📚 Electrophilic Addition in GCSE CIE Chemistry | GCSE CIE 化学亲电加成考点精讲
Electrophilic addition is a fundamental reaction type in organic chemistry, central to the CIE IGCSE Chemistry syllabus. It explains how alkenes – hydrocarbons containing a carbon‑carbon double bond – react with a range of substances to form saturated products. Mastering this topic not only helps you answer exam questions confidently but also builds a bridge to A‑level concepts. In this revision guide, we unpack the essential ideas, mechanisms, patterns, and common pitfalls in electrophilic addition for GCSE candidates.
亲电加成是有机化学中的基本反应类型，在 CIE IGCSE 化学课程中占据核心地位。它解释了含有碳碳双键的烯烃如何与多种物质反应生成饱和产物。掌握这一主题不仅能让你自信应对考试，还为 A‑level 的学习打下桥梁。本复习指南将为 GCSE 考生拆解亲电加成的基本概念、机理、规律以及常见易错点。
1. What is Electrophilic Addition? | 什么是亲电加成？
Electrophilic addition is a reaction in which an electrophile attacks an electron‑rich part of a molecule and two new bonds are formed. In the context of alkenes, the C=C double bond is an area of high electron density, making it attractive to electrophiles – species that are electron‑deficient or carry a full positive charge. Under suitable conditions, the π bond breaks and the electrophile adds across the two carbon atoms, turning the unsaturated alkene into a saturated compound.
亲电加成是一种由亲电试剂进攻分子中电子云密度较高的部位，从而形成两个新键的反应。对烯烃而言，C=C 双键是一个高电子密度区域，对亲电试剂——缺电子或带正电的物种——具有强烈吸引力。在合适条件下，π 键断裂，亲电试剂加成到两个碳原子上，使不饱和烯烃转变为饱和化合物。
2. The Alkenes: C=C as a Reaction Centre | 烯烃：C=C 作为反应中心
Alkenes are hydrocarbons with the general formula CₙH₂ₙ. Their defining feature is the C=C double bond, consisting of a strong σ bond and a weaker π bond. The π bond is formed by sideways overlap of p orbitals, creating a cloud of electron density above and below the plane of the molecule. This cloud is what electrophiles target. The double bond also restricts rotation, giving rise to geometrical isomerism (cis‑trans), which is tested at IGCSE level.
烯烃是通式为 CₙH₂ₙ 的碳氢化合物。其标志性结构是 C=C 双键，由一个强 σ 键和一个较弱的 π 键组成。π 键由 p 轨道侧向重叠形成，在分子平面上方和下方形成电子云。这个电子云正是亲电试剂的目标。双键还限制了旋转，导致顺反几何异构，这在 IGCSE 考试中也会涉及。
3. What is an Electrophile? | 什么是亲电试剂？
An electrophile is a reagent that accepts an electron pair to form a new covalent bond. Common electrophiles in IGCSE chemistry include H⁺ (from HBr or H₂SO₄), Br⁺ (from Br₂ with induction), and sometimes the slightly positive end of a polarised molecule. Electrophiles can be cations or neutral molecules that possess an electron‑deficient atom. For example, in the addition of hydrogen bromide, the H⁺ acts as the electrophile, attracted to the double bond.
亲电试剂是一种能够接受电子对从而形成新的共价键的试剂。IGCSE 化学中常见的亲电试剂有 H⁺（来自 HBr 或 H₂SO₄）、Br⁺（由 Br₂ 极化产生），以及极性分子中略带正电的一端。亲电试剂可以是阳离子，也可以是含有缺电子原子的中性分子。例如，在溴化氢加成反应中，H⁺ 作为亲电试剂，被双键吸引。
4. General Mechanism of Electrophilic Addition | 亲电加成的一般机理
The mechanism can be described in two stages. First, the electrophile is attracted to the π electron cloud and forms a bond with one of the carbon atoms, breaking the π bond. This creates a carbocation – a positively charged carbon intermediate. In the second step, a nucleophile (often the leftover part of the reagent) attacks the carbocation, donating an electron pair and forming a new covalent bond. The overall transformation is addition, with no atom lost.
