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 of alkenes, where the carbon–carbon double bond opens up to allow two new groups to add across the unsaturated carbons. Mastering this topic is essential for GCSE Chemistry, as it bridges structure, bonding, organic reaction mechanisms, and practical tests for unsaturation.

亲电加成是烯烃的特征反应，碳碳双键打开，让两个新基团加成到不饱和碳原子上。掌握这个主题对 GCSE 化学至关重要，因为它将结构、键合、有机反应机理和不饱和度的检测实验联系在一起。

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

Electrophilic addition is a reaction in which an electrophile attacks a region of high electron density in an unsaturated molecule, leading to the addition of two atoms or groups across a double bond. The pi bond breaks, and two new sigma bonds are formed.

亲电加成是一种反应，亲电试剂进攻不饱和分子中电子密度高的区域，导致两个原子或基团加成到双键上。π键断裂，形成两个新的σ键。

Because alkenes have a double bond consisting of a strong sigma bond and a weaker pi bond, the pi electrons are exposed and readily attract species that are electron-deficient. This makes alkenes much more reactive than alkanes.

由于烯烃的双键由一个牢固的σ键和一个较弱的π键组成，π电子暴露在外，容易吸引缺电子的物种。这使得烯烃的反应活性远高于烷烃。

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2. The Carbon–Carbon Double Bond | 碳碳双键

The C=C double bond in ethene (C₂H₄) and other alkenes is made up of one sigma (σ) bond and one pi (π) bond. The sigma bond is formed by the overlap of sp² hybrid orbitals head‑on, while the pi bond comes from the sideways overlap of adjacent p orbitals.

乙烯（C₂H₄）及其他烯烃中的 C=C 双键由一个σ键和一个π键组成。σ键由 sp² 杂化轨道正面重叠形成，而π键来自相邻 p 轨道的侧面重叠。

The pi bond projects above and below the plane of the molecule, creating a cloud of loosely held electrons. This cloud is the source of the high electron density that makes the alkene vulnerable to attack by electrophiles.

π键分布在该分子平面的上方和下方，形成一片束缚较松的电子云。这片电子云正是高电子密度的来源，使烯烃容易被亲电试剂进攻。

• Sigma bond: strong, symmetrical, holds the carbons close.
• Pi bond: weaker, exposed, easily broken during addition.
• σ键： 强，对称，使碳原子紧密结合。
• π键： 较弱，暴露在外，容易在加成反应中断裂。

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3. Electrophiles: Electron Seekers | 亲电试剂：电子寻求者

An electrophile is a species that is attracted to regions of high electron density. Electrophiles are often positively charged ions (e.g. H⁺, NO₂⁺) or neutral molecules with a partial positive charge on one atom (e.g. the bromine molecule Br₂ polarised by a nearby double bond).

亲电试剂是被高电子密度区域吸引的物种。亲电试剂通常是带正电的离子（如 H⁺、NO₂⁺）或在一个原子上带有部分正电荷的中性分子（如被附近双键极化的溴分子 Br₂）。

Common electrophiles in GCSE chemistry include:

• Bromine, Br₂ (polarised or as Br⁺ in polar solvents)
• Hydrogen halides: HBr, HCl, HI (the H⁺ part acts as the electrophile)
• Concentrated sulfuric acid: H₂SO₄ (H⁺ attacks the double bond)
• 溴，Br₂（在极性溶剂中被极化或形成 Br⁺）
• 卤化氢：HBr、HCl、HI（H⁺ 部分起亲电试剂作用）
• 浓硫酸：H₂SO₄（H⁺ 进攻双键）

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4. Mechanism of Electrophilic Addition – Step 1: Attack by the Electrophile | 亲电加成机理 – 第一步：亲电进攻

The first step involves the pi electrons of the double bond moving towards the electrophile to form a new bond. This step requires the breaking of the pi bond and the generation of a positively charged intermediate called a carbocation.

第一步涉及双键的π电子流向亲电试剂，形成一个新键。这一步需要 π 键断裂并生成一个带正电的中间体，称为碳正离子。

For example, when HBr adds to ethene:

CH₂=CH₂ + H⁺ → CH₃–C⁺H₂

The hydrogen ion (the electrophile) attaches to one carbon, leaving the other carbon with a positive charge. The bromide ion, Br⁻, is released but not yet bonded.

