: CH₃COCH₃ + HCN ⇌ CH₃C(OH)(CN)CH₃.
机理：亲核试剂 CN⁻ 进攻羰基碳，将 π 电子推到氧上，形成烷氧负离子中间体。然后该中间体被弱酸（如 HCN 或水）质子化，得到氰醇。对于丙酮：CH₃COCH₃ + HCN ⇌ CH₃C(OH)(CN)CH₃。
This addition is reversible under basic conditions. Other nucleophiles, such as hydride (H⁻ from NaBH₄) and amines, follow a similar pattern. The rate is often increased by acid catalysis, which protonates the carbonyl oxygen and makes the carbon more electrophilic.
该加成在碱性条件下是可逆的。其他亲核试剂，如氢负离子（来自 NaBH₄ 的 H⁻）和胺，遵循相似的模式。酸催化通常可提高速率，酸使羰基氧质子化，使碳更具亲电性。
8. Elimination Reactions | 消除反应
Elimination reactions produce alkenes by removing atoms or groups from adjacent carbon atoms. For haloalkanes, hot ethanolic potassium hydroxide promotes dehydrohalogenation via an E2 mechanism.
消除反应通过从相邻碳原子上除去原子或基团来生成烯烃。对于卤代烷，热的氢氧化钾乙醇溶液通过 E2 机理促进脱卤化氢。
In E2, the hydroxide ion acts as a base and abstracts a β‑hydrogen at the same time as the leaving group departs. The electron pair from the C–H bond moves to form a π bond. This concerted process gives anti-periplanar stereochemistry. For bromoethane: CH₃CH₂Br + OH⁻ → CH₂=CH₂ + Br⁻ + H₂O.
在 E2 中，氢氧根离子作为碱，在离去基团离去的同时夺取 β-氢。C–
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