该机理可分两步描述。第一步，亲电试剂被 π 电子云吸引，与其中一个碳原子成键，导致 π 键断裂，生成一个碳正离子——带正电的碳中间体。第二步，亲核试剂（通常是试剂剩余部分）进攻碳正离子，提供电子对形成新的共价键。总体转变为加成反应，没有原子丢失。
5. Addition of Hydrogen: Hydrogenation | 氢气加成：加氢反应
When an alkene reacts with hydrogen gas (H₂) in the presence of a nickel catalyst at 150 °C, the double bond opens and two hydrogen atoms add across it, producing an alkane. This is called hydrogenation or catalytic hydrogenation. The reaction is important in the food industry to convert liquid vegetable oils into solid margarine. In the exam, you may be asked to write the equation, e.g. C₂H₄ + H₂ → C₂H₆, or to identify the role of nickel as a heterogeneous catalyst providing a surface for the reaction.
烯烃与氢气在镍催化剂、150 °C 条件下反应时，双键打开，两个氢原子加成到双键两端，生成相应的烷烃，称为加氢反应或催化加氢。该反应在食品工业中用于将液态植物油转化为固态人造黄油。考试中可能要求书写方程式，如 C₂H₄ + H₂ → C₂H₆，或指出镍作为多相催化剂提供反应表面的作用。
6. Addition of Halogens: Bromination Test | 卤素加成：溴水试验
Alkenes react rapidly with bromine (Br₂) at room temperature to form a dibromoalkane. The reddish‑brown colour of bromine water is discharged immediately, providing the classic test for unsaturation. The mechanism involves the polarisation of the Br–Br bond as it approaches the electron‑rich double bond: Br₂ behaves as an electrophile, with Brᵟ⁺ attacking first. The product for ethene is 1,2‑dibromoethane. The reaction also works with chlorine, but bromine water is the standard laboratory test.
烯烃在室温下能与溴迅速反应，生成二溴代烷。溴水的红棕色立即褪去，这是检验不饱和键的经典方法。其机理为：Br–Br 键在接近富电子双键时发生极化，Br₂ 作为亲电试剂，Brᵟ⁺ 先进攻。以乙烯为例，产物是 1,2‑二溴乙烷。该反应也适用于氯，但溴水是标准的实验室检验方法。
7. Addition of Hydrogen Halides: HX | 卤化氢加成：HX
Hydrogen halides such as HCl, HBr and HI add to alkenes to form halogenoalkanes. The hydrogen atom acts as the electrophile, adding first to one carbon, followed by the halide ion. With unsymmetrical alkenes like propene, two products can form, depending on which carbon the hydrogen attaches to. The major product is predicted by Markovnikov’s rule: the hydrogen adds to the carbon with more hydrogen atoms already attached, so the more stable carbocation intermediate forms.
卤化氢（如 HCl、HBr、HI）可与烯烃加成生成卤代烷。氢原子作为亲电试剂，首先加成到一个碳上，随后卤离子进攻。对于不对称烯烃（如丙烯），根据氢原子连接位置不同可生成两种产物。主要产物可由马尔科夫尼科夫规则预测：氢加到原本连有较多氢原子的碳上，这样能形成更稳定的碳正离子中间体。
8. Addition of Water: Hydration of Alkenes | 水的加成：烯烃水合
Alkenes can add water (H₂O) in the presence of an acid catalyst, such as concentrated sulfuric acid or phosphoric acid. This hydration produces alcohols. For example, ethene reacts with steam over a phosphoric acid catalyst at high temperature and pressure to yield ethanol. The reaction is a reversible electrophilic addition where H⁺ first adds to the double bond to form a carbocation, which is then attacked by water. In the lab, a two‑step process via alkyl hydrogen sulfate can also be used.
