Electrophilic Addition | 亲电加成考点精讲

📚 Electrophilic Addition | 亲电加成考点精讲

Electrophilic addition is the characteristic reaction mechanism of alkenes, one of the most important organic reaction types in OCR GCSE Chemistry. Understanding how molecules with electron‑deficient species attack the carbon–carbon double bond helps you predict products and explain experimental observations, such as the decolourisation of bromine water. This guide walks you through the key concepts, mechanisms and examples you need to master the topic.

亲电加成是烯烃的特征反应机理，也是 OCR GCSE 化学中有机化学最重要的反应类型之一。理解缺电子物种如何进攻碳碳双键，能帮助你预测产物并解释实验现象，比如溴水褪色。本指南将带你梳理必须掌握的核心概念、机理和实例。

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

Electrophilic addition is a reaction in which an electrophile – a species that loves electrons and is electron‑deficient – attacks a region of high electron density, breaking a π bond and forming two new σ bonds. Alkenes have a double bond consisting of a σ bond and a π bond; the exposed π electrons make alkenes nucleophilic centres, attracting electrophiles. The process results in a saturated product where atoms or groups add across the double bond.

亲电加成是一种反应，其中亲电试剂（一种缺电子、热爱电子的物种）进攻电子密度高的区域，断裂 π 键并形成两个新的 σ 键。烯烃的双键由一个 σ 键和一个 π 键组成；暴露的 π 电子使烯烃成为亲核中心，吸引亲电试剂。反应生成饱和产物，原子或基团加成到双键两端。

The term ‘electrophilic’ comes from ‘electron‑loving’, and ‘addition’ means two reactant molecules combine to form a single product without any small molecule being lost. This contrasts with substitution, where an atom is replaced. In GCSE OCR, you are expected to recognise electrophilic addition as the key reactivity of alkenes with halogens, hydrogen halides and hydrogen.

“亲电”一词来自“亲电子”，而“加成”意味着两个反应物分子结合成一个产物，没有小分子离去。这与取代反应（原子被替换）不同。在 OCR GCSE 中，你需要认识到亲电加成是烯烃与卤素、卤化氢和氢气反应的核心反应类型。

2. Structure of Alkenes: The Electron‑Rich Double Bond | 烯烃的结构：富含电子的双键

Alkenes are hydrocarbons containing at least one carbon–carbon double bond, C=C. The double bond is the functional group that governs their chemistry. It consists of a strong σ (sigma) bond formed by head‑on overlap of sp² hybrid orbitals, and a weaker π (pi) bond formed by sideways overlap of p‑orbitals. The π bond sits above and below the plane of the molecule, giving a region of high electron density.

烯烃是至少含有一个碳碳双键 C=C 的烃。双键是决定其化学性质的官能团。它由一个强 σ 键（由 sp² 杂化轨道头对头重叠形成）和一个较弱的 π 键（由 p 轨道侧面重叠形成）组成。π 键位于分子平面的上下方，形成电子密度高的区域。

This electron cloud is polarisable and accessible, making alkenes very attractive to electrophiles. The simplest alkene, ethene (C₂H₄), has a planar structure around the double‑bonded carbons, with bond angles of approximately 120° due to sp² hybridisation.

这团电子云可极化且易于接近，使烯烃对亲电试剂极有吸引力。最简单的烯烃乙烯（C₂H₄）在双键碳周围呈平面结构，由于 sp² 杂化，键角约为 120°。

In the OCR specification, you need to be able to draw and interpret the displayed formula of ethene and recognise the C=C double bond. The high electron density of the π bond is the chemical reason why alkenes undergo electrophilic addition rather than electrophilic substitution, which is common in arenes.

在 OCR 大纲中，你需要能画出并解读乙烯的结构式，识别 C=C 双键。π 键的高电子密度是烯烃发生亲电加成而非亲电取代的化学原因，后者常见于芳烃。

3. Electrophiles: The Attacking Species | 亲电试剂：进攻物种

An electrophile is a species that accepts an electron pair from a nucleophile. Electrophiles are electron‑deficient – they carry a positive charge or have an incomplete octet. Common electrophiles in alkene addition include halogens (e.g. Br₂), hydrogen halides (e.g. HBr), and the proton H⁺ from concentrated sulfuric acid or water with an acid catalyst.

