📚 IGCSE Chemistry 0620 Reaction Mechanisms (2026-2028 Syllabus) | IGCSE 化学 0620 反应机理 (2026-2028 大纲)
Reaction mechanisms describe, step by step, how bonds are broken and formed during a chemical reaction. For Cambridge IGCSE Chemistry 0620 (2026-2028), you need to understand two key organic mechanisms: free radical substitution in alkanes and electrophilic addition in alkenes. This knowledge helps you predict products and explain observations such as the decolourisation of bromine water.
反应机理逐步描述了化学反应中化学键的断裂和形成过程。针对剑桥 IGCSE 化学 0620（2026-2028 年大纲），你需要掌握两种关键的有机反应机理：烷烃的自由基取代反应和烯烃的亲电加成反应。这些知识能帮助你预测产物，并解释如溴水褪色等实验现象。
1. What are Reaction Mechanisms? | 什么是反应机理？
A reaction mechanism is a detailed sequence of elementary steps that shows the movement of electrons and the breaking/forming of bonds. It uses curly arrows to indicate electron pair movement, and half-headed (fish-hook) arrows for single electron movement. Understanding mechanisms allows chemists to control reactions and design new synthetic pathways.
反应机理是一系列基元步骤的详细顺序，展示了电子的移动以及化学键的断裂和形成。它使用弯曲箭头表示电子对的移动，用半箭头（鱼钩箭头）表示单个电子的移动。理解反应机理使化学家能够控制反应并设计新的合成路线。
In IGCSE, mechanism questions focus on applying these concepts to specific reactions: chlorination of methane and bromination of alkenes. You must be able to draw or interpret curly arrows and identify the type of bond fission involved.
在 IGCSE 中，机理题的重点是将这些概念应用到具体反应中：甲烷的氯化和烯烃的溴化。你必须能够画出或识别弯曲箭头，并判断涉及的是哪种类型的键断裂。
2. Homolytic and Heterolytic Fission | 均裂与异裂
Bond breaking is the first step in any reaction. When a covalent bond breaks, it can do so in two ways. Homolytic fission occurs when the bond breaks symmetrically, with each atom taking one electron from the shared pair, forming two free radicals. Free radicals are neutral, highly reactive species with an unpaired electron, shown by a dot (e.g., Cl•).
键的断裂是任何反应的第一步。共价键断裂有两种方式。均裂发生在键对称断裂时，每个原子从共用电子对中取走一个电子，形成两个自由基。自由基是中性、高活性、具有未成对电子的物种，用一个圆点表示（如 Cl•）。
Heterolytic fission is unsymmetrical: both electrons from the bond move to one atom, creating a positive ion (cation) and a negative ion (anion). This type of fission is common in electrophilic addition, where a polarised molecule like Br₂ generates Br⁺ and Br⁻ in the presence of a double bond.
异裂是不对称的：键中的两个电子都转移到其中一个原子上，形成一个正离子（阳离子）和一个负离子（阴离子）。这种断裂方式常见于亲电加成反应中，例如极化的溴分子 Br₂ 在双键存在时会产生 Br⁺ 和 Br⁻。
Cl-Cl → 2Cl• (homolytic) H-Cl → H⁺ + Cl⁻ (heterolytic)
Cl-Cl → 2Cl•（均裂） H-Cl → H⁺ + Cl⁻（异裂）
3. Free Radical Substitution: Overview | 自由基取代反应概述
Alkanes are generally unreactive, but they undergo substitution with halogens in the presence of ultraviolet (UV) light. This reaction is a chain reaction involving free radicals, with three stages: initiation, propagation, and termination. The overall equation for methane and chlorine is:
烷烃通常不活泼，但在紫外光照射下能与卤素发生取代反应。这是一个涉及自由基的链式反应，包含三个阶段：链引发、链增长和链终止。甲烷与氯气的总反应方程式为：
CH₄ + Cl₂ → CH₃Cl + HCl
The reaction requires UV light to break the Cl-Cl bond. Without UV light, the reaction does not occur, or is extremely slow. You must learn to write equations for each stage using displayed formulae or structural diagrams.
反应需要紫外光来断裂 Cl-Cl 键。没有紫外光，反应不会发生或极其缓慢。你必须学会用展示式或结构式写出每个阶段的方程式。
4. Initiation Step | 链引发步骤
Initiation starts the chain. A chlorine molecule absorbs UV energy and undergoes homolytic fission to form two chlorine free radicals. This step requires only a small amount of energy and is the reason UV light is necessary.
链引发启动了整个反应链。一个氯气分子吸收紫外光能量，发生均裂生成两个氯自由基。这一步只需要少量能量，这也是为什么必须使用紫外光。
Cl₂ → 2Cl• (shown with a half-headed curly arrow from the bond to each Cl atom)
Cl₂ → 2Cl• （用从键指向每个 Cl 原子的半箭头表示）
In an exam, you may be asked to draw this step using fish-hook arrows. Remember that each arrow represents the movement of one electron. Start the arrow at the bond and point to the chlorine atom it joins.
