📚 Electrophilic Addition: IGCSE AQA Chemistry Essentials | IGCSE AQA 化学：亲电加成 考点精讲
Welcome to this focused revision guide on electrophilic addition, a core reaction type for alkenes in IGCSE AQA Chemistry. Understanding this mechanism and its applications is essential for mastering organic chemistry.
欢迎阅读这篇关于亲电加成的重点复习指南，这是IGCSE AQA化学中烯烃的核心反应类型。理解这一机理及其应用对于掌握有机化学至关重要。
1. What Is Electrophilic Addition? | 什么是亲电加成？
Electrophilic addition is a reaction in which an electrophile attacks an electron-rich double bond in an alkene, resulting in the addition of atoms or groups across the double bond to form a saturated compound.
亲电加成是一种反应，其中亲电试剂进攻烯烃中富电子的双键，导致原子或基团加成到双键上，形成饱和化合物。
The term 'electrophile' means 'electron-loving'. Electrophiles are electron-deficient species that seek out regions of high electron density.
“亲电试剂”意为“喜好电子的”。亲电试剂是缺电子的物种，寻找电子密度高的区域。
ich is a region of high electron density due to the presence of π electrons. This makes them reactive towards electrophiles.
烯烃含有C=C双键，由于π电子的存在，这是一个高电子密度区域。这使得它们对亲电试剂具有反应活性。
2. Structure and Reactivity of Alkenes | 烯烃的结构与反应性
nsaturated hydrocarbons with the general formula CₙH₂ₙ. The functional group is the carbon–carbon double bond, C=C.
烯烃是不饱和烃，通式为CₙH₂ₙ。官能团是碳碳双键，C=C。
ng σ (sigma) bond and a weaker π (pi) bond. The π electrons sit above and below the plane of the molecule, forming an electron-rich cloud that is easily attacked by electrophiles.
C=C双键由一个强σ键和一个较弱的π键组成。π电子位于分子平面的上方和下方，形成一个容易受亲电试剂攻击的富电子云。
Because the π bond is weaker than the σ bond, it breaks first during addition reactions, allowing the alkene to become saturated without losing any atoms – typical of addition.
由于π键比σ键弱，它在加成反应中首先断裂，使烯烃变成饱和化合物而不会丢失原子——这是加成反应的典型特征。
3. Common Electrophiles | 常见的亲电试剂
In IGCSE AQA Chemistry, the key electrophiles you need to know are:
在IGCSE AQA化学中，你需要了解的关键亲电试剂有：
- Br₂ (bromine) – often used to test for unsaturation
- HBr (hydrogen bromide) – an example of a hydrogen halide
- H₂O (water) – but only in the presence of an acid catalyst (usually H₃PO₄ or H₂SO₄)
- H₂SO₄ (concentrated sulfuric acid) – adds across the double bond to form an alkyl hydrogensulfate
- Br₂（溴）——常用于不饱和度测试
- HBr（溴化氢）——卤化氢的一个例子
- H₂O（水）——但需要酸催化剂（通常是H₃PO₄或H₂SO₄）存在
- H₂SO₄（浓硫酸）——加成到双键上形成硫酸氢烷基酯
Each of these electrophiles is either already polarised or becomes polarised upon approaching the electron-rich double bond. For symmetrical molecules like Br₂, an induced dipole occurs when it approaches the alkene.
这些亲电试剂中的每一个要么已经是极化的，要么在接近富电子双键时被极化。对于像Br₂这样的对称分子，当它接近烯烃时会产生诱导偶极。
4. General Mechanism of Electrophilic Addition | 亲电加成的一般机理
The mechanism can be broken down into two main steps:
该机理可分为两个主要步骤：
Step 1: The electrophile is attracted to the double bond. The π electrons form a bond to the electrophile, generating a carbocation (a positively charged carbon intermediate) and a negatively charged leaving group.
第一步：亲电试剂被双键吸引。π电子与亲电试剂形成键，产生碳正离子（带正电的碳中间体）和一个带负电的离去基团。
Step 2: The negatively charged species (or nucleophile) rapidly attacks the carbocation, donating a pair of electrons to form a new σ bond. This completes the addition.
第二步：带负电的物种（或亲核试剂）迅速攻击碳正离子，提供一对电子形成一个新的σ键。这完成了加成。
This two-step process is typical for addition of asymmetrical electrophiles such as HBr. The formation of the carbocation intermediate influences the regioselectivity explained by Markovnikov's rule.
这种两步过程对于不对称亲电试剂（如HBr）的加成是典型的。碳正离子中间体的形成影响了区域选择性，这可由马氏规则解释。
5. Addition of Halogens: Bromination | 卤素加成：溴化
The addition of bromine to an alkene is an important visual test for unsaturation. Ethene reacts with bromine (Br₂) to form 1,2-dibromoethane.
溴对烯烃的加成是不饱和度的重要直观测试。乙烯与溴反应生成1,2-二溴乙烷。
Equation: C₂H₄ + Br₂ → C₂H₄Br₂
方程式：C₂H₄ + Br₂ → C₂H₄Br₂
When bromine water (orange-brown) is shaken with an alkene, it is decolourised. This is because the bromine adds across the double bond, consuming the coloured bromine molecules.
