Reaction Mechanisms in AS Chemistry: Free-Radical Substitution, Electrophilic Addition, and Nucleophilic Substitution | AS 化学反应机理：自由基取代、亲电加成与亲核取代

📚 Reaction Mechanisms in AS Chemistry: Free-Radical Substitution, Electrophilic Addition, and Nucleophilic Substitution | AS 化学反应机理：自由基取代、亲电加成与亲核取代

Reaction mechanisms are the heart of organic chemistry. They describe the step-by-step movement of electrons during bond breaking and bond making, explaining how reactants transform into products. In AS Chemistry Unit 1, you need to master three key types of reaction mechanisms: free-radical substitution of alkanes, electrophilic addition of alkenes, and nucleophilic substitution of halogenoalkanes. These mechanisms appear frequently in exam questions, including the June 2019 Unit 1 paper, where students were asked to draw curly arrows, identify types of bond fission, and predict major products.

反应机理是有机化学的核心。它们描述了化学键断裂和形成过程中电子逐步移动的细节，从而解释了反应物如何转化为产物。在 AS 化学 Unit 1 中，你需要掌握三种关键的反应机理类型：烷烃的自由基取代、烯烃的亲电加成以及卤代烷的亲核取代。这些机理在考试中频繁出现,包括 2019 年 6 月的 Unit 1 试卷，其中要求学生画出弯箭头、识别键断裂的类型并预测主要产物。

1. What is a Reaction Mechanism? | 什么是反应机理？

A reaction mechanism shows the detailed pathway of a reaction, including the intermediates, transition states, and the movement of electrons shown by curly arrows. It helps chemists understand why certain products are favoured and how to control reaction conditions.

反应机理展示了反应的详细路径，包括中间体、过渡态以及用弯箭头表示的电子移动。它帮助化学家理解为何某种产物更易生成以及如何控制反应条件。

2. Curly Arrows and Bond Fission | 弯箭头与键的断裂

In mechanisms, a curly arrow starts from a lone pair or a bond and points to an electron-deficient site. A full arrow (↷) represents the movement of an electron pair, while a half-headed arrow (‘fish-hook’) shows the movement of a single electron, used in free-radical reactions. Bond breaking can be homolytic (each atom gets one electron) or heterolytic (one atom takes both electrons).

在机理中，弯箭头从孤对电子或化学键出发，指向缺电子的位置。全箭头（↷）表示一对电子的移动，而半箭头（鱼钩箭头）则用于表示自由基反应中单个电子的移动。键的断裂可以是均裂（每个原子得到一个电子）或异裂（一个原子带走两个电子）。

3. Free-Radical Substitution: The Basics | 自由基取代反应基础

Free-radical substitution occurs when alkanes react with halogens in the presence of ultraviolet (UV) light. For example, methane reacts with chlorine to form chloromethane and hydrogen chloride. The mechanism involves three stages: initiation, propagation, and termination.

自由基取代反应发生在烷烃与卤素在紫外光（UV）存在的条件下。例如，甲烷与氯气反应生成氯甲烷和氯化氢。该机理包含三个阶段：引发、增长和终止。

4. Initiation, Propagation, and Termination | 引发、增长与终止

Initiation: UV light supplies energy to break the weakest bond (Cl–Cl) homolytically, forming two chlorine radicals.

Cl–Cl → 2 Cl• (UV light)

Propagation: A chlorine radical abstracts a hydrogen atom from methane, producing an alkyl radical and HCl. The alkyl radical then reacts with another chlorine molecule, forming the product and regenerating a chlorine radical. These two steps repeat in a chain reaction.

Cl• + CH₄ → •CH₃ + HCl

•CH₃ + Cl₂ → CH₃Cl + Cl•

Termination: Two radicals combine to form a stable molecule, ending the chain. Possible terminations include:

Cl• + Cl• → Cl₂
•CH₃ + Cl• → CH₃Cl
•CH₃ + •CH₃ → C₂H₆

引发：紫外光提供能量使最弱的键（Cl–Cl）发生均裂，生成两个氯自由基。

增长：一个氯自由基从甲烷中夺取一个氢原子，生成烷基自由基和氯化氢。然后该烷基自由基与另一个氯分子反应，生成产物并再生一个氯自由基。这两个步骤循环进行，构成链式反应。

终止：两个自由基结合形成稳定分子，从而终止链式反应。可能的终止反应包括 Cl• + Cl• → Cl₂, •CH₃ + Cl• → CH₃Cl 和 •CH₃ + •CH₃ → C₂H₆。

5. Mechanism of Methane Chlorination in Detail | 甲烷氯代的详细机理

The overall reaction is a substitution because one hydrogen atom is replaced by chlorine. However, side reactions can produce a mixture of products such as dichloromethane, trichloromethane, and tetrachloromethane. In an exam, you must be able to write the propagation steps with the correct curly arrows and identify the homolytic bond-breaking steps.

总反应是取代反应，因为一个氢原子被氯原子取代。然而，副反应会产生二氯甲烷、三氯甲烷和四氯甲烷等混合物。在考试中，你必须能够用正确的弯箭头写出增长步骤，并识别均裂键断裂的步骤。

6. Electrophilic Addition of Alkenes | 烯烃的亲电加成

Electrophilic addition is the characteristic reaction of alkenes due to the electron-rich π bond. An electrophile (electron-loving species) attacks the double bond, leading to addition across it. Typical electrophiles include H⁺ (from HBr or H₂SO₄) and Brδ⁺ (from Br₂).

由于存在电子丰富的 π 键，亲电加成是烯烃的特征反应。亲电试剂（爱电子物种）进攻双键，导致加成反应发生在双键两端。典型的亲

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