Electrophilic Addition for IGCSE WJEC Chemistry: Key Exam Points | IGCSE WJEC 化学：亲电加成考点精讲

📚 Electrophilic Addition for IGCSE WJEC Chemistry: Key Exam Points | IGCSE WJEC 化学：亲电加成考点精讲

Electrophilic addition is a fundamental reaction type for unsaturated hydrocarbons such as alkenes. In IGCSE WJEC Chemistry, understanding the mechanism and conditions for these reactions is crucial for exam success. This article summarises the key points, common pitfalls, and essential facts you need to know.

亲电加成是不饱和烃（如烯烃）的基本反应类型。在IGCSE WJEC化学中，理解这类反应的机理和条件对考试至关重要。本文总结了核心考点、常见错误和必背知识点。

1. What is Electrophilic Addition? | 什么是亲电加成？

Electrophilic addition is a reaction in which an electron-poor species (electrophile) attacks a region of high electron density in an organic molecule, typically the carbon-carbon double bond (C=C) of an alkene. The π bond breaks, and two new sigma bonds are formed as the electrophile and a nucleophile add across the double bond.

亲电加成是一种反应，其中缺电子物种（亲电试剂）攻击有机分子中电子密度高的区域，通常是烯烃的碳碳双键（C=C）。π键断裂，亲电试剂和亲核试剂分别加到双键两侧，形成两个新的σ键。

2. Structure of Alkenes and Reactivity | 烯烃的结构与反应性

Alkenes contain at least one C=C double bond, consisting of one sigma (σ) bond and one pi (π) bond. The π bond is formed by sideways overlap of p-orbitals, resulting in a region of high electron density above and below the plane of the atoms. This electron-rich area makes alkenes susceptible to attack by electrophiles.

烯烃至少含有一个C=C双键，由一个σ键和一个π键组成。π键由p轨道侧面重叠形成，在原子平面上方和下方产生高电子密度区域。这个富电子区域使得烯烃容易被亲电试剂进攻。

3. What is an Electrophile? | 什么是亲电试剂？

An electrophile (electron-lover) is a species that is attracted to electron-rich centres and can accept a pair of electrons to form a new covalent bond. Common electrophiles in IGCSE WJEC Chemistry include bromine (Br₂), hydrogen bromide (HBr), and the H⁺ ion from acids or water under acidic conditions.

亲电试剂（亲电子体）是被富电子中心吸引并能接受一对电子形成新共价键的物种。IGCSE WJEC化学中常见的亲电试剂包括溴（Br₂）、溴化氢（HBr）以及酸性条件下从酸或水中产生的H⁺离子。

4. Addition of Bromine – Test for Unsaturation | 溴的加成——不饱和检验

When an alkene reacts with bromine dissolved in an organic solvent (or bromine water), the reddish-brown colour disappears as bromine adds across the double bond. The reaction forms a colourless dibromoalkane.

CH₂=CH₂ + Br₂ → CH₂BrCH₂Br (1,2-dibromoethane)

This decolourisation is the classic chemical test for unsaturation (C=C).

当烯烃与溶于有机溶剂的溴（或溴水）反应时，红棕色褪去，因为溴加成到双键上，生成无色的二溴代烷。

CH₂=CH₂ + Br₂ → CH₂BrCH₂Br (1,2-二溴乙烷)

褪色现象是检验不饱和度（C=C双键）的经典化学方法。

5. Addition of Hydrogen Halides (e.g., HBr) | 卤化氢的加成（如HBr）

Hydrogen halides such as HBr also undergo electrophilic addition with alkenes. The H–Br bond is polarised, with H carrying a partial positive charge and acting as the electrophile. In the case of symmetrical alkenes, only one product is formed.

CH₂=CH₂ + HBr → CH₃CH₂Br (bromoethane)

For unsymmetrical alkenes (e.g., propene), two products are possible, but the major product follows the rule: the hydrogen atom attaches to the carbon atom that already has more hydrogen atoms.

卤化氢如HBr也能与烯烃发生亲电加成。H–Br键是极性的，H带有部分正电荷，充当亲电试剂。对于对称烯烃，只生成一种产物。

CH₂=CH₂ + HBr → CH₃CH₂Br (溴乙烷)

对于不对称烯烃（如丙烯），可能有两种产物，但主要产物遵循以下规律：氢原子加到已连接较多氢原子的碳原子上。

6. Hydrogenation of Alkenes | 烯烃的加氢反应

Addition of hydrogen (H₂) to an alkene, called hydrogenation, converts it to an alkane. This reaction requires a nickel catalyst and a temperature of around 150°C. It is used industrially to harden vegetable oils into margarine.