烯烃在酸催化剂（如浓硫酸或磷酸）存在下可与水加成，该水合反应生成醇。例如，乙烯与蒸汽在磷酸催化、高温高压下反应生成乙醇。这是一个可逆的亲电加成：H⁺ 先加成到双键形成碳正离子，随后水分子进攻。在实验室中，也可通过烷基硫酸氢酯的两步法来实现。
9. Electrophilic Addition and Polymerisation | 亲电加成与聚合反应
Addition polymerisation is a special case of electrophilic addition, though often initiated by free radicals or catalysts. In this process, many alkene monomers join together by opening their double bonds, forming long saturated polymer chains. CIE IGCSE expects you to know the formation of poly(ethene), poly(propene), and poly(chloroethene) (PVC). You should be able to draw repeating units from monomers and vice versa, understanding that the double bond breaks and new single bonds form between monomers.
加聚反应是亲电加成的特例，虽然常通过自由基或催化剂引发。在此过程中，许多烯烃单体打开双键连接在一起，形成长的饱和聚合物链。CIE IGCSE 要求掌握聚乙烯、聚丙烯和聚氯乙烯（PVC）的生成。你需要能够从单体画出重复单元，反之亦然，理解双键断裂、单体间形成新单键的过程。
10. Worked Example: Predicting Products | 案例分析：预测产物
Consider the reaction between propene and HBr. The first step is electrophilic attack by H⁺, which can add to either carbon of the double bond. Addition to the terminal carbon (CH₂) leads to a secondary carbocation, while addition to the middle carbon (CH) gives a primary carbocation. Secondary carbocations are more stable, so the major product is 2‑bromopropane (CH₃CHBrCH₃), not 1‑bromopropane. In exams, marks are given for correct structural formulas and naming the major product.
以丙烯与 HBr 的反应为例。第一步是 H⁺ 亲电进攻，可加成到双键任一碳上。加成到末端碳（CH₂）生成二级碳正离子，加成到中间碳（CH）生成一级碳正离子。二级碳正离子更稳定，因此主要产物是 2‑溴丙烷（CH₃CHBrCH₃），而不是 1‑溴丙烷。考试中，写出正确结构式并命名主要产物就能得分。
11. Common Mistakes and How to Avoid Them | 常见错误与避坑指南
Students often confuse addition with substitution reactions, or forget that a double bond contains both a σ and a π bond. Avoid saying ‘the double bond breaks completely’ – only the π bond breaks, while the σ bond remains. When drawing mechanisms, make sure curly arrows start from a bond or a lone pair and point precisely to the electrophilic atom. For polymerisation, never write ‘n’ as a subscript – always show it in front of the repeating unit and brackets. Also, check your carbocation stability order: tertiary > secondary > primary > methyl.
学生常将加成与取代混淆，或忘记双键同时含有 σ 和 π 键。避免说“双键完全断裂”——只有 π 键断裂，σ 键保留。画机理时，确保弯箭头从化学键或孤对电子出发，准确指向亲电原子。在聚合反应中，切勿将“n”写成下标——它应置于括号和重复单元之前。此外，要核对碳正离子稳定性顺序：三级 > 二级 > 一级 > 甲基。
12. Exam Tips and Summary | 考试技巧与总结
Electrophilic addition questions frequently appear in CIE IGCSE Paper 4 (Theory) and Paper 2 (Multiple Choice). Learn the colour change for the bromine water test (orange/brown to colourless). Be ready to draw displayed or structural formulas for products. Understand the role of catalysts and reaction conditions. The key takeaway: alkenes undergo electrophilic addition because of their electron‑rich π bond, and the addition pattern depends on the stability of the intermediate carbocation.
亲电加成题目频繁出现在 CIE IGCSE 试卷 4（理论）和试卷 2（选择题）中。记住溴水试验的颜色变化（橙/棕色变为无色）。准备好画出产物的结构式或显示式。理解催化剂的作用和反应条件。关键要点是：烯烃因其富电子的 π 键而发生亲电加成，而其加成模式取决于中间体碳正离子的稳定性。
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