在考试中，你可能需要画出这一步骤并使用鱼钩箭头。请记住每个箭头代表一个电子的移动。箭头起始于化学键，指向结合的氯原子。
5. Propagation Steps | 链增长步骤
Propagation steps are those in which one radical reacts to produce another radical, keeping the chain going. For methane chlorination, there are two propagation steps. First, a chlorine radical attacks a methane molecule, abstracting a hydrogen atom. This forms hydrogen chloride and a methyl radical.
链增长步骤是自由基反应产生另一个自由基的步骤，使反应链持续进行。在甲烷氯化中，有两个链增长步骤。首先，一个氯自由基攻击甲烷分子，夺取一个氢原子，生成氯化氢和一个甲基自由基。
Cl• + CH₄ → HCl + •CH₃
Second, the methyl radical reacts with a chlorine molecule, producing chloromethane and regenerating a chlorine radical. This new Cl• can then react with another CH₄, continuing the chain.
其次，甲基自由基与一个氯气分子反应，生成一氯甲烷并重新生成一个氯自由基。这个新生成的 Cl• 可以继续与另一个 CH₄ 反应，使链持续下去。
•CH₃ + Cl₂ → CH₃Cl + Cl•
Notice that radicals are regenerated in each step, which is why a single initiation event can lead to thousands of product molecules. The propagation phase is the heart of the chain reaction.
注意在每个步骤中自由基都会被重新生成，因此一次引发事件可以导致成千上万个产物分子的生成。链增长阶段是整个链式反应的核心。
6. Termination Steps | 链终止步骤
Termination occurs when two radicals combine to form a stable molecule, removing radicals from the system and stopping the chain. Several termination reactions are possible:
当两个自由基结合形成一个稳定分子时，就会发生链终止，从体系中移除自由基并使反应链停止。可能存在多种终止反应：
Cl• + Cl• → Cl₂
Cl• + •CH₃ → CH₃Cl
•CH₃ + •CH₃ → C₂H₆
These reactions explain why minor products like ethane are sometimes detected. In the exam, you may be asked to suggest other possible termination products, so always consider combinations of the radicals present.
这些反应可以解释为什么会检测到如乙烷等副产物。在考试中，你可能会被要求提出其他可能的链终止产物，因此要始终从体系中存在的自由基的组合来思考。
7. Electrophilic Addition: Introduction | 亲电加成反应简介
Alkenes contain a carbon–carbon double bond, which is a region of high electron density. This makes alkenes attractive to electrophiles (electron-loving species). An electrophile is an electron-deficient atom or ion that can accept a pair of electrons to form a new bond. In IGCSE, the main electrophilic addition reactions are with bromine (Br₂) and hydrogen halides (e.g., HBr).
烯烃含有碳碳双键，这是一个电子密度较高的区域。这使得烯烃对亲电试剂（缺电子的物种）具有吸引力。亲电试剂是一种缺电子的原子或离子，能够接受一对电子形成新键。在 IGCSE 中，主要的亲电加成反应是与溴（Br₂）和卤化氢（如 HBr）的反应。
The reaction with bromine is used as a test for unsaturation: orange bromine water is decolourised when shaken with an alkene. The product is a colourless dibromoalkane.
与溴的反应被用作不饱和键的检验：当与烯烃一起振荡时，橙色的溴水会褪色。产物是无色的二溴代烷烃。
8. Mechanism of Electrophilic Addition with Bromine | 与溴的亲电加成机理
The addition of Br₂ to ethene proceeds via heterolytic fission, not homolytic fission. As the Br₂ molecule approaches the electron-rich double bond, the bond polarises, and the electrons in the double bond attack the slightly positive bromine atom. This causes the Br–Br bond to break heterolytically: one bromine takes both electrons, becoming Br⁻, while the other bromine attaches to a carbon, forming a carbocation intermediate.
Br₂ 与乙烯的加成反应通过异裂而非均裂进行。当 Br₂ 分子靠近富电子的双键时，Br－Br 键发生极化，双键中的电子攻击略带正电的溴原子，导致 Br－Br 键发生异裂：一个溴原子带走一对电子形成 Br⁻，另一个溴原子连接到一个碳上，形成一个碳正离子中间体。
The curly arrow starts from the centre of the C=C bond and points to the Br atom that will become Br⁻. A second arrow shows the Br–Br bond electrons moving entirely onto that same bromine. Then, the bromide ion, Br⁻, acts as a nucleophile and attacks the carbocation, forming the second C–Br bond.
弯曲箭头从 C=C 双键的中间出发，指向将成为 Br⁻ 的那个溴原子。第二个箭头表示 Br–Br 键的电子完全转移到同一个溴原子上。然后，溴离子 Br⁻ 作为亲核试剂进攻碳正离子，形成第二个 C－Br 键。
C₂H₄ + Br₂ → BrCH₂CH₂⁺ + Br⁻ → CH₂BrCH₂Br
This stepwise mechanism explains the anti-addition stereochemistry often observed, but IGCSE does not require stereochemistry details. You should be able to draw curly arrows and the structure of the carbocation for symmetrical alkenes.