亲电试剂是从亲核试剂接受电子对的物种。亲电试剂是缺电子的——它们带正电荷或具有不完整的八隅体。烯烃加成中常见的亲电试剂包括卤素（如 Br₂）、卤化氢（如 HBr），以及来自浓硫酸或在酸催化下水的质子 H⁺。

In the case of bromine, the molecule Br₂ is non‑polar overall, but as it approaches the electron‑rich double bond, an induced dipole forms, with the nearer bromine atom becoming partially positive (δ⁺) and acting as the electrophile. For HBr, the hydrogen carries a partial positive charge due to the electronegativity difference, making it the electrophile.

就溴而言，Br₂ 分子整体非极性，但当它靠近富电子的双键时，会产生诱导偶极，靠近双键的溴原子带部分正电荷（δ⁺），充当亲电试剂。对于 HBr，由于电负性差异，氢带部分正电荷，使其成为亲电试剂。

Recognising electrophiles is a core skill for predicting whether a reagent will add to an alkene. You should remember that electrophiles are electron‑poor; they seek out electron‑rich centres like the C=C π bond.

识别亲电试剂是预测某种试剂是否会与烯烃加成的核心技能。你应记住亲电试剂是缺电子的；它们寻找像 C=C π 键这样富电子的中心。

4. Electrophilic Addition of Bromine: The Bromine Water Test | 溴的亲电加成：溴水试验

The reaction between an alkene and bromine (Br₂) dissolved in an organic solvent or in water (bromine water) is one of the most iconic electrophilic additions. The orange‑brown colour of bromine disappears instantly as the alkene decolourises it, producing a colourless dibromoalkane. This serves as the classical test for unsaturation.

烯烃与溶解在有机溶剂或水中的溴（溴水）反应，是最具代表性的亲电加成之一。溴的橙棕色瞬间消失，因烯烃使其褪色，生成无色的二溴代烷。这就是典型的不饱和性检验方法。

CH₂=CH₂ + Br₂ → CH₂Br–CH₂Br

Mechanistically, the π electrons attack the slightly positive bromine atom, forming a cyclic bromonium ion (in advanced terms) or a carbocation intermediate, but for GCSE you can describe the process in simpler terms: the double bond opens and each carbon forms a new single bond to a bromine atom. The product is 1,2‑dibromoethane in the case of ethene.

机理上，π 电子进攻略带正电荷的溴原子，形成环状溴鎓离子（高级概念）或碳正离子中间体，但在 GCSE 阶段，你可以用更简单的方式描述：双键打开，每个碳与一个溴原子形成新的单键。对于乙烯，产物是 1,2‑二溴乙烷。

When bromine water is used, a competing reaction with water can give a mixture of products, but the decolourisation still reliably indicates the presence of a C=C double bond. This electrophilic addition is stereospecific, but stereochemistry is not required at GCSE level.

当使用溴水时，水引起的竞争反应可能产生混合物，但褪色仍能可靠指示 C=C 双键的存在。这种亲电加成是立体专一的，但 GCSE 不要求立体化学。

5. Addition of Hydrogen Halides: HBr, HCl and HI | 卤化氢加成：HBr、HCl 和 HI

Hydrogen halides, such as HBr, add across the double bond of alkenes to produce haloalkanes. The hydrogen atom, being δ⁺ due to the polarity of the H–X bond, acts as the electrophile and attaches to one carbon of the double bond. The halide ion (Br⁻, Cl⁻, I⁻) then bonds to the other carbon.

卤化氢，如 HBr，加在烯烃双键上生成卤代烷。由于 H–X 键的极性，氢原子带部分正电荷（δ⁺），充当亲电试剂，连接到双键的一个碳上。随后卤离子（Br⁻, Cl⁻, I⁻）与另一个碳成键。

For symmetrical alkenes like ethene, only one product is formed. However, with unsymmetrical alkenes such as propene (CH₃CH=CH₂), the reaction can yield two possible products depending on which carbon the hydrogen adds to. The major product is predicted by Markovnikov’s rule (see Section 6).

对于对称烯烃如乙烯，只生成一种产物。但对于不对称烯烃如丙烯（CH₃CH=CH₂），根据氢原子加在哪个碳上，反应可能得到两种可能的产物。主要产物由马氏规则预测（见第 6 节）。

The general equation for an alkene with HX is: CnH2n + HX → CnH2n+1X. This addition is important in organic synthesis for introducing halogen atoms into hydrocarbon chains under mild conditions.

