Electrophilic Addition in A-Level Chemistry | A-Level 化学：亲电加成考点精讲

📚 Electrophilic Addition in A-Level Chemistry | A-Level 化学：亲电加成考点精讲

Electrophilic addition is the most important reaction pathway for alkenes, underpinning the synthesis of alcohols, halogenoalkanes, and polymers. It is a classic example of how an electron-rich π bond can be attacked by an electron-deficient species. Mastering the mechanism, regioselectivity, and carbocation stability is essential for A-Level success.

亲电加成是烯烃最重要的反应途径，是合成醇、卤代烷和聚合物的基础。它是富电子的 π 键被缺电子物种进攻的经典范例。掌握其机理、区域选择性和碳正离子稳定性对于 A-Level 化学考试至关重要。

1. Introduction to Electrophilic Addition | 亲电加成简介

Alkenes contain a double bond consisting of a σ bond and a π bond. The π bond is formed by the sideways overlap of p orbitals, resulting in a region of high electron density above and below the plane of the molecule. This makes alkenes nucleophilic and highly susceptible to attack by electrophiles – species that are electron-deficient and can accept a pair of electrons.

烯烃含有由一个 σ 键和一个 π 键组成的双键。π 键由 p 轨道侧面重叠形成，在分子平面上下形成一个高电子密度区域。这使得烯烃具有亲核性，极易受到亲电试剂的进攻——亲电试剂是缺电子且能接受一对电子的物种。

The general mechanism involves the breaking of the π bond and formation of two new σ bonds, with the electrophile adding to one carbon and a nucleophile (often the counterion or solvent) adding to the other. The reaction is called an addition because the double bond is lost and two atoms or groups are added across the original double bond.

一般机理涉及 π 键断裂和两个新 σ 键的形成，亲电试剂加到一个碳上，而亲核试剂（通常是抗衡离子或溶剂）加到另一个碳上。该反应被称为加成，因为双键消失，两个原子或基团加在原双键的两端。

2. The Electrophilic Addition Mechanism | 亲电加成机理

The mechanism proceeds in two key steps. In the first, rate-determining step, the electrophile (E⁺) accepts a pair of electrons from the π bond, forming a covalent bond to one carbon. This heterolytic fission of the π bond leaves the other carbon with a positive charge, generating a carbocation intermediate.

该机理分两个关键步骤。在第一步速控步中，亲电试剂（E⁺）接受来自 π 键的一对电子，与其中一个碳原子形成共价键。π 键的异裂使得另一个碳原子带正电荷，生成碳正离子中间体。

In the second, fast step, the carbocation is attacked by a nucleophile (Nu⁻), which donates a pair of electrons to the positively charged carbon, forming the final addition product. The overall reaction is regioselective and, in some cases, stereoselective.

在第二步快步骤中，碳正离子被亲核试剂（Nu⁻）进攻，后者提供一对电子给带正电的碳，形成最终加成产物。总反应具有区域选择性，在某些情况下还具有立体选择性。

3. Markovnikov’s Rule | 马氏规则

When an unsymmetrical alkene undergoes electrophilic addition with an unsymmetrical reagent (e.g. HBr, H₂O/H⁺), two constitutional isomers can form. Markovnikov’s rule states that the hydrogen atom of the reagent adds to the carbon of the double bond that already has the greater number of hydrogen atoms. In other words, the electrophilic H⁺ attaches to the less substituted carbon, while the nucleophile attaches to the more substituted carbon (the one that forms the more stable carbocation).

当不对称烯烃与不对称试剂（如 HBr、H₂O/H⁺）发生亲电加成时，可能形成两种结构异构体。马氏规则指出，试剂中的氢原子加在双键上含氢较多的碳原子上。换句话说，亲电的 H⁺ 连在取代较少的碳上，而亲核试剂连在取代较多的碳上（即能形成更稳定碳正离子的碳）。

A modern interpretation is based on carbocation stability: the reaction proceeds via the most stable carbocation intermediate. The more alkyl groups attached to the positively charged carbon, the more stable the carbocation, due to inductive electron-donating effects.

现代解释基于碳正离子的稳定性：反应经过最稳定的碳正离子中间体进行。与带正电碳相连的烷基越多，由于诱导给电子效应，碳正离子越稳定。

4. Carbocation Stability and Inductive Effects | 碳正离子稳定性和诱导效应

Carbocations are classified as primary (1°), secondary (2°), or tertiary (3°) based on the number of carbon atoms directly bonded to the positively charged carbon. Stability increases in the order: 1° < 2° < 3°. This is because alkyl groups are electron-donating through the inductive effect (+I effect), which reduces the positive charge density on the carbon and stabilises the ion.

