Electrophilic Addition in Alkenes (GCSE CCEA) | 烯烃的亲电加成考点精讲

📚 Electrophilic Addition in Alkenes (GCSE CCEA) | 烯烃的亲电加成考点精讲

Electrophilic addition is the characteristic reaction of alkenes, where an electron‑deficient species (electrophile) attacks the carbon‑carbon double bond. This topic appears frequently in GCSE CCEA Chemistry, so mastering the mechanism, conditions, and major products will secure top marks.

亲电加成是烯烃的特征反应，缺电子的物种（亲电试剂）进攻碳碳双键。这一主题在 GCSE CCEA 化学中频繁出现，掌握反应机理、条件与主产物是拿高分的关键。

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

Electrophilic addition is a two‑step reaction in which the π bond of an alkene attracts an electrophile, leading to the formation of a more saturated product. The electrophile (‘electron‑lover’) is a species that accepts an electron pair from the alkene.

亲电加成是一种两步反应，烯烃的π键吸引亲电试剂，生成更饱和的产物。亲电试剂（‘电子爱好者’）是从烯烃接受一对电子的物种。

Typical electrophiles include Br⁺ (from polarised Br₂), H⁺ (from HBr), and H⁺ (from H₂O with an acid catalyst). The reaction involves breaking the weak π bond and forming two new σ bonds.

典型的亲电试剂包括 Br⁺（来自极化的 Br₂）、H⁺（来自 HBr）和 H⁺（酸催化下的 H₂O）。反应过程中弱的π键断裂，形成两个新的σ键。

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2. Alkenes and the Pi Bond | 烯烃与π键

Alkenes contain a C═C double bond made of one σ bond and one π bond. The π bond is formed by sideways overlap of p orbitals and is weaker and more exposed than the σ bond. This makes the π electrons available for attack by electrophiles.

烯烃含有 C═C 双键，由一个σ键和一个π键组成。π键由 p 轨道的侧面重叠形成，比σ键更弱且更暴露，因此π电子容易被亲电试剂进攻。

Because the π bond is an area of high electron density, alkenes behave as nucleophiles in electrophilic addition, donating electrons to the electrophile.

由于π键区域具有高电子密度，烯烃在亲电加成中充当亲核试剂，向亲电试剂提供电子。

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3. The General Two‑Step Mechanism | 一般两步反应机理

Step 1: The electrophile (E⁺) is attracted to the π electrons. A pair of π electrons moves to form a new bond between the electrophile and a carbon atom. The other carbon becomes electron‑deficient, forming a carbocation intermediate. The electrophile often comes from a polarised reagent, e.g. H–Br → H⁺ + Br⁻.

第一步：亲电试剂（E⁺）被π电子吸引。一对π电子移动，在亲电试剂和一个碳原子之间形成新键。另一个碳原子变得缺电子，形成碳正离子中间体。亲电试剂通常来自极化试剂，如 H–Br → H⁺ + Br⁻。

Step 2: The negative ion (or nucleophile) quickly donates an electron pair to the carbocation, forming a second new bond. This completes the addition and gives the saturated product.

第二步：负离子（或亲核试剂）迅速向碳正离子提供一对电子，形成第二个新键，完成加成生成饱和产物。

For symmetrical alkenes (e.g. ethene), the carbocation formed is the same regardless of which carbon is attacked first. For unsymmetrical alkenes, different carbocations can be formed, which is where Markovnikov’s rule applies.

对于对称烯烃（如乙烯），无论首先进攻哪个碳，生成的碳正离子都相同。对于不对称烯烃，可以生成不同的碳正离子，这时就要应用马氏规则。

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4. Addition of Bromine (Br₂) | 与溴的加成

Alkenes react rapidly with bromine at room temperature in the dark, converting orange‑brown bromine to a colourless dibromoalkane. This reaction is used as a test for unsaturation (the bromine water test).

烯烃在室温、避光条件下与溴迅速反应，将橙棕色的溴转变为无色的二溴代烷。该反应用于不饱和性检验（溴水试验）。

The Br–Br molecule becomes polarised as it approaches the π electrons, inducing a dipole: Brᵟ⁺–Brᵟ⁻. The Brᵟ⁺ acts as the electrophile.

Br–Br 分子靠近π电子时发生极化，诱导出偶极：Brᵟ⁺–Brᵟ⁻。其中 Brᵟ⁺ 作为亲电试剂。

CH₂=CH₂ + Br₂ → CH₂BrCH₂Br

The mechanism proceeds via a cyclic bromonium ion for GCSE CCEA? Actually, at GCSE level, the simple carbocation pathway is often accepted, but some specifications may include the cyclic bromonium ion as the intermediate. For CCEA GCSE, the key point is the colour change and the formation of the dibromo product.

在 GCSE CCEA 层级，通常接受简单的碳正离子路径，但有些教学大纲可能引入环溴翁离子。对 CCEA 而言，重点是颜色变化和生成二溴代产物。

When bromine water is used, the competing reaction with water can produce a bromoalcohol as a minor product, but this is not required at GCSE level.

使用溴水时，与水的竞争反应可能生成少量溴代醇，但 GCSE 阶段不需要掌握。

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5. Addition of Hydrogen Halides (e.g. HBr) | 与卤化氢的加成（如 HBr）

Hydrogen halides (HCl, HBr, HI) add across the double bond. The H–X bond is polar, with hydrogen carrying a partial positive charge, making it the electrophile.