C₂H₄ + H₂ → C₂H₆ (ethane)

氢气（H₂）对烯烃的加成称为加氢反应，可将烯烃转变为烷烃。该反应需要镍催化剂和约150°C的温度。工业上用于将植物油硬化成人造黄油。

C₂H₄ + H₂ → C₂H₆ (乙烷)

7. Hydration of Alkenes (Addition of Water) | 烯烃的水合反应（加水）

Alkenes react with steam (water) to form alcohols. This hydration reaction requires a phosphoric acid catalyst, high temperature (about 300°C), and high pressure (60–70 atm). For ethene, ethanol is produced; for unsymmetrical alkenes, the major alcohol product is the one where the –OH group attaches to the more substituted carbon.

CH₂=CH₂ + H₂O (g) ⇌ CH₃CH₂OH (ethanol)

烯烃与水蒸气反应生成醇。这个水合反应需要磷酸催化剂、高温（约300°C）和高压（60–70 atm）。乙烯水合生成乙醇；对于不对称烯烃，主要醇产物是–OH基团加到取代较多的碳原子上的产物。

CH₂=CH₂ + H₂O (g) ⇌ CH₃CH₂OH (乙醇)

8. Simplified Mechanism of Electrophilic Addition | 亲电加成的简化机理

Although IGCSE does not require drawing the full mechanism with curly arrows, it is helpful to understand the steps: (1) The π electrons of the alkene are attracted to the electrophile (e.g., Br₂), inducing a dipole and causing heterolytic fission of the Br–Br bond. (2) A carbocation intermediate forms as the electrophile attaches to one carbon. (3) The remaining bromide ion (nucleophile) quickly attacks the carbocation, completing the addition.

虽然IGCSE不要求画出完整的弯箭头机理，但理解以下步骤有助于记忆：(1) 烯烃的π电子被亲电试剂（如Br₂）吸引，诱导偶极并导致Br–Br键的异裂。(2) 形成碳正离子中间体，亲电试剂连到一个碳上。(3) 剩余的溴离子（亲核试剂）迅速进攻碳正离子，完成加成。

9. Electrophilic Addition vs Addition Polymerisation | 亲电加成与加成聚合的区别

Both processes involve alkenes, but electrophilic addition adds a small molecule (X–Y) across the double bond, forming a single product. Addition polymerisation links many alkene monomers together to form a long-chain polymer without eliminating any small molecule. The monomer’s double bond opens up to form single bonds between monomers.

两种过程都涉及烯烃，但亲电加成是将小分子（X–Y）加到双键两侧，形成单一产物。加聚反应则是将许多烯烃单体连接形成长链聚合物，不脱除任何小分子。单体的双键打开，在单体之间形成单键连接。

10. Key Exam Points and Common Mistakes | 考点总结与常见错误

Here are the essentials you must remember for the exam:

Colour change: alkenes decolourise bromine water (orange/brown to colourless). The bromine is not removed; it adds across the double bond.
Conditions: hydrogenation uses H₂/Ni/150°C; hydration uses H₂O(g)/H₃PO₄/300°C/60 atm. Do not mix them up.
For unsymmetrical alkenes, the hydrogen adds to the carbon with more hydrogens already attached (and the other part adds to the more substituted carbon).
Always draw displayed formulae clearly, showing all atoms and bonds, especially in addition reaction equations.
Avoid vague language: say ‘the double bond opens up and adds the electrophile’, not ‘it absorbs bromine’.

以下是考试必须牢记的要点：

颜色变化：烯烃使溴水褪色（由橙色/棕色变为无色）。溴并没有被去除，而是加成到双键上。
条件：加氢反应用H₂/Ni/150°C；水合反应用H₂O(g)/H₃PO₄/300°C/60 atm。切勿混淆。
对于不对称烯烃，氢加到已连接较多氢的碳上（另一部分则加到取代较多的碳上）。
始终清晰地绘制结构式，标出所有原子和化学键，尤其是在书写加成反应方程式时。
避免模糊表述：要说‘双键打开并与亲电试剂加成’，而不要说‘吸收了溴’。

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Electrophilic Addition for IGCSE WJEC Chemistry: Key Exam Points | IGCSE WJEC 化学：亲电加成 考点精讲