📚 Electrophilic Addition in A-Level AQA Chemistry | A-Level AQA 化学：亲电加成 考点精讲
Electrophilic addition is the characteristic reaction of alkenes and a cornerstone of organic chemistry in AQA A-Level. This article covers the key mechanistic details, the role of the π bond, how to draw curly arrows, carbocation stability, Markovnikov’s rule, and the evidence for the two‑step mechanism. Work through each section to master this topic fully.
亲电加成是烯烃的特征反应，也是 AQA A-Level 有机化学的核心内容。本文涵盖关键机理细节、π 键的作用、如何画弯箭头、碳正离子稳定性、马氏规则以及两步机理的证据。逐节学习，彻底掌握这一考点。
1. Why Alkenes Undergo Electrophilic Addition | 烯烃为何发生亲电加成
Alkenes contain a C=C double bond made of a σ bond and a π bond. The π bond sits above and below the plane of the molecule, with its electrons exposed. These electrons form a region of high electron density that attracts electrophiles – electron‑deficient species that seek an electron pair.
烯烃含有一个由 σ 键和 π 键组成的 C=C 双键。π 键位于分子平面的上方和下方，电子暴露在外。这些电子形成一个高电子密度区域，吸引亲电试剂——即缺乏电子并寻求电子对的物种。
The π bond is weaker than the σ bond and breaks relatively easily during the reaction. This allows the two carbon atoms to form new σ bonds with the attacking electrophile and a nucleophile, resulting in an addition product where the double bond becomes a single bond.
π 键比 σ 键弱，反应时相对容易断裂。这使得两个碳原子能与进攻的亲电试剂和亲核试剂形成新的 σ 键，产生加成产物，双键变为单键。
2. Curly Arrows and Mechanism Drawing | 弯箭头与机理绘制
In AQA exams you must use curly arrows to show electron movement. A curly arrow starts from an electron‑rich site (a bond or a lone pair) and points to an electron‑poor site. For electrophilic addition, the first arrow goes from the centre of the π bond towards the electrophile (e.g. H⁺ or Brᵟ⁺).
在 AQA 考试中，你必须使用弯箭头表示电子移动。弯箭头从富电子位点（键或孤对电子）出发，指向缺电子位点。对于亲电加成，第一支箭头从 π 键中央指向亲电试剂（如 H⁺ 或 Brᵟ⁺）。
The second curly arrow shows the breaking of a bond in the electrophile (e.g. H–Br bond breaking onto the Br atom). Finally, a third arrow is used when the negative part of the reagent (Br⁻) donates its lone pair to the carbocation to form the final product.
第二支弯箭头表示亲电试剂中键的断裂（例如 H–Br 键的电子对归 Br 原子）。最后，第三支箭头用于表示试剂的负性部分（Br⁻）将其孤对电子提供给碳正离子，形成最终产物。
Example – Mechanism of ethene with HBr:
示例 – 乙烯与 HBr 的机理：
- Arrow 1: from C=C π bond to the H atom of H–Br.
弯箭头1：从 C=C π 键指向 H–Br 的 H 原子。 - Arrow 2: from the H–Br bond to the Br atom, showing heterolytic fission.
弯箭头2：从 H–Br 键指向 Br 原子，表示异裂。 - Intermediate: a carbocation CH₃–C⁺H₂ and Br⁻.
中间体：碳正离子 CH₃–C⁺H₂ 和 Br⁻。 - Arrow 3: from a lone pair on Br⁻ to the positively charged carbon of the carbocation.
弯箭头3：从 Br⁻ 上的一对孤对电子指向碳正离子上带正电的碳。
3. Heterolytic Fission and Polarisation | 异裂与极化
The electrophile must be polarised or become polarised before attacking. For a hydrogen halide H–X, the bond is polar because X is more electronegative than H. The δ+ hydrogen acts as the electrophile. Bromine, Br₂, is non‑polar, but as it approaches the electron‑rich π bond, the bromine molecule becomes polarised: Brᵟ⁺–Brᵟ⁻, enabling electrophilic attack.