当溴水（橙棕色）与烯烃一起振摇时，会被脱色。这是因为溴加成到双键上，消耗了有色的溴分子。
The mechanism for Br₂ addition involves the polarisation of Br–Br as it approaches the double bond. The δ⁺ Br atom acts as the electrophile, forming a cyclic bromonium ion or proceeding via a carbocation, depending on conditions. For IGCSE, a simplified carbocation mechanism is often accepted.
Br₂加成的机理涉及Br–Br在接近双键时被极化。带δ⁺的Br原子充当亲电试剂，形成环状溴鎓离子或通过碳正离子进行，取决于条件。在IGCSE中，简化的碳正离子机理通常被接受。
6. Addition of Hydrogen Halides | 卤化氢的加成
omide (HBr) adds to alkenes to form bromoalkanes. For example, ethene + HBr → bromoethane.
溴化氢（HBr）加成到烯烃上形成溴代烷。例如，乙烯 + HBr → 溴乙烷。
HBr is already a polar molecule: H–Br is polarised as Hᵟ⁺–Brᵟ⁻. The Hᵟ⁺ acts as the electrophile. After the π bond attacks the H⁺, a carbocation is formed. The Br⁻ then combines with the carbocation.
HBr已经是极性分子：H–Br极化，带Hᵟ⁺–Brᵟ⁻。Hᵟ⁺作为亲电试剂。在π键攻击H⁺之后，形成碳正离子。然后Br⁻与碳正离子结合。
With unsymmetrical alkenes like propene, addition of HBr can produce two isomeric products: 1-bromopropane and 2-bromopropane. The major product is predicted by Markovnikov's rule.
对于不对称烯烃如丙烯，HBr的加成可产生两种异构产物：1-溴丙烷和2-溴丙烷。主要产物由马氏规则预测。
7. Markovnikov's Rule | 马氏规则
Markovnikov's rule states that in the addition of a protic acid HX to an unsymmetrical alkene, the hydrogen atom attaches to the carbon with the greater number of hydrogen atoms already attached. In other words, 'the rich get richer'.
马氏规则指出，在质子酸HX加成到不对称烯烃时，氢原子连接到已经连接了较多氢原子的碳上。换句话说，“富者愈富”。
This effect occurs because the reaction proceeds via the most stable carbocation intermediate. Carbocations are stabilised by alkyl groups that donate electron density. The stability order is: tertiary (3°) > secondary (2°) > primary (1°).
这种效应发生是因为反应经过最稳定的碳正离子中间体。碳正离子通过提供电子密度的烷基而稳定。稳定性顺序为：叔(3°) > 仲(2°) > 伯(1°)。
For propene: CH₃–CH=CH₂ + HBr → CH₃–CHBr–CH₃ (2-bromopropane) as the major product, not CH₃–CH₂–r (1-bromopropane), because the secondary carbocation is more stable than the primary one.
对于丙烯：CH₃–CH=CH₂ + HBr → CH₃–CHBr–CH₃（2-溴丙烷）作为主要产物，而不是CH₃–CH₂–CH₂Br（1-溴丙烷），因为二级碳正离子比一级碳正离子更稳定。
8. Addition of Water: Hydration of Alkenes | 水的加成：烯烃的水合
Alkenes can be converted into alcohols by direct hydration with steam in the presence of an acid catalyst (usually phosphoric acid, H₃PO₄, on a solid support).
烯烃可以通过在酸催化剂（通常是磷酸，H₃PO₄，于固体载体上）存在下与水蒸气直接水合转化为醇。
Conditions: temperature around 300 °C, pressure of 60–70 atm. The reaction is reversible, so high pressure and excess steam are used to favour the forward reaction.
条件：温度约300°C，压力60–70 atm。该反应是可逆的，因此使用高压和过量水蒸气来促进正反应。
Equation: C₂H₄ + H₂O ⇌ C₂H₅OH (ethanol)
方程式：C₂H₄ + H₂O ⇌ C₂H₅OH（乙醇）
This is a key industrial process for making ethanol. The mechanism follows electrophilic addition: H⁺ from the acid catalyst attaches to one carbon of the double bond, forming a carbocation. Water then attacks the carbocation, and deprotonation gives the alcohol, regenerating the H⁺ catalyst.
这是工业制乙醇的关键过程。机理遵循亲电加成：来自酸催化剂的H⁺连接到双键的一个碳上，形成碳正离子。然后水攻击碳正离子，去质子化得到醇，同时再生H⁺催化剂。
9. Addition of Concentrated Sulfuric Acid | 浓硫酸的加成
Concentrated sulfuric acid (H₂SO₄) reacts with alkenes in an electrophilic addition reaction. For example, ethene reacts to form ethyl hydrogensulfate (C₂H₅OSO₃H).
浓硫酸（H₂SO₄）与烯烃发生亲电加成反应。例如，乙烯反应生成硫酸氢乙酯（C₂H₅OSO₃H）。
The mechanism: the alkene π bond attacks a proton from H₂SO₄, generating a carbocation and HSO₄⁻. The HSO₄⁻ then attacks the carbocation, forming the alkyl hydrogensulfate.
机理：烯
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