卤化氢（HCl、HBr、HI）会加在双键上。H–X 键是极性的，氢带部分正电荷，因此作为亲电试剂。

For ethene (symmetrical alkene), the product is simply a haloalkane:

对于乙烯（对称烯烃），产物简单生成卤代烷：

CH₂=CH₂ + HBr → CH₃CH₂Br

For unsymmetrical alkenes (e.g. propene), two possible products exist. Markovnikov’s rule is used to predict the major product.

对于不对称烯烃（如丙烯），有两种可能的产物。需要用马氏规则预测主要产物。

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6. Markovnikov’s Rule | 马氏规则

Markovnikov’s rule states: in the addition of HX or H₂O to an unsymmetrical alkene, the hydrogen atom adds to the carbon atom that already has the greater number of hydrogen atoms. In other words, the electrophile (H⁺) adds to the less substituted carbon, and the nucleophile adds to the more substituted carbon (the one bearing more carbon groups).

马氏规则指出：在 HX 或 H₂O 与不对称烯烃的加成中，氢原子加到氢原子数目较多的碳原子上。也就是说，亲电试剂 H⁺ 加到取代较少的碳上，亲核试剂加到取代较多的碳（带有更多碳基团）上。

This can be explained by the stability of the carbocation intermediate: tertiary carbocations (3° > 2° > 1°) are more stable than secondary or primary ones due to inductive effects and hyperconjugation. The pathway forming the more stable carbocation is favoured, leading to the Markovnikov product.

这可以用碳正离子中间体的稳定性来解释：由于诱导效应和超共轭，三级碳正离子（3° > 2° > 1°）比二级或一级更稳定。形成更稳定碳正离子的途径占优，从而得到马氏产物。

Example with propene and HBr:

以丙烯与 HBr 为例：

CH₃CH=CH₂ + HBr → CH₃CHBrCH₃ (major) + CH₃CH₂CH₂Br (minor)

The major product is 2‑bromopropane, not 1‑bromopropane, because the secondary carbocation is more stable than the primary one.

主要产物是 2-溴丙烷，而不是 1-溴丙烷，因为二级碳正离子比一级更稳定。

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7. Addition of Water (Hydration of Alkenes) | 与水的加成（烯烃水合）

Alkenes react with steam in the presence of a phosphoric(V) acid catalyst (or sulfuric acid) to produce alcohols. This is an industrially important reaction, e.g. manufacture of ethanol from ethene.

烯烃在磷酸（V）（或硫酸）催化剂存在下与水蒸气反应生成醇。这是一个重要的工业反应，如从乙烯制乙醇。

Conditions: concentrated phosphoric acid catalyst, high temperature (around 300 °C) and high pressure (60–70 atm). The mechanism is electrophilic addition with H⁺ acting as the electrophile.

条件：浓磷酸催化剂，高温（约 300 °C）和高压（60–70 atm）。机理是亲电加成，H⁺ 作为亲电试剂。

CH₂=CH₂ + H₂O (g) ⇌ CH₃CH₂OH

For unsymmetrical alkenes, Markovnikov’s rule applies: the OH group attaches to the more substituted carbon. For propene, propan‑2‑ol is the major product.

对于不对称烯烃，同样适用马氏规则：OH 基团接在取代更多的碳上。以丙烯为例，主要产物是 2-丙醇。

This reaction is reversible; the yield of alcohol is increased by using high pressure and removing the product as it forms.

该反应可逆；提高压力和及时移除产物可提高醇的产率。

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8. Testing for Unsaturation | 不饱和性检验

Alkenes decolourise bromine water (orange‑brown to colourless) rapidly. This is a standard test to distinguish alkenes from alkanes. No UV light or heating is required, unlike with alkanes.

烯烃能使溴水（橙棕色）迅速褪为无色。这是区分烯烃与烷烃的标准检验方法。与烷烃不同，无需紫外线或加热。

The decolourisation occurs because bromine adds across the double bond to form a colourless dibromo product. If the compound is an alkane, the colour persists until UV light initiates a radical substitution.

褪色是因为溴加成到双键上，生成无色的二溴代产物。若化合物为烷烃，则需要紫外光引发自由基取代，颜色才会变化。

In the CCEA exam, you may be asked to describe this test and give the expected colour change for a given unsaturated compound.

在 CCEA 考试中，可能要求描述该检验方法，并给出某一不饱和化合物预期的颜色变化。

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9. Summary and Common Exam Tips | 总结与常见答题技巧

Reaction	Reagent	Conditions	Product
Bromination	Br₂ (or bromine water)	Room temp., dark	Dibromoalkane
Hydrohalogenation	HBr/HCl/HI	Room temp.	Haloalkane (follow Markovnikov)
Hydration	H₂O (steam)	H₃PO₄ catalyst, 300 °C, 60‑70 atm	Alcohol (follow Markovnikov)

Exam tips: Always identify the electrophile clearly. For unsymmetrical alkenes, draw the carbocations to justify the major product. Do not confuse electrophilic addition with free radical substitution of alkanes. Balance equations properly and state the colour change when required.

答题技巧：始终明确亲电试剂。对于不对称烯烃，画出碳正离子以说明主要产物的合理性。不要将亲电加成与烷烃的自由基取代混淆。正确配平方程式，并在需要时描述颜色变化。

Remember, in GCSE CCEA Chemistry, the core electrophilic addition reactions involve ethene and propene as the main examples. Practise the mechanism using curly arrows in your diagrams — even if not always required, they help to secure understanding.

请记住，在 GCSE CCEA 化学中，核心的亲电加成反应主要以乙烯和丙烯为例。用图示练习弯箭头机理——即使不要求画出，也有助于巩固理解。

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