烯烃与 HX 反应的通式为：CnH2n + HX → CnH2n+1X。该加成在有机合成中用于在温和条件下将卤原子引入烃链，非常重要。

6. Markovnikov’s Rule: Predicting the Major Product | 马氏规则：预测主要产物

When a hydrogen halide or water (with acid) adds to an unsymmetrical alkene, the hydrogen atom tends to attach to the carbon of the double bond that already has more hydrogen atoms attached. This is known as Markovnikov’s rule: “the rich get richer” – the carbon richer in hydrogen gets the hydrogen.

当卤化氢或水（酸催化）与不对称烯烃加成时，氢原子倾向于加到双键上本身连接较多氢原子的碳上。这就是马氏规则：“富者愈富”——含氢较多的碳得到氢。

For propene, CH₃CH=CH₂, the addition of HBr gives mainly 2‑bromopropane (CH₃CHBrCH₃), not 1‑bromopropane (CH₂BrCH₂CH₃). This is because the intermediate carbocation is more stable when the positive charge is on the secondary carbon rather than on the primary carbon.

对于丙烯 CH₃CH=CH₂，与 HBr 加成主要得到 2‑溴丙烷（CH₃CHBrCH₃），而不是 1‑溴丙烷（CH₂BrCH₂CH₃）。这是因为中间体碳正离子的正电荷位于仲碳上比位于伯碳上更稳定。

While the full mechanistic reasoning requires knowledge of carbocation stability (see Section 7), you can often apply the rule simply: in HX addition, the H goes to the less substituted carbon of the double bond, and the X goes to the more substituted carbon.

虽然完全理解机理需要碳正离子稳定性知识（见第 7 节），你通常可以简单套用规则：在 HX 加成中，H 加在双键取代基较少的碳上，X 加在取代基较多的碳上。

7. Carbocation Intermediate and Stability | 碳正离子中间体及其稳定性

The electrophilic addition of HX proceeds via a carbocation intermediate. After the electrophilic H⁺ attaches to one carbon of the double bond, the other carbon loses the π electrons and becomes positively charged, forming a carbocation. The halide ion then quickly attacks this carbocation to complete the addition.

HX 的亲电加成经过碳正离子中间体。亲电的 H⁺ 连接到双键的一个碳上后，另一个碳失去 π 电子并带上正电荷，形成碳正离子。然后卤离子迅速进攻这个碳正离子，完成加成。

Carbocations are classified as primary (1°), secondary (2°) or tertiary (3°) based on the number of carbon atoms directly attached to the positively charged carbon. Stability increases in the order: 1° < 2° < 3°. This is because alkyl groups donate electron density through hyperconjugation, dispersing the positive charge.

碳正离子根据直接连在带正电碳上的碳原子数目，分为伯（1°）、仲（2°）或叔（3°）。稳定性顺序为：1° < 2° < 3°。这是因为烷基通过超共轭效应提供电子密度，分散了正电荷。

For Markovnikov addition to propene, the intermediate formed via H⁺ adding to the terminal CH₂ gives a secondary carbocation (CH₃CH⁺CH₃), which is more stable than the primary carbocation (CH₃CH₂CH₂⁺) formed if H⁺ adds to the internal carbon. The more stable carbocation leads to the major product.

对于丙烯的马氏加成，H⁺ 加在末端 CH₂ 上形成的中间体是仲碳正离子（CH₃CH⁺CH₃），这比 H⁺ 加在内部碳上形成的伯碳正离子（CH₃CH₂CH₂⁺）更稳定。更稳定的碳正离子导致主要产物。

8. Catalytic Hydrogenation of Alkenes | 烯烃的催化加氢

Addition of hydrogen (H₂) to an alkene is also an electrophilic addition, but it requires a metal catalyst, typically nickel, platinum or palladium, to lower the activation energy. The reaction converts an alkene into an alkane: the C=C double bond is reduced to a C–C single bond.

氢气（H₂）对烯烃的加成也是一种亲电加成，但需要金属催化剂，如镍、铂或钯，来降低活化能。反应将烯烃转化为烷烃：C=C 双键被还原为 C–C 单键。

C₂H₄ + H₂ → C₂H₆

The mechanism is heterogeneous – the reactants adsorb onto the catalyst surface, where the H–H bond breaks and the hydrogen atoms add stepwise to the alkene. For GCSE, you simply need to know the reagents (hydrogen gas, H₂), the conditions (nickel catalyst, 150°C), and that it is an addition reaction used industrially to harden unsaturated vegetable oils into margarine.

