Electrophilic Addition in AS Chemistry: A Comprehensive Guide | AS化学亲电加成考点精讲

📚 Electrophilic Addition in AS Chemistry: A Comprehensive Guide | AS化学亲电加成考点精讲

Electrophilic addition is a cornerstone of organic reaction mechanisms in AS Chemistry. It explains how alkenes, with their electron-rich double bonds, react with species that seek electrons. Mastering this topic is essential for students aiming to excel in their exams, as it underpins organic synthesis, reaction conditions, and mechanistic reasoning. This guide will break down the key concepts, mechanisms, and test-taking strategies for electrophilic addition reactions.

亲电加成是AS化学有机反应机理的基石。它解释了具有富电子双键的烯烃如何与寻找电子的物种发生反应。掌握这一专题是学生考试取得优异成绩的关键，因为它贯穿有机合成、反应条件和机理推理。本文将系统梳理亲电加成反应的核心概念、机理及应试策略。

1. Definition of Electrophilic Addition | 亲电加成的定义

An electrophilic addition reaction is one in which an electrophile (electron-loving species) attacks an unsaturated centre, typically the electron-rich π bond of an alkene, resulting in the breaking of the π bond and the formation of two new sigma (σ) bonds. The unsaturated molecule becomes saturated.

亲电加成反应是指亲电试剂（缺电子物种）进攻不饱和中心，通常是烯烃富电子的π键，导致π键断裂并形成两个新的σ键。不饱和分子由此变为饱和分子。

2. Why Alkenes Undergo Electrophilic Addition | 为什么烯烃发生亲电加成？

The carbon-carbon double bond comprises a strong σ bond and a weaker π bond. The π electrons are held less tightly by the carbon nuclei and lie above and below the plane of the molecule, making them readily available to an electrophile. Thus, alkenes act as electron-pair donors (nucleophiles) toward electrophiles.

碳碳双键由一个较强的σ键和一个较弱的π键构成。π电子被碳核束缚较弱，位于分子平面的上方和下方，因此容易提供给亲电试剂。因此，烯烃对亲电试剂充当电子对供体（亲核试剂）。

3. Common Electrophiles | 常见亲电试剂

In AS Chemistry, the typical electrophiles encountered are: hydrogen halides (HBr, HCl, HI), halogens (Br₂, Cl₂), concentrated sulfuric acid (H₂SO₄), and the proton (H⁺) in acid-catalysed hydration. Some of these molecules may need to be polarised or carry a formal positive charge to act as electrophiles.

AS化学中常见的亲电试剂有：卤化氢（HBr、HCl、HI）、卤素（Br₂、Cl₂）、浓硫酸（H₂SO₄）以及在酸催化水合反应中的质子（H⁺）。这些分子中的一些需要被极化或带有形式正电荷才能作为亲电试剂起作用。

4. Mechanism with Symmetrical Alkenes (e.g., Ethene + Br₂) | 对称烯烃的机理（如乙烯与溴）

The mechanism between ethene and bromine (Br₂) is a classic example. As Br₂ approaches the ethene molecule, the π electrons repel the electron cloud of the Br–Br bond, inducing a dipole. The bond

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