这种逐步进行的机理解释了通常观察到的反式加成立体化学，但 IGCSE 不要求立体化学细节。你应该能画出对称烯烃的弯曲箭头以及碳正离子的结构。
9. Electrophilic Addition with Hydrogen Halides | 与卤化氢的亲电加成
Hydrogen halides such as HBr add across the double bond of an alkene. HBr is a polar molecule: Hδ⁺–Brδ⁻. The hydrogen atom, being electron-deficient, acts as the electrophile. The double bond attacks the H atom, breaking the H–Br bond heterolytically. The H adds to one carbon of the double bond, forming a carbocation, and the bromide ion then attacks the carbocation to give the haloalkane.
卤化氢如 HBr 可以加成到烯烃的双键上。HBr 是一个极性分子：Hδ⁺–Brδ⁻。缺电子的氢原子作为亲电试剂。双键进攻 H 原子，使 H–Br 键发生异裂。H 加成到双键的一个碳上形成碳正离子，然后溴离子进攻碳正离子生成卤代烷烃。
CH₂=CH₂ + HBr → CH₃CH₂Br
When ethene reacts with HBr, the only possible product is bromoethane. However, with unsymmetrical alkenes such as propene, you may need to consider which carbon gets the hydrogen. IGCSE does not require Markovnikov’s rule in depth, but you may be asked to predict the structure based on the stability of the carbocation formed.
当乙烯与 HBr 反应时，唯一的产物是溴乙烷。然而，对于不对称烯烃如丙烯，你可能需要考虑氢加在哪个碳上。IGCSE 不要求深入掌握马氏规则，但可能要求你根据所形成的碳正离子稳定性来预测产物结构。
10. Drawing Curly Arrows Correctly | 正确绘制弯曲箭头
Mastering curly arrows is essential for scoring full marks in mechanism questions. For ionic steps (heterolytic fission), use double-headed curly arrows starting from a bond or from a lone pair, pointing exactly to the atom receiving the electrons. For radical steps (homolytic fission), use single-headed fish-hook arrows, each accounting for one electron.
掌握弯曲箭头对于在机理题中取得满分至关重要。对于离子型步骤（异裂），使用双头弯曲箭头，从化学键或孤对电子出发，精确指向接受电子的原子。对于自由基步骤（均裂），使用单头鱼钩箭头，每个箭头代表一个电子。
Common exam mistakes include: arrows pointing from the electrophile to the double bond (should be the reverse), forgetting to show the Br–Br bond breaking step in addition, drawing arrows from H instead of the double bond, and using the wrong type of arrow. Always label charges on intermediates.
常见考试错误包括：箭头从亲电试剂指向双键（应为相反方向），在加成反应中忘记展示 Br–Br 键的断裂步骤，从 H 原子而不是从双键画箭头，以及使用错误的箭头类型。始终要给中间体标上电荷。
11. Comparing Free Radical Substitution and Electrophilic Addition | 自由基取代与亲电加成的比较
It is important to distinguish between these two mechanism types. Substitution replaces an H atom with a halogen atom in alkanes, requires UV light, and involves neutral free radicals. Addition adds two atoms across a double bond of an alkene, occurs readily in the dark at room temperature, and involves ions (electrophiles and nucleophiles).
区分这两种机理类型非常重要。取代反应是将烷烃中的 H 原子替换为卤原子，需要紫外光，并涉及中性的自由基。加成反应是在烯烃的双键上添加两个原子，在室温避光条件下即可发生，并涉及离子（亲电试剂与亲核试剂）。
In substitution, the product still has single bonds (a haloalkane), whereas in addition, the double bond is converted to a single bond, producing a saturated compound. Both mechanisms are tested on drawing curly arrows and writing equations for each step.
在取代反应中，产物仍为单键（卤代烷烃），而在加成反应中，双键转变为单键，生成饱和化合物。两种机理在考试中都会考查弯曲箭头的绘制以及各步骤方程式的书写。
12. Summary and Exam Tips | 总结与考试技巧
For IGCSE Chemistry 0620, focus on: the definitions of homolytic and heterolytic fission; the three stages of free radical substitution with methane and chlorine; the curly arrow mechanism for electrophilic addition of Br₂ and HBr to ethene; and the tests that distinguish alkanes from alkenes. Practise drawing all steps with correct electron movement.
对于 IGCSE 化学 0620，重点掌握：均裂与异裂的定义；甲烷与氯气自由基取代反应的三个阶段；乙烯与 Br₂ 和 HBr 的亲电加成弯曲箭头机理；以及区分烷烃和烯烃的检验方法。练习用正确的电子移动画出所有步骤。
Memorise the key conditions: UV light for chlorination of methane; no catalyst needed for bromine water test. Be ready to identify the role of each species: Cl• as a free radical, Br⁺ or H⁺ as electrophiles, Br⁻ as a nucleophile. Use clear, precise curly arrows and label partial charges where appropriate.
记住关键条件：甲烷氯化需要紫外光；溴水检验无需催化剂。要能识别每种粒子的角色：Cl• 是自由基，Br⁺ 或 H⁺ 是亲电试剂，Br⁻ 是亲核试剂。使用清晰准确的弯曲箭头，并在适当位置标出部分电荷。
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