机理为非均相反应——反应物吸附在催化剂表面，H–H 键断裂，氢原子逐步加成到烯烃上。在 GCSE 阶段，你只需知道试剂（氢气，H₂）、条件（镍催化剂，150°C），以及这是一种用于将不饱和植物油硬化成人造黄油工业加成的反应。

Hydrogenation is not usually used as a test for unsaturation because there is no obvious colour change. Instead, it is a crucial transformation for modifying the physical properties of organic molecules.

加氢反应通常不用作不饱和性检验，因为没有明显的颜色变化。相反，它是改变有机分子物理性质的重要转化。

9. Addition of Concentrated Sulfuric Acid | 浓硫酸的加成

Alkenes react with cold concentrated sulfuric acid (H₂SO₄) to form alkyl hydrogen sulfates. The electrophile is the H⁺ from the acid, which adds to one carbon of the double bond, and the HSO₄⁻ ion adds to the other. This is another example of electrophilic addition that follows Markovnikov’s rule for unsymmetrical alkenes.

烯烃与冷的浓硫酸（H₂SO₄）反应生成硫酸氢烷基酯。亲电试剂是酸中的 H⁺，它加成到双键的一个碳上，HSO₄⁻ 离子加成到另一个碳上。这是亲电加成的又一个例子，对于不对称烯烃遵循马氏规则。

For ethene, the reaction gives ethyl hydrogen sulfate (CH₃CH₂OSO₂OH). This product can then be hydrolysed by warming with water to produce ethanol. This two‑step process is a historical method for industrial ethanol production, although nowadays direct hydration of ethene with steam and a phosphoric acid catalyst is preferred.

对于乙烯，反应生成硫酸氢乙酯（CH₃CH₂OSO₂OH）。该产物随后可与水加热水解生成乙醇。该两步法曾是工业制乙醇的历史方法，不过如今更倾向使用乙烯与蒸汽在磷酸催化下的直接水合。

In the OCR specification, you might see this reaction as an example of how alkenes can be converted into alcohols indirectly. It emphasises the versatility of electrophilic addition for making valuable functional groups.

在 OCR 大纲中，你可能会见到这个反应，作为烯烃如何间接转化为醇的例子。它突显了亲电加成在制备有用官能团方面的多样性。

10. Summary: Key Points to Remember | 总结：需牢记的要点

Electrophilic addition is the typical reaction of alkenes, driven by the high electron density of the C=C π bond. The attacking species is an electrophile – an electron‑deficient molecule or ion. Common examples include Br₂, HBr, H₂SO₄ and H₂ (with catalyst). The addition always leads to a saturated product where two species have added across the original double bond.

亲电加成是烯烃的典型反应，由 C=C π 键的高电子密度驱动。进攻物种是亲电试剂——一种缺电子的分子或离子。常见例子包括 Br₂、HBr、H₂SO₄ 和 H₂（有催化剂）。加成总是得到饱和产物，其中两个物种加在原双键两边。

Markovnikov’s rule helps predict the major product when unsymmetrical alkenes react with HX or water. The decolourisation of bromine water is a reliable test for the carbon–carbon double bond. Understanding the stepwise mechanism involving a carbocation intermediate provides a deeper insight into why the rule works, even if the GCSE syllabus may not demand a full electron‑pushing mechanism.

马氏规则有助于预测不对称烯烃与 HX 或水反应的主要产物。溴水褪色是检验碳碳双键的可靠方法。理解包含碳正离子中间体的分步机理，能提供对规则为何有效的更深洞见，即便 GCSE 大纲可能不要求完整的电子转移机理。

Remember the conditions for hydrogenation, the use of the bromine water test, and the significance of electrophilic addition in converting unsaturated compounds into useful products like alcohols and haloalkanes. Always identify the electrophile first when tackling an addition problem.

牢记加氢的条件、溴水试验的用途，以及亲电加成在将不饱和化合物转化为醇和卤代烷等有用产物方面的意义。在处理加成问题时，务必首先识别亲电试剂。

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Electrophilic Addition | 亲电加成考点精讲