碳正离子根据直接与带正电碳相连的碳原子数分为伯（1°）、仲（2°）或叔（3°）。稳定性依次增加：1° < 2° < 3°。这是因为烷基通过诱导效应（+I 效应）给电子，降低了碳上的正电荷密度，从而稳定离子。

The methyl carbocation (CH₃⁺) is the least stable and is rarely formed in typical A-Level reactions. In exam explanations, always refer to the number of alkyl groups donating electron density towards the positive centre. The more substituted the carbocation, the lower its energy, and the faster it forms in the rate-determining step.

甲基碳正离子（CH₃⁺）最不稳定，在典型的 A-Level 反应中几乎不生成。在考试解释中，务必提及烷基向正电中心提供电子密度的数量。碳正离子取代越多，能量越低，在速控步中形成得越快。

5. Addition of Hydrogen Halides (HX) | 卤化氢加成

Alkenes react with hydrogen halides (HCl, HBr, HI) at room temperature to form halogenoalkanes. The H–X bond is polar, with the hydrogen bearing a partial positive charge and acting as the electrophile. For example, ethene reacts with HBr to form bromoethane.

烯烃在室温下与卤化氢（HCl、HBr、HI）反应生成卤代烷。H–X 键是极性的，氢带部分正电荷，作为亲电试剂。例如，乙烯与 HBr 反应生成溴乙烷。

With unsymmetrical alkenes, Markovnikov’s rule determines the major product. Propene plus HBr yields primarily 2-bromopropane, not 1-bromopropane. The mechanism involves protonation of the double bond to form the more stable secondary carbocation, followed by nucleophilic attack by Br⁻.

对于不对称烯烃，马氏规则决定主产物。丙烯加 HBr 主要生成 2-溴丙烷，而非 1-溴丙烷。机理为双键质子化形成更稳定的二级碳正离子，随后 Br⁻ 进行亲核进攻。

CH₃CH=CH₂ + HBr → CH₃CHBrCH₃ (major product)

CH₃CH=CH₂ + HBr → CH₃CHBrCH₃ （主要产物）

6. Addition of Water: Hydration | 水加成：水合反应

Alkenes can be converted into alcohols by the acid-catalysed addition of water. This is known as hydration. The reaction requires a strong acid catalyst, typically concentrated phosphoric acid (H₃PO₄) or sulfuric acid, and is carried out at high temperature and pressure. Industrially, this is used to manufacture ethanol from ethene.

烯烃可通过酸催化的加水反应转变为醇，这被称为水合反应。反应需要强酸催化剂，通常是浓磷酸（H₃PO₄）或硫酸，并在高温高压下进行。工业上用此法从乙烯制备乙醇。

The mechanism involves protonation of the double bond by H⁺ (from acid) to generate a carbocation, followed by nucleophilic attack by water. Deprotonation of the oxonium ion regenerates the acid catalyst and yields the alcohol. Markovnikov’s rule applies: propene gives propan-2-ol as the major product.

机理为 H⁺（来自酸）使双键质子化生成碳正离子，然后水作为亲核试剂进攻。氧鎓离子去质子化再生酸催化剂并得到醇。马氏规则适用：丙烯主要得到丙-2-醇。

CH₂=CH₂ + H₂O →(H⁺, high T,P) CH₃CH₂OH

CH₂=CH₂ + H₂O →（H⁺, 高温高压）CH₃CH₂OH

7. Addition of Halogens: Bromination | 卤素加成：溴化反应

Alkenes react rapidly with halogens (Cl₂, Br₂) at room temperature in the dark to form vicinal dihalogenoalkanes. Bromination is a key test for unsaturation. Although Br₂ is non-polar, an approaching alkene π bond induces a dipole, making the nearer Br atom electrophilic (Brδ⁺) and the farther one nucleophilic (Brδ⁻).

烯烃在室温避光下与卤素（Cl₂、Br₂）迅速反应，生成邻二卤代烷。溴化反应是检验不饱和性的关键方法。虽然 Br₂ 是非极性分子，但靠近的烯烃 π 键会诱导偶极，使较近的 Br 原子成为亲电的（Brδ⁺），较远的成为亲核的（Brδ⁻）。

The mechanism begins with the electrophilic attack of Brδ⁺ on the π bond, forming a cyclic bromonium ion intermediate (a three-membered ring containing a positively charged Br). This is followed by backside attack of Br⁻ on the bromonium ion, resulting in anti addition. The two Br atoms add to opposite faces of the original double bond.

机理始于 Brδ⁺ 对 π 键的亲电进攻，形成环状溴鎓离子中间体（含有一个带正电 Br 的三元环）。随后 Br⁻ 从背面进攻溴鎓离子，导致反式加成。两个 Br 原子加在原双键的两侧。

CH₂=CH₂ + Br₂ → CH₂Br–CH₂Br

The stereochemical consequence – anti addition – can be examined with cyclic alkenes, where the trans product is formed exclusively. This contrasts with the HX addition, which often gives a mixture of stereoisomers.

