📚 Electrophilic Addition | 亲电加成考点精讲
Electrophilic addition is a fundamental reaction mechanism in organic chemistry, especially crucial for understanding the reactivity of alkenes. This topic frequently appears in CCEA IGCSE Chemistry, testing students on mechanism, conditions, and product prediction. Mastering electrophilic addition will not only secure marks in organic chemistry but also lay a solid foundation for further studies at A-Level.
亲电加成是有机化学中的一个基本反应机理，对于理解烯烃的反应活性尤为重要。该专题在 CCEA IGCSE 化学中经常出现，考查学生对机理、反应条件和产物预测的掌握。熟练掌握亲电加成不仅能保证有机化学部分的得分，也为 A-Level 的进一步学习打下坚实基础。
1. What is Electrophilic Addition? | 什么是亲电加成？
Electrophilic addition refers to a reaction in which an electrophile – a species that accepts an electron pair – attacks an electron-rich unsaturated centre, such as the carbon-carbon double bond (C=C). The pi electrons of the double bond are especially attractive to electrophiles, leading to the addition of atoms or groups across the bond.
亲电加成是指亲电试剂（能接受电子对的物种）进攻富电子的不饱和中心（如碳碳双键）的反应。双键中的π电子对亲电试剂具有特别的吸引力，从而使得原子或基团加成到双键两端。
2. Structure of Alkenes and the C=C Bond | 烯烃的结构与碳碳双键
To understand electrophilic addition, students must be familiar with the bonding in alkenes. A C=C double bond consists of one sigma (σ) bond and one pi (π) bond. The σ bond lies directly between the two carbon nuclei, while the π bond results from sideways overlap of p orbitals, placing electron density above and below the plane of the molecule. This exposed π electron cloud is highly susceptible to attack by electrophiles.
要理解亲电加成，学生必须熟悉烯烃的成键方式。碳碳双键由一个σ键和一个π键组成。σ键位于两个碳原子核之间，而π键由p轨道的侧向重叠形成，使得电子云分布在分子平面的上方和下方。这团暴露的π电子云极易受到亲电试剂的进攻。
3. The General Mechanism: Heterolytic Fission and Carbocations | 通用机理：异裂与碳正离子
The electrophilic addition of hydrogen halides to alkenes illustrates the key steps. Consider hydrogen bromide (HBr). The mechanism begins with heterolytic fission: the H–Br bond breaks unevenly, producing a hydrogen ion (H⁺) and a bromide ion (Br⁻). The hydrogen ion acts as an electrophile.
卤化氢与烯烃的亲电加成反应体现出关键步骤。以溴化氢(HBr)为例。机理从异裂开始：H–Br键不均匀断裂，产生氢离子(H⁺)和溴离子(Br⁻)。氢离子作为亲电试剂。
Step 1: The electrophile H⁺ attacks the C=C double bond, using the π electron pair to form a new C–H bond. This leaves the adjacent carbon electron-deficient, resulting in a carbocation intermediate.
步骤1：亲电试剂 H⁺ 进攻C=C双键，利用π电子对形成新的C–H键。这使得邻近的碳原子缺电子，生成碳正离子中间体。
Step 2: The bromide ion (Br⁻) now acts as a nucleophile, donating a pair of electrons to the positively charged carbon of the carbocation, completing the addition and forming the final saturated product.
步骤2：溴离子(Br⁻)此时充当亲核试剂，提供一对电子给碳正离子上带正电的碳原子，完成加成反应，生成最终饱和产物。
General equation: C₂H₄ + HBr → CH₃CH₂Br
注：在CCEA考试中，需要能够描述异裂并指出碳正离子中间体，但无需使用弯曲箭头表示电子移动。
4. Addition of Halogens: Bromine Water Test | 与卤素加成：溴水测试
When an alkene is shaken with orange-brown bromine water, the solution rapidly decolourises. This is an electrophilic addition reaction, often used as a chemical test for unsaturation. The mechanism proceeds via heterolytic fission of Br₂ to Br⁺ and Br⁻, followed by electrophilic attack and carbocation formation, and then nucleophilic capture by Br⁻.
当烯烃与橙褐色的溴水一同振荡时，溶液会迅速褪色。这是一种亲电加成反应，常被用作不饱和烃的化学检验。其机理包括：Br₂ 异裂为 Br⁺ 和 Br⁻，然后亲电进攻形成碳正离子，接着 Br⁻ 进行亲核捕获。
The colour change observed is from orange-brown to colourless. No catalyst or special conditions are required; the reaction occurs readily at room temperature. It is important to state the colour change explicitly in exam answers.
