📚 IGCSE Chemistry: Electrophilic Addition Key Points | IGCSE 化学：亲电加成 考点精讲
Electrophilic addition is a fundamental reaction type in organic chemistry, especially for alkenes. It involves the addition of an electrophile across a carbon–carbon double bond, converting unsaturated molecules into saturated ones. Mastering this topic will help you predict products, apply Markovnikov’s rule and understand key industrial processes.
亲电加成是有机化学中一类基本反应，特别适用于烯烃。它涉及亲电试剂对碳碳双键的加成，将不饱和分子转变为饱和分子。掌握本专题将有助于你预测产物、应用马氏规则并理解重要的工业过程。
1. Introduction to Electrophilic Addition | 亲电加成简介
Alkenes contain a C=C double bond, which is an area of high electron density. This makes the alkene attractive to species that seek electrons, known as electrophiles. In an electrophilic addition, the π bond breaks and two new σ bonds form, adding atoms or groups across the original double bond.
烯烃含有 C=C 双键，这是一个电子密度高的区域。这使得烯烃容易被那些寻求电子的物种（即亲电试剂）进攻。在亲电加成中，π 键断裂并形成两个新的 σ 键，原子或基团加成到原来的双键两端。
2. What Is an Electrophile? | 什么是亲电试剂？
An electrophile (electron-loving) is a species that is electron-deficient and can accept a pair of electrons to form a new covalent bond. Common electrophiles in addition reactions include H⁺ (from HX), Br⁺ (from polarised Br₂), or the partially positive end of a polar molecule like H–Br.
亲电试剂（爱电子物种）是一种缺电子的物种，可以接受一对电子以形成新的共价键。加成反应中常见的亲电试剂包括 H⁺（来自 HX）、Br⁺（来自极化的 Br₂）或极性分子（如 H–Br）中带部分正电荷的一端。
3. General Mechanism of Electrophilic Addition | 亲电加成的一般机理
The mechanism can be divided into three key stages: (1) The electrophile attacks the double bond, causing heterolytic fission of the electrophilic reagent. (2) A carbocation intermediate forms on the most stable carbon. (3) The negatively charged species (nucleophile) quickly bonds with the carbocation to complete the addition.
机理可分为三个关键阶段：(1) 亲电试剂进攻双键，导致亲电试剂的异裂。(2) 在最稳定的碳原子上形成碳正离子中间体。(3) 带负电的物种（亲核试剂）迅速与碳正离子结合，完成加成。
Example with HBr: CH₂=CH₂ + HBr → CH₃–CH₂Br
HBr 示例：CH₂=CH₂ + HBr → CH₃–CH₂Br
4. Addition of Hydrogen Halides (HX) | 卤化氢的加成
Hydrogen halides such as HCl, HBr and HI add across the double bond. The H atom acts as the electrophile. With symmetrical alkenes like ethene, only one product is possible. With unsymmetrical alkenes, two products can form, and we need Markovnikov’s rule to predict the major product.
卤化氢如 HCl、HBr 和 HI 可以加成到双键上。氢原子充当亲电试剂。对于对称烯烃（如乙烯），只可能生成一种产物。对于不对称烯烃，可能生成两种产物，我们需要用马氏规则来预测主要产物。
5. Markovnikov’s Rule | 马氏规则
Markovnikov’s rule states: when an unsymmetrical alkene reacts with a hydrogen halide, the hydrogen atom adds to the carbon that already has the greater number of hydrogen atoms. Effectively, the electrophile (H⁺) bonds to the less substituted carbon, while the nucleophile bonds to the more substituted carbon, where the carbocation is more stable.
马氏规则指出：当不对称烯烃与卤化氢反应时，氢原子加在原有氢原子较多的碳原子上。实际上，亲电试剂（H⁺）加在取代较少的碳上，而亲核试剂加在取代较多的碳上，因为那里的碳正离子更稳定。
Example: CH₃–CH=CH₂ + HBr → CH₃–CHBr–CH₃ (major) rather than CH₃–CH₂–CH₂Br
示例：CH₃–CH=CH₂ + HBr → CH₃–CHBr–CH₃（主产物）而不是 CH₃–CH₂–CH₂Br
6. Addition of Halogens (Br₂, Cl₂) | 卤素的加成
Halogens add to alkenes at room temperature, without requiring a catalyst. The reaction is useful as a test for unsaturation: bromine water (orange) is decolourised when shaken with an alkene. The mechanism involves the polarisation of the halogen molecule as it approaches the electron-rich double bond, generating a transient Br⁺ electrophile.