亲电试剂在进攻前必须是极化的或被极化。对于卤化氢 H–X，由于 X 的电负性大于 H，键有极性。δ+ 的氢充当亲电试剂。溴 Br₂ 是非极性的，但当它靠近富电子的 π 键时，溴分子被极化：Brᵟ⁺–Brᵟ⁻，进而发生亲电进攻。
Heterolytic fission is the breaking of a covalent bond where both electrons from the bond go to the more electronegative atom. This generates ions: for H–Br → H⁺ + Br⁻. This step is crucial for generating the electrophile in some cases (e.g. with AlBr₃ catalyst).
异裂是指共价键断裂时，两个成键电子都归于电负性更大的原子，从而生成离子：H–Br → H⁺ + Br⁻。在某些情况下（例如使用 AlBr₃ 催化剂），此步骤对产生亲电试剂至关重要。
4. Carbocation Formation and Stability | 碳正离子的生成与稳定性
The intermediate formed after the electrophile adds to one carbon of the double bond is a carbocation – a carbon atom with only three bonds and a positive charge. The stability of the carbocation dictates the outcome of the reaction when more than one product is possible.
亲电试剂加成到双键的一个碳上后，形成的中间体是碳正离子——一个只有三个键且带正电荷的碳原子。当可能有多个产物时，碳正离子的稳定性决定反应的结果。
Carbocation stability order: tertiary (3°) > secondary (2°) > primary (1°) > methyl. This is because alkyl groups are electron‑donating and spread the positive charge through positive inductive effect and hyperconjugation.
碳正离子稳定性顺序：三级 (3°) > 二级 (2°) > 一级 (1°) > 甲基。这是因为烷基具有供电性，通过正诱导效应和超共轭作用分散正电荷。
| Type 类型 | Formula 示例 | Stability 稳定性 |
|---|---|---|
| 3° tertiary | (CH₃)₃C⁺ | Most stable |
| 2° secondary | (CH₃)₂CH⁺ | Medium |
| 1° primary | CH₃CH₂⁺ | Least stable among common ones |
The greater the stability of the carbocation, the lower its activation energy of formation and the more likely it is to form. Therefore, when an unsymmetrical alkene reacts with an electrophile, the more stable carbocation intermediate is preferred.
碳正离子越稳定，其形成的活化能越低，越容易生成。因此，当不对称烯烃与亲电试剂反应时，倾向于形成更稳定的碳正离子中间体。
5. Markovnikov’s Rule (Regioselectivity) | 马尔科夫尼科夫规则（区域选择性）
When hydrogen halides add to unsymmetrical alkenes, two products are mathematically possible, but one predominates. Markovnikov’s rule states that the hydrogen atom (the electrophile) attaches to the carbon that already has the greater number of hydrogen atoms, i.e. the less substituted carbon. Another way to put it: the halogen attaches to the more substituted carbon.
当卤化氢与不对称烯烃加成时，数学上可能有两种产物，但其中一种占主导。马氏规则指出，氢原子（亲电试剂）加成到原本连有较多氢原子的碳上，即取代度较低的碳。另一种表述：卤素加成到取代度较高的碳上。
This rule is a consequence of carbocation stability. The mechanism proceeds via the more stable carbocation. For example, propene (CH₃–CH=CH₂) plus HBr: the carbocation that forms is CH₃–C⁺H–CH₃ (secondary, more stable) rather than CH₃–CH₂–C⁺H₂ (primary). Therefore the major product is 2‑bromopropane, not 1‑bromopropane.
这一规则源于碳正离子的稳定性。机理经过更稳定的碳正离子进行。例如，丙烯 (CH₃–CH=CH₂) 加 HBr：形成的碳正离子是 CH₃–C⁺H–CH₃（二级，更稳定），而不是 CH₃–CH₂–C⁺H₂（一级）。因此主要产物是 2‑溴丙烷，而非 1‑溴丙烷。
6. Addition of Hydrogen Halides (HBr, HCl, HI) | 卤化氢的加成 (HBr, HCl, HI)
Hydrogen halides add rapidly to alkenes at room temperature. The reactivity order is HI > HBr > HCl, related to the bond strength: H–I is weakest and breaks most easily. The reaction is heterolytic: the π electrons attack the Hᵟ⁺, the H–X bond breaks heterolytically, and the halide ion then combines with the carbocation.