例如，HBr 与乙烯加成时：氢离子（亲电试剂）连接到一个碳上，使另一个碳带正电荷。溴离子 Br⁻ 释放出来，但尚未成键。

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5. Mechanism of Electrophilic Addition – Step 2: Attack by the Nucleophile | 亲电加成机理 – 第二步：亲核进攻

The negatively charged ion (or the nucleophilic part of the reagent) quickly attacks the carbocation, donating a pair of electrons to form the second new sigma bond. This step completes the addition.

带负电的离子（或试剂的亲核部分）迅速进攻碳正离子，提供一对电子形成第二个新的σ键。这一步完成了加成。

Using HBr and ethene again:

CH₃–C⁺H₂ + Br⁻ → CH₃–CH₂Br

The product is bromoethane. Overall, the double bond has been converted to a single bond, and two substituents have been added across the original two carbons.

产物是溴乙烷。总体上，双键转变为单键，两个取代基加到了原来的两个碳上。

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6. Reaction with Bromine (Br₂) | 与溴的反应

When an alkene is bubbled through bromine water (orange-brown), the solution rapidly decolourises. This happens because bromine adds across the C=C bond to form a dibromoalkane, which is colourless.

当烯烃通入溴水（橙棕色）时，溶液迅速褪色。这是因为溴加成到 C=C 双键上，生成无色的二溴代烷烃。

Mechanism: The approaching double bond polarises the Br–Br molecule. The pi electrons attack the slightly positive bromine atom. This forms a cyclic bromonium ion intermediate (or a carbocation in simpler GCSE terms), which then is opened by the Br⁻ ion to yield 1,2-dibromoethane from ethene.

机理：靠近的双键使 Br–Br 分子极化。π电子进攻略带正电的溴原子，形成环状溴鎓离子中间体（在更简单的 GCSE 表述中为碳正离子），然后被 Br⁻ 离子进攻开环，从乙烯得到 1,2-二溴乙烷。

C₂H₄ + Br₂ → C₂H₄Br₂

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7. Bromine Water Test for Unsaturation | 溴水试验检测不饱和度

The bromine water test is a simple and reliable chemical test to distinguish alkenes from alkanes. Alkanes do not react with bromine water under normal conditions, so the orange colour persists. Alkenes, on the other hand, cause an instant colour change from orange to colourless.

溴水试验是一种简单可靠的化学试验，可区分烯烃和烷烃。烷烃在正常条件下不与溴水反应，因此橙色保持不变。而烯烃会使溶液瞬间从橙色变为无色。

Substance | 物质	Result with bromine water | 溴水试验结果	Explanation | 解释
Alkane (e.g. hexane) | 烷烃（如己烷）	Remains orange | 保持橙棕色	No C=C bond, no addition | 无 C=C 键，不发生加成
Alkene (e.g. ethene) | 烯烃（如乙烯）	Turns colourless immediately | 立即变为无色	Electrophilic addition across C=C | 在 C=C 双键上发生亲电加成

This test is often used in GCSE practicals to identify unsaturated oils or to monitor the conversion of alkenes during cracking.

该试验常用于 GCSE 实验中鉴定不饱和油脂或监测裂化过程中烯烃的转化。

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8. Reaction with Hydrogen Halides (HX) | 与卤化氢的反应

Alkenes react with hydrogen halides such as HCl, HBr, and HI at room temperature to form halogenoalkanes. The hydrogen ion (H⁺) from the H–X molecule acts as the electrophile, and the halide ion (X⁻) adds in the second step.

烯烃在室温下与 HCl、HBr、HI 等卤化氢反应生成卤代烷。H–X 分子中的氢离子（H⁺）作为亲电试剂，卤离子（X⁻）在第二步加成。

General equation:

CₙH₂ₙ + HX → CₙH₂ₙ₊₁X

With an unsymmetrical alkene like propene (CH₃–CH=CH₂), the addition of HX can give two possible products. The major product is determined by the stability of the intermediate carbocation.

对于不对称烯烃如丙烯（CH₃–CH=CH₂），HX 的加成可能得到两种产物。主要产物取决于中间体碳正离子的稳定性。

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9. Markovnikov’s Rule – Who Gets the Hydrogen? | 马氏规则 – 氢加到谁身上？

When adding HX to an unsymmetrical alkene, the hydrogen atom attaches to the carbon that already has the greater number of hydrogen atoms (the less substituted carbon). This is Markovnikov’s rule and arises because the more stable carbocation is formed preferentially during the mechanism.