立体化学结果——反式加成——可以用环状烯烃来考察，此时只生成反式产物。这与 HX 加成不同，后者常给出立体异构体的混合物。

8. Addition of Sulfuric Acid | 硫酸加成

Cold, concentrated sulfuric acid reacts with alkenes to form alkyl hydrogensulfates. The electrophile is H⁺ from the slightly dissociated H₂SO₄. The remaining HSO₄⁻ acts as the nucleophile. For ethene, the product is ethyl hydrogensulfate, CH₃CH₂OSO₃H.

冷的浓硫酸与烯烃反应生成硫酸氢烷基酯。亲电试剂是来自 H₂SO₄ 微弱电离的 H⁺。生成的 HSO₄⁻ 作为亲核试剂。对于乙烯，产物是硫酸氢乙酯，CH₃CH₂OSO₃H。

Subsequent hydrolysis with water, upon heating, cleaves the alkyl hydrogensulfate to yield the corresponding alcohol and regenerate sulfuric acid. This two-step process is an alternative route to manufacture ethanol from ethene.

随后在加热下加水水解，使硫酸氢烷基酯断裂，得到相应的醇并再生硫酸。该两步法是乙烯制乙醇的另一种途径。

CH₂=CH₂ + H₂SO₄ → CH₃CH₂OSO₃H →(H₂O, heat) CH₃CH₂OH + H₂SO₄

CH₂=CH₂ + H₂SO₄ → CH₃CH₂OSO₃H →（H₂O, 加热）CH₃CH₂OH + H₂SO₄

9. Testing for Unsaturation | 不饱和性检验

The bromine water test is a simple qualitative test for alkenes. When orange-brown bromine water is added to an alkene, the solution rapidly decolourises as the bromine is consumed in the addition reaction to form a colourless dibromo compound. This does not occur with alkanes under normal conditions.

溴水试验是检验烯烃的简单定性方法。当橙棕色的溴水加入烯烃中，溶液迅速褪色，因为溴在加成反应中被消耗，生成无色的二溴化合物。烷烃在通常条件下不会发生此反应。

It is important that the reaction is carried out in the dark to avoid competing free-radical substitution reactions that could also decolourise bromine. In the exam, you may be asked to distinguish between an alkane and an alkene, or to explain why alkenes decolourise bromine water whereas alkanes require UV light.

重要的是反应应在避光下进行，以避免竞争的自由基取代反应也可能使溴褪色。在考试中，可能被要求区分烷烃和烯烃，或解释为何烯烃使溴水褪色而烷烃需要紫外光。

10. Summary and Exam Tips | 总结与考试技巧

Electrophilic addition is a central topic that integrates mechanism, regioselectivity, carbocation stability, and stereochemistry. Always draw curly arrows from the π bond to the electrophile, and from the nucleophile to the carbocation. Remember that the first step is rate-determining and determines the product distribution via the stability of the carbocation intermediate.

亲电加成是融合机理、区域选择性、碳正离子稳定性和立体化学的核心主题。始终画出从 π 键指向亲电试剂的弯箭头，以及从亲核试剂指向碳正离子的弯箭头。记住第一步是速控步，并通过碳正离子中间体的稳定性决定产物分布。

For Markovnikov addition, clearly state the ‘the hydrogen attaches to the carbon with the most hydrogens’ and support it with carbocation stability reasoning. In bromination, do not forget the bromonium ion intermediate and the concept of anti addition. Use the bromine water test as a functional group identification tool, but note its limitations (e.g., phenols also decolourise bromine water).

对于马氏加成，明确说明“氢加在含氢最多的碳上”，并用碳正离子稳定性原理解释。在溴化反应中，不要忘记溴鎓离子中间体和反式加成的概念。利用溴水试验作为官能团鉴定工具，但注意其局限性（例如，酚类也能使溴水褪色）。

Practice drawing detailed mechanisms for ethene, propene, and symmetrical/unsymmetrical alkenes with HBr, H₂O/H⁺, Br₂, and H₂SO₄. Familiarity with the curly arrow notation and the energy profile of a two-step electrophilic addition will give you confidence in structured and multiple-choice questions.

练习绘制乙烯、丙烯以及对称/不对称烯烃与 HBr、H₂O/H⁺、Br₂ 和 H₂SO₄ 反应的详细机理。熟练掌握弯箭头表示法和两步亲电加成的能量曲线，将使你在结构题和选择题中充满信心。

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Electrophilic Addition in A-Level Chemistry | A-Level 化学：亲电加成 考点精讲