卤化氢在室温下与烯烃迅速加成。反应活性顺序为 HI > HBr > HCl，这与键能有关：H–I 键最弱，最容易断裂。反应是异裂过程：π 电子进攻 Hᵟ⁺，H–X 键异裂，卤离子随后与碳正离子结合。
These reactions are important in synthesis and also illustrate the principle of electrophilic addition. For symmetrical alkenes such as ethene, only one product forms. For unsymmetrical alkenes, the product distribution depends on the stability of the possible carbocations.
这些反应在合成中很重要，也阐明了亲电加成的原理。对于对称烯烃如乙烯，只生成一种产物。对于不对称烯烃，产物分布取决于可能碳正离子的稳定性。
7. Addition of Halogens (Br₂, Cl₂) | 卤素的加成 (Br₂, Cl₂)
Bromine and chlorine also add to alkenes via electrophilic addition. The Br₂ molecule is non‑polar, but upon approaching the π bond, an induced dipole arises: Brᵟ⁺–Brᵟ⁻. The π electrons attack the Brᵟ⁺, forming a cyclic bromonium ion (or a carbocation, depending on the level of detail required by AQA).
溴和氯也能通过亲电加成与烯烃反应。Br₂ 分子是非极性的，但当其靠近 π 键时，产生诱导偶极：Brᵟ⁺–Brᵟ⁻。π 电子进攻 Brᵟ⁺，形成环状溴鎓离子（或碳正离子，视 AQA 要求的详细程度而定）。
AQA typically accepts a mechanism via a carbocation for bromine, showing the first step: Brᵟ⁺ adding to one carbon with simultaneous Br⁻ release, followed by the bromide ion attacking the carbocation from the opposite side, leading to anti addition (not always specified at A‑Level but good to know).
AQA 通常接受通过碳正离子的溴加成机理，显示第一步：Brᵟ⁺ 加到一个碳上同时释放 Br⁻，然后溴离子从背面进攻碳正离子，导致反式加成（A-Level 不一定明确要求但了解有好处）。
This reaction is used as a test for unsaturation: bromine water (orange) turns colourless in the presence of an alkene.
该反应用作不饱和键的检验：溴水（橙色）遇烯烃褪为无色。
8. Addition of Sulfuric Acid and Hydration | 硫酸加成与水合反应
Alkenes react with cold concentrated sulfuric acid to form alkyl hydrogensulfates. The electrophile is H⁺ from H₂SO₄ (or effectively H–OSO₃H). The π electrons attack Hᵟ⁺, forming a carbocation, which is then attacked by the HSO₄⁻ ion. This is an electrophilic addition.
烯烃与冷的浓硫酸反应生成硫酸氢烷基酯。亲电试剂是来自 H₂SO₄ 的 H⁺（或实质上的 H–OSO₃H）。π 电子进攻 Hᵟ⁺，生成碳正离子，随后被 HSO₄⁻ 离子进攻。这是一个亲电加成反应。
On warming with water (hydrolysis), the alkyl hydrogensulfate gives the corresponding alcohol. This two‑step process is the industrial hydration of alkenes to produce alcohols.
加水加热（水解），硫酸氢烷基酯生成相应的醇。这一两步过程是工业上烯烃水合制醇的方法。
Overall: CH₂=CH₂ + H₂SO₄ → CH₃CH₂OSO₃H, then + H₂O → CH₃CH₂OH + H₂SO₄.
总反应：CH₂=CH₂ + H₂SO₄ → CH₃CH₂OSO₃H，然后 + H₂O → CH₃CH₂OH + H₂SO₄。
9. Evidence for the Two‑Step Mechanism | 两步机理的证据
Kinetic studies show that the electrophilic addition of HBr to ethene is second order overall: rate = k[alkene][HBr]. This suggests that both species are involved in the rate‑determining step – which is the first step, formation of the carbocation.
动力学研究表明，HBr 与乙烯的亲电加成是二级反应：速率 = k[烯烃][HBr]。这表明两种物质参与了决速步骤——即第一步，碳正离子的生成。
The second step (attack of Br⁻ on carbocation) is fast. Carbocation rearrangements, observed when possible (e.g. 3‑methyl‑1‑butene with HCl giving mainly 2‑chloro‑2‑methylbutane via a 1,2‑methyl shift), provide further evidence for a carbocation intermediate. Such rearrangements would not occur if the mechanism were concerted.