将 HX 加成到不对称烯烃上时，氢原子会连接到原来连有更多氢原子的碳上（取代较少的碳）。这就是马氏规则，其原因是反应机理中优先形成更稳定的碳正离子。

For propene with HBr:

CH₃–CH=CH₂ + HBr → CH₃–CHBr–CH₃ (major) + CH₃–CH₂–CH₂Br (minor)

The major product is 2-bromopropane because the secondary carbocation (CH₃–C⁺H–CH₃) is more stable than the primary carbocation (CH₃–CH₂–C⁺H₂).

主要产物是 2-溴丙烷，因为二级碳正离子（CH₃–C⁺H–CH₃）比一级碳正离子（CH₃–CH₂–C⁺H₂）更稳定。

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10. Reaction with Concentrated Sulfuric Acid | 与浓硫酸的反应

Alkenes can react with cold concentrated sulfuric acid (H₂SO₄) to form alkyl hydrogen sulfates. This is also an electrophilic addition where the H⁺ from the acid acts as the electrophile, adding across the double bond, followed by the HSO₄⁻ ion attacking the carbocation.

烯烃能与冷的浓硫酸（H₂SO₄）反应生成硫酸氢烷基酯。这也是一种亲电加成，酸中的 H⁺ 作为亲电试剂加成到双键上，然后 HSO₄⁻ 离子进攻碳正离子。

Ethene with H₂SO₄:

C₂H₄ + H₂SO₄ → C₂H₅OSO₃H (ethyl hydrogen sulfate)

This product can later be hydrolysed by water to form ethanol. This indirect hydration of ethene is an important industrial route to alcohols.

该产物随后可通过水解生成乙醇。这种乙烯的间接水合作用是工业上制备醇的重要途径。

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11. Summary of Electrophilic Addition Reactions | 亲电加成反应总结

Reactant | 反应物	Reagent | 试剂	Product type | 产物类型	Key condition | 关键条件
Alkene	Br₂ in water or organic solvent	Dibromoalkane	Room temp., dark (to avoid free radical substitution)
Alkene	HBr / HCl	Halogenoalkane	Room temp.
Alkene	Conc. H₂SO₄	Alkyl hydrogen sulfate	Cold
Alkene	H₂O (with acid catalyst)	Alcohol	High temp., pressure, H₃PO₄ catalyst

All these reactions share the same general mechanistic pattern: electrophilic attack on the pi bond, formation of a carbocation (or similar intermediate), and rapid nucleophilic capture of the intermediate. The overall result is the conversion of a C=C double bond to a C–C single bond with two new sigma bonds.

所有这些反应都遵循相同的基本机理模式：亲电试剂进攻π键，形成碳正离子（或类似中间体），然后亲核试剂快速捕获中间体。总体结果是 C=C 双键转变为 C–C 单键并形成两个新的σ键。

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12. Exam Tips and Common Mistakes | 考试技巧与常见错误

• Always show the intermediate carbocation if the question asks for a mechanism. GCSE marks are awarded for correct charges and curly arrows from the pi bond to the electrophile and from the nucleophile to the carbocation. Use clear diagrams in your exam.
• 如果题目要求画出机理，一定要显示碳正离子中间体。 GCSE 评分会奖励正确的电荷标记以及从π键到亲电试剂、从亲核试剂到碳正离子的弯曲箭头。考试中务必使用清晰的图示。
• Don’t confuse addition with substitution. Alkanes undergo free‑radical substitution with halogens (requires UV light), whereas alkenes undergo electrophilic addition rapidly in the dark.
• 不要混淆加成与取代。 烷烃与卤素发生自由基取代（需要紫外光），而烯烃在黑暗条件下迅速发生亲电加成。
• Watch out for unsymmetrical alkenes and Markovnikov products. Practise drawing both possible carbocations and identify which is more stable (tertiary > secondary > primary).
• 注意不对称烯烃和马氏产物。 练习画出两种可能的碳正离子，并判断哪种更稳定（三级 > 二级 > 一级）。
• The colour change of bromine water is a favourite GCSE test. Describe it accurately: ‘orange‑brown to colourless’ for alkenes. Say ‘remains orange‑brown’ for alkanes.
• 溴水的颜色变化 是 GCSE 常考的热点。准确描述：烯烃使溴水“从橙棕色变为无色”，烷烃则“保持橙棕色”。
• Balance equations carefully. Count the carbons and halogens atoms correctly in addition products.
• 仔细配平方程式。 加成产物的碳原子和卤原子数量要数对。

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