Electrophilic Addition in IB Edexcel Chemistry | IB Edexcel 化学：亲电加成 考点精讲

📚 Electrophilic Addition in IB Edexcel Chemistry | IB Edexcel 化学：亲电加成 考点精讲

Electrophilic addition is the cornerstone reaction of unsaturated organic compounds, especially alkenes. In the IB and Edexcel A-Level Chemistry syllabus, this mechanism is assessed repeatedly, from drawing curly arrows to predicting major products using Markovnikov’s rule. Mastering this topic is essential for understanding how π-bonds react with electrophiles to form saturated products. This article provides a comprehensive breakdown of the electrophilic addition mechanism, its variations, stereochemical outcomes, and the key points examiners look for.

亲电加成反应是不饱和有机化合物（尤其是烯烃）的基石反应。在 IB 和 Edexcel A-Level 化学大纲中，这一机理被反复考查，从绘制弯箭头到运用马氏规则预测主产物。掌握这一主题对于理解 π 键如何与亲电试剂反应生成饱和产物至关重要。本文将全面解析亲电加成机理、各种变化形式、立体化学结果以及考官们关注的要点。

1. The π-Bond as an Electron-Rich Source | 作为富电子来源的 π 键

The C=C double bond in alkenes consists of a σ bond and a π bond. The π bond is formed by sideways overlap of p orbitals, placing electron density above and below the plane of the molecule. This exposed electron cloud makes alkenes nucleophilic and susceptible to attack by electrophiles — electron-deficient species that seek electron pairs.

烯烃中的 C=C 双键由一个 σ 键和一个 π 键组成。π 键由 p 轨道的侧向重叠形成，电子云分布在分子平面的上方和下方。这种暴露的电子云使烯烃具有亲核性，容易受到亲电试剂（缺电子的物种，寻找电子对）的攻击。

2. Defining Electrophile and Reaction Mechanism | 亲电试剂的定义与反应机理

An electrophile is a reagent that accepts a pair of electrons to form a new covalent bond. Common electrophiles in addition reactions include H⁺ (from HBr, HCl, H₂ SO₄), Br₂ (polarised by an approaching alkene), and Hg(OAc)₂. The general mechanism proceeds in two steps: (1) electrophilic attack to form a carbocation intermediate, and (2) nucleophilic capture by the counter-ion or solvent.

亲电试剂是接受一电子对以形成新共价键的试剂。加成反应中常见的亲电试剂包括 H⁺（来自 HBr、HCl、H₂ SO₄）、Br₂（被靠近的烯烃极化）和 Hg(OAc)₂。一般机理分为两步：(1) 亲电进攻形成碳正离子中间体，(2) 抗衡离子或溶剂作为亲核试剂捕获碳正离子。

3. Curly Arrow Notation in IB/Edexcel Exams | IB/Edexcel 考试中的弯箭头表示法

Curly arrows must start from the bond or lone pair of electrons and point towards the atom or bond being formed. In the first step, an arrow starts from the C=C π bond and goes to the electrophilic atom (e.g., H in HBr). A second arrow shows the H–Br bond breaking heterolytically, with the electron pair going to Br. In the second step, the bromide ion lone pair attacks the carbocation, forming a C–Br bond.

弯箭头必须从化学键或孤对电子出发，指向正在形成的原子或键。在第一步中，箭头从 C=C π 键出发指向亲电原子（如 HBr 中的 H）。第二个箭头表示 H–Br 键发生异裂，电子对转移给 Br。第二步中，溴离子的孤对电子进攻碳正离子，形成 C–Br 键。

4. Markovnikov’s Rule and Carbocation Stability | 马氏规则与碳正离子稳定性

When an unsymmetrical alkene reacts with a hydrogen halide, the hydrogen atom attaches to the carbon with the greater number of hydrogen atoms already present (the ‘rich get richer’). This regioselectivity is explained by carbocation stability: tertiary > secondary > primary > methyl. The more substituted carbocation is more stable due to hyperconjugation and the inductive effect of alkyl groups.

当不对称烯烃与卤化氢反应时，氢原子加在原本氢原子数目较多的碳上（“富的更富”）。这种区域选择性可用碳正离子稳定性解释：三级 > 二级 > 一级 > 甲基。烷基的超共轭效应和诱导效应使取代较多的碳正离子更稳定。

5. Addition of Hydrogen Halides (HBr, HCl, HI) | 卤化氢的加成 (HBr, HCl, HI)

Gaseous hydrogen halides react readily with alkenes at room temperature to form haloalkanes. The reactivity order is HI > HBr > HCl, parallel to acid strength and bond enthalpy. For unsymmetrical alkenes, the major product follows Markovnikov’s rule. Note: HCl is often less reactive and may require a catalyst or heating.

气态卤化氢在室温下即可与烯烃快速反应生成卤代烷。反应活性顺序为 HI > HBr > HCl，这与酸性强度和键焓一致。对于不对称烯烃，主产物遵循马氏规则。注意：HCl 的反应活性通常较低，可能需要催化剂或加热。

6. Acid-Catalysed Hydration of Alkenes | 烯烃的酸催化水合

Alkenes react with steam in the presence of a phosphoric acid catalyst (H₃PO₄) on a solid silica support, at 300–330 °C and 60–70 atm. This is an industrial method to produce alcohols like ethanol from ethene. The mechanism still follows electrophilic addition, with H⁺ attacking the double bond to form the most stable carbocation, then water adding as a nucleophile, followed by deprotonation.