观察到的颜色变化是由橙褐色变为无色。该反应不需要催化剂或特殊条件；室温下即可迅速发生。考试中必须明确指出颜色变化。
C₂H₄ + Br₂ → CH₂BrCH₂Br
5. Addition of Hydrogen Halides: Markovnikov’s Rule | 与卤化氢加成：马氏规则
When an unsymmetrical alkene (such as propene, CH₃CH=CH₂) reacts with a hydrogen halide like HBr, two products are theoretically possible. However, one isomer is formed in much greater yield. Markovnikov’s rule predicts the major product: the hydrogen atom of H–X attaches to the carbon atom that already has the greater number of hydrogen atoms directly bonded to it (the less substituted carbon), while the halide attaches to the more substituted carbon.
当不对称烯烃（如丙烯 CH₃CH=CH₂）与卤化氢（如 HBr）反应时，理论上有两种可能的产物。但其中一种异构体的产率远高于另一种。马氏规则可以预测主要产物：H–X 的氢原子加在直接连接氢原子数目较多的碳原子上（即取代较少的碳），而卤素原子则加在取代较多的碳上。
For example, the addition of HBr to propene gives mainly 2-bromopropane, not 1-bromopropane. This is because the reaction proceeds via a carbocation intermediate; the more stable carbocation is preferentially formed. This mechanistic basis is examined in depth by CCEA.
例如，丙烯与 HBr 加成的主要产物是 2-溴丙烷，而不是 1-溴丙烷。这是因为反应经历了碳正离子中间体；更稳定的碳正离子更容易形成。CCEA 会对这一机理基础进行深入考查。
CH₃CH=CH₂ + HBr → CH₃CHBrCH₃ (major product)
6. Carbocation Stability and Product Prediction | 碳正离子稳定性与产物预测
The key to applying Markovnikov’s rule is understanding carbocation stability. Carbocations are classified as primary (1°), secondary (2°), or tertiary (3°) based on the number of carbon atoms attached to the positively charged carbon. Alkyl groups have an electron-donating effect, which stabilises the positive charge. Therefore, the stability order is: tertiary > secondary > primary > methyl. The more stable the carbocation, the more likely it is to form, and thus the major product derives from the most stable carbocation intermediate.
应用马氏规则的关键在于理解碳正离子的稳定性。碳正离子根据带正电的碳原子上连接的碳原子数目分为一级(1°)、二级(2°)或三级(3°)。烷基具有供电效应，能够稳定正电荷。因此稳定性顺序为：三级 > 二级 > 一级 > 甲基。碳正离子越稳定就越容易生成，因此主要产物来自于最稳定的碳正离子中间体。
When predicting the product for an unsymmetrical alkene, draw out the two possible carbocations that could arise from adding H⁺ to either end of the double bond. Compare their stabilities and select the major product by trapping the more stable carbocation with the halide ion.
在预测不对称烯烃的产物时，画出 H⁺ 分别加到双键两端时可能生成的两种碳正离子，比较它们的稳定性，然后用卤离子捕获较稳定的碳正离子，得出主要产物。
7. Addition of Sulfuric Acid: Hydration via Alkyl Hydrogensulfate | 与硫酸加成：通过硫酸氢酯的水合
Alkenes react with cold concentrated sulfuric acid to form alkyl hydrogensulfates. For example, ethene yields ethyl hydrogensulfate. This is another electrophilic addition: the electrophile H⁺ (from H₂SO₄) adds to the C=C bond, generating a carbocation, which is then attacked by the hydrogensulfate ion (HSO₄⁻).
烯烃与冷的浓硫酸反应生成硫酸氢酯。例如，乙烯生成硫酸氢乙酯。这是另一种亲电加成反应：来自硫酸的亲电试剂 H⁺ 加成到 C=C 键上产生碳正离子，随后碳正离子被硫酸氢根离子(HSO₄⁻)进攻。
Subsequent warming with water hydrolyses the alkyl hydrogensulfate to give the corresponding alcohol and regenerate sulfuric acid. This two-step process is the indirect hydration of alkenes.