卤素在室温下就能与烯烃加成，无需催化剂。该反应可用于检验不饱和键：当溴水（橙色）与烯烃一同振荡时会褪色。机理涉及卤素分子靠近富电子双键时发生极化，产生瞬态的 Br⁺ 亲电试剂。
Br₂ + CH₂=CH₂ → CH₂Br–CH₂Br (addition of bromine forms 1,2-dibromoethane)
Br₂ + CH₂=CH₂ → CH₂Br–CH₂Br（溴加成生成 1,2-二溴乙烷）
7. Addition of Water: Hydration | 水的加成：水合反应
Water can add to alkenes to produce alcohols. This hydration reaction requires a strong acid catalyst (e.g., concentrated H₃PO₄ or H₂SO₄) and high temperature (around 300°C) for the direct route. Industrially, ethene is hydrated to make ethanol. The addition follows Markovnikov’s rule: the H atom joins the less substituted carbon, generating the more substituted alcohol as the major product.
水可以加成到烯烃上生成醇。这一水合反应通过直接法需要强酸催化剂（如浓磷酸或浓硫酸）和高温（约 300°C）。工业上通过乙烯水合生产乙醇。加成遵循马氏规则：氢原子加在取代较少的碳上，生成的醇中羟基连在取代较多的碳上为主产物。
CH₂=CH₂ + H₂O → CH₃CH₂OH (ethene → ethanol)
CH₂=CH₂ + H₂O → CH₃CH₂OH（乙烯 → 乙醇）
8. Addition of Hydrogen: Hydrogenation | 氢气的加成：加氢反应
Alkenes react with hydrogen gas in the presence of a metal catalyst (nickel at 150°C, or platinum/palladium at room temperature) to form alkanes. This is not strictly electrophilic addition but a catalytic hydrogenation; however, it is often taught alongside electrophilic addition. The reaction is used in the margarine industry to turn liquid vegetable oils into solid spreads.
烯烃在金属催化剂（镍，150°C；或铂/钯，室温）存在下与氢气反应生成烷烃。严格来说这不属于亲电加成而是催化加氢，但常常与亲电加成一同讲授。该反应用于人造黄油产业，将液态植物油转变为固态涂抹脂。
9. Carbocation Stability | 碳正离子稳定性
A key concept behind regioselectivity is carbocation stability. Alkyl groups are electron‑releasing, so the more alkyl groups attached to the positively charged carbon, the more stable the carbocation. The order is: tertiary (3°) > secondary (2°) > primary (1°) > methyl. This explains why in unsymmetrical additions, the nucleophile attaches to the carbon that forms the more stable carbocation intermediate.
区域选择性的关键概念是碳正离子稳定性。烷基是给电子的，因此带正电荷的碳上连接的烷基越多，碳正离子越稳定。顺序为：叔碳正离子 (3°) > 仲碳正离子 (2°) > 伯碳正离子 (1°) > 甲基。这就解释了为什么在不对称加成中，亲核试剂连接到能形成更稳定碳正离子中间体的碳原子上。
10. Key Reaction Summary Table | 关键反应总结表
| Reaction / 反应 | Reagents & Conditions / 试剂与条件 | Product / 产物 |
| Addition of HX (e.g. HBr) / 卤化氢加成 | HBr gas, room temp. / 溴化氢气体，室温 | Haloalkane / 卤代烷 |
| Halogenation (Br₂) / 卤素加成 | Br₂ (aq or neat), r.t. / 溴水或纯溴，室温 | Vicinal dihalide / 邻二卤代物 |
| Hydration / 水合 | H₂O(g), H₃PO₄ cat., 300°C / 水蒸气，磷酸催化，300°C | Alcohol / 醇 |
| Hydrogenation / 加氢 | H₂, Ni cat., 150°C / 氢气，镍催化，150°C | Alkane / 烷烃 |
11. Common Mistakes and Tips | 常见错误与应试技巧
Students often forget that the decolourisation of bromine water confirms the presence of a C=C double bond, not simply that the compound is unsaturated. Another common error is reversing the orientation when applying Markovnikov’s rule: the hydrogen goes to the carbon with more hydrogens, not less. Practice drawing curly‑arrow mechanisms where required, and always show the heterolytic fission step.
学生常忘记溴水褪色是证实存在 C=C 双键，而不仅仅是分子不饱和。另一个常见错误是在应用马氏规则时颠倒了加成方向：氢加到含氢较多的碳上，而非较少的碳上。在要求画弯曲箭头机理时务必多练习，并且永远要画出异裂这一步。
12. Summary | 总结
Electrophilic addition is the characteristic reaction of alkenes. Remember the electrophile attacks the π bond, a carbocation forms on the more stable carbon, and then the nucleophile attacks. Markovnikov’s rule and carbocation stability are your guides for predicting major products. Use the summary table to recall conditions for each specific addition reaction.
亲电加成是烯烃的特征反应。请记住：亲电试剂进攻 π 键，碳正离子在较稳定的碳上形成，然后亲核试剂进攻。马氏规则和碳正离子稳定性是你预测主要产物的指南。利用总结表回忆每种特定加成反应的条件。
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