第二步（Br⁻ 进攻碳正离子）是快速的。当可能时观察到的碳正离子重排（例如 3‑甲基‑1‑丁烯与 HCl 反应，主要通过 1,2‑甲基迁移生成 2‑氯‑2‑甲基丁烷），进一步提供了碳正离子中间体的证据。如果机理是协同的，就不会发生这种重排。
Additionally, anti addition in bromination can be explained through a bridged bromonium ion (beyond AQA but supports the idea of a two‑step process).
此外，溴化反应中的反式加成可以通过桥环溴鎓离子解释（超出 AQA 范围，但支持了两步过程的理念）。
10. Key Definitions and Terminology | 关键定义与术语
- Electrophile: An electron‑pair acceptor, such as H⁺, NO₂⁺, SO₃, Brᵟ⁺. Must be attracted to a region of high electron density.
亲电试剂：电子对接受体，如 H⁺、NO₂⁺、SO₃、Brᵟ⁺。必须被高电子密度区域吸引。 - Addition reaction: A reaction where two molecules combine to form a single product, with the loss of unsaturation.
加成反应：两个分子结合成一个产物，不饱和度降低的反应。 - Carbocation: A species with a positively charged carbon atom bonded to three groups and an empty p orbital.
碳正离子：带正电荷的碳原子与三个基团键合、具有一个空 p 轨道的物种。 - Heterolytic fission: Bond breaking in which both electrons move to one atom, forming a cation and an anion.
异裂：键断裂时两个电子都归于一个原子，形成阳离子和阴离子。 - Regioselectivity: Preference for the formation of one constitutional isomer over another in an addition reaction.
区域选择性：加成反应中优先形成一种构造异构体而非另一种的偏好。
11. Common Mistakes and Exam Tips | 常见错误与考试技巧
Many students forget to draw the negative charge on the halide ion after heterolytic fission. Always show the bromide ion as Br⁻ with a full negative charge and a lone pair.
许多考生忘记在异裂后画出卤离子的负电荷。务必把溴离子画成带有完整负电荷和孤对电子的 Br⁻。
Curly arrows must start from a bond or a lone pair – not from a positive charge. The arrow from the π bond should start from the centre of the double bond, not from one carbon atom.
弯箭头必须从键或孤对电子出发——不能从正电荷出发。来自 π 键的弯箭头应从双键中央出发，而不是从某个碳原子出发。
When predicting products, always consider carbocation stability. Never write a primary carbocation if a more stable secondary or tertiary one can form through the same type of addition.
在预测产物时，始终要考虑碳正离子稳定性。如果能通过相同类型的加成形成更稳定的二级或三级碳正离子，就绝不写一级碳正离子。
Be careful with sulfuric acid‑catalysed hydration: you must show the regeneration of H₂SO₄ in the overall equation if required, but the mechanism steps only go as far as the alkyl hydrogensulfate and then hydrolysis.
注意硫酸催化的水合反应：如果要求总方程式，必须体现 H₂SO₄ 的再生，但机理步骤只需画至硫酸氢烷基酯及随后的水解。
12. Summary and Practice Suggestions | 总结与练习建议
Electrophilic addition of alkenes is a unified mechanistic theme: π bond attacks electrophile → carbocation formed → nucleophile attacks carbocation. Understanding this three‑arrow process and the factors that control regiochemistry (carbocation stability) is essential for success in AQA Chemistry.
烯烃的亲电加成是一个统一的机理主题：π 键进攻亲电试剂 → 形成碳正离子 → 亲核试剂进攻碳正离子。理解这个三箭头流程以及控制区域化学的因素（碳正离子稳定性）对 AQA 化学考试的成功至关重要。
Practise drawing mechanisms for HBr, Br₂, and H₂SO₄ with ethene, propene, and 2‑methylpropene. Be able to identify major and minor products, name them, and explain your reasoning using carbocation stability.
练习绘制乙烯、丙烯和 2‑甲基丙烯与 HBr、Br₂ 和 H₂SO₄ 反应的机理。能够鉴别主要和次要产物，命名它们，并用碳正离子稳定性解释你的推理。
Review kinetic evidence and rearrangement examples to deepen your grasp of the two‑step nature of the reaction. This will prepare you for both structured questions and synoptic essays.
复习动力学证据和重排示例，加深对反应两步性质的理解。这将为你应对结构化问题和综合论述做好准备。
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