随后与水一起加热，硫酸氢酯水解生成相应的醇并再生硫酸。这个两步过程是烯烃的间接水合法。
C₂H₄ + H₂SO₄ → CH₃CH₂OSO₃H
CH₃CH₂OSO₃H + H₂O → C₂H₅OH + H₂SO₄
8. Direct Hydration of Ethene to Ethanol | 乙烯直接水合制乙醇
Industrially, ethanol can be produced directly from ethene and steam. The electrophilic addition requires a phosphoric acid (H₃PO₄) catalyst, high temperature (approximately 300 °C), and high pressure (60–70 atm). The reaction is reversible, so ethanol vapour is constantly removed to shift the equilibrium to the right.
工业上，乙醇可以由乙烯和水蒸气直接反应制得。这种亲电加成需要磷酸(H₃PO₄)催化剂、高温（约 300 °C）和高压（60–70 atm）。该反应是可逆的，因此需不断移除乙醇蒸气以使平衡向右移动。
The mechanism still involves electrophilic attack by a proton (H⁺) from the acid catalyst, carbocation formation, and nucleophilic attack by water. The regenerated H⁺ ensures catalytic action.
其机理仍然包含酸催化剂产生的质子(H⁺)进行亲电进攻、形成碳正离子，然后水分子进行亲核进攻。再生的 H⁺ 使催化剂得以循环。
C₂H₄ + H₂O ⇌ C₂H₅OH
9. Summary of Electrophilic Addition Reactions | 亲电加成反应总结
The table below summarises the key addition reactions of alkenes required for CCEA IGCSE Chemistry:
下表总结了 CCEA IGCSE 化学要求的烯烃关键加成反应：
| Reagent | Conditions | Product Type | Example |
|---|---|---|---|
| Br₂ (or Cl₂) | Room temp, no catalyst | dihalogenoalkane | C₂H₄ + Br₂ → CH₂BrCH₂Br |
| HBr or HCl | Room temp | halogenoalkane | CH₂=CH₂ + HBr → CH₃CH₂Br |
| Cold conc. H₂SO₄ | Cold, then warm with water | alcohol (indirect hydration) | C₂H₄ + H₂SO₄ → CH₃CH₂OSO₃H; then hydrolysis to C₂H₅OH |
| Steam (H₂O) | H₃PO₄ catalyst, 300 °C, 60–70 atm | alcohol (direct hydration) | C₂H₄ + H₂O ⇌ C₂H₅OH |
Note: For unsymmetrical alkenes reacting with HX or H₂SO₄ (and subsequent water), Markovnikov’s rule must be applied to determine the major product.
注意：对于不对称烯烃与 HX 或 H₂SO₄（随后加水）的反应，必须应用马氏规则来确定主要产物。
10. Exam Tips and Common Mistakes | 考试技巧与常见错误
-
Always state the full colour change when describing the bromine water test: “from orange-brown to colourless”. Saying “decolourised” alone may not gain full marks.
描述溴水测试时，务必写出完整的颜色变化：”由橙褐色变为无色”。仅说”褪色”可能无法得到满分。
-
When drawing or describing the mechanism for HX addition, clearly show heterolytic fission and label the carbocation as an intermediate. No curly arrows are required at IGCSE, but the sequence of bond breaking and forming must be logical.
在绘制或描述 HX 加成机理时，要清晰表示异裂并标出碳正离子中间体。IGCSE 不要求画箭头，但键的断裂与形成顺序必须合理。
-
For Markovnikov’s rule, always justify your answer with reference to carbocation stability – e.g. “the secondary carbocation is more stable than the primary”.
在应用马氏规则时，务必引用碳正离子稳定性进行解释 – 例如”二级碳正离子比一级碳正离子更稳定”。
-
Distinguish between direct and indirect hydration: indirect hydration uses concentrated sulfuric acid followed by water, while direct hydration uses steam with a phosphoric acid catalyst at high temperature and pressure.
区分直接水合与间接水合：间接水合先使用浓硫酸再加水，直接水合使用水蒸气与磷酸催化剂在高温高压下反应。
-
When writing equations, ensure that carbon skeletons and hydrogen counts are correct, especially after applying Markovnikov’s rule to unsymmetrical alkenes.
书写方程式时，要保证碳骨架和氢原子数目正确，尤其是对不对称烯烃应用马氏规则后。
-
Remember that the electrophile in HX addition is H⁺, not the whole HX molecule. This helps in understanding why the mechanism proceeds via a carbocation.
记住
Published by TutorHao | IGCSE Chemistry Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导