烯烃在水蒸气和磷酸催化剂（H₃PO₄，负载于固体二氧化硅上）存在下，于 300–330 °C 和 60–70 atm 下反应。这是从乙烯生产乙醇的工业方法。机理依然遵循亲电加成，H⁺ 优先进攻双键形成最稳定的碳正离子，然后水作为亲核试剂加成，再经过去质子化得到醇。

7. Halogenation: Addition of Br₂ and Cl₂ | 卤素加成：Br₂ 与 Cl₂

When an alkene is bubbled through a solution of bromine in an inert solvent (e.g., CCl₄) at room temperature, the reddish-brown colour decolourises instantly. This is the classic test for unsaturation. The mechanism involves the alkene’s π electrons polarising the Br–Br bond, making one Br atom electrophilic. A cyclic bromonium ion intermediate forms, which gives anti-addition stereochemistry (trans product).

将烯烃气体通入溴的惰性溶剂（如 CCl₄）溶液，室温下红棕色迅速褪去。这是检验不饱和度的经典方法。机理中，烯烃的 π 电子极化 Br–Br 键，使一个溴原子成为亲电试剂。形成环状溴鎓离子中间体，导致反式加成立体化学（得到反式产物）。

8. Stereochemistry: Syn vs. Anti Addition | 立体化学：同面加成与反式加成

Electrophilic addition can produce stereoisomers when the product contains chiral centres. Hydrogen halide addition: planar carbocation allows nucleophile attack from either side, giving a racemic mixture if a chiral centre forms. Bromination: the cyclic bromonium ion blocks one face, forcing the second Br⁻ to attack from the opposite side (anti addition). This gives exclusive trans products from cycloalkenes.

当产物含有手性中心时，亲电加成可能产生立体异构体。卤化氢加成：平面型碳正离子允许亲核试剂从任一面进攻，若生成手性中心，则得到外消旋混合物。溴化：环状溴鎓离子阻挡了一面，迫使第二个 Br⁻ 从反面进攻（反式加成）。环烯烃因此专一生成反式产物。

9. Hydrogenation of Alkenes | 烯烃的氢化反应

Alkenes react with hydrogen gas in the presence of a metal catalyst (Pd/C, Pt, or Ni) at room temperature and pressure to form alkanes. This is a syn addition — both H atoms add to the same face of the double bond. The reaction is exothermic and provides data for enthalpy of hydrogenation, which is used to compare alkene stability.

烯烃在金属催化剂（Pd/C、Pt 或 Ni）存在下与氢气反应，常温常压即可生成烷烃。此为同面加成——两个氢原子加在双键的同一侧。该反应放热，提供氢化焓数据，可用于比较烯烃稳定性。

10. Electrophilic Addition in Polymerisation | 亲电加成在聚合反应中的应用

Addition polymerisation of ethene and substituted alkenes (e.g., propene, styrene, vinyl chloride) proceeds via a free-radical or cationic mechanism, both involving electrophilic species. In the industrial high-pressure process, an initiator radical attacks the C=C bond, generating an active site that propagates the chain. Many textbook exam questions ask for repeating units of poly(ethene), poly(propene), and poly(tetrafluoroethene) (PTFE).

乙烯及取代烯烃（如丙烯、苯乙烯、氯乙烯）的加聚反应通过自由基或阳离子机理进行，其中都包含亲电物种。在工业高压工艺中，引发剂自由基进攻 C=C 键，生成一个活性位点并不断扩展链增长。许多考试题会要求写出聚（乙烯）、聚（丙烯）和聚四氟乙烯（PTFE）的重复单元。

11. Competitive Reactions and Rearrangements | 竞争反应与重排现象

Sometimes, the initially formed carbocation can rearrange via a hydride or alkyl shift to form a more stable carbocation before nucleophilic attack. This leads to unexpected products, e.g., the addition of HCl to 3-methyl-1-butene gives some 2-chloro-2-methylbutane via a 1,2-methyl shift. High-level exam questions may test this if a more stable carbocation can be formed by rearrangement.

有时，最初形成的碳正离子可能通过氢负离子或烷基迁移发生重排，形成更稳定的碳正离子后再被捕获。这导致意料之外的产物，例如 3-甲基-1-丁烯与 HCl 加成会经由 1,2-甲基迁移生成部分 2-氯-2-甲基丁烷。高难度考题若存在重排得到更稳定碳正离子的情况，可能考查此知识点。

12. Key Exam Tips and Common Mistakes | 关键答题技巧与常见错误

Always show heterolytic bond fission with full curly arrows. Mark the δ+ and δ− on the electrophile when using the induced dipole model for Br₂. For Markovnikov addition, explicitly state ‘more stable carbocation’ rather than just quoting the rule. Do not forget to include the regeneration of the acid catalyst in hydration mechanisms. When drawing the bromonium ion, show the positive charge on the three-membered ring.

务必使用完整的弯箭头表示异裂。在处理 Br₂ 的诱导偶极模型时，标出 δ+ 和 δ−。对于马氏加成，要明确说明“更稳定的碳正离子”，而非仅仅引用规则。在水合机理中别忘了催化酸的再生。绘制溴鎓离子时，正电荷应标在三元环上。

Published by TutorHao | Chemistry Revision Series | aleveler.com

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