Free Radical Substitution Mechanism Explained | 自由基取代反应机理详解

📚 Free Radical Substitution Mechanism Explained | 自由基取代反应机理详解

Free radical substitution is one of the most important reaction mechanisms in IB Chemistry HL, typically introduced within the organic chemistry chapter on alkanes. This mechanism explains how alkanes, which are otherwise unreactive, can undergo substitution with halogens under specific conditions.

自由基取代反应是 IB 化学高级水平（HL）中最重要的反应机理之一，通常在烷烃的有机化学章节中介绍。该机理解释了原本不活泼的烷烃如何在特定条件下与卤素发生取代反应。

1. What Is Free Radical Substitution? | 什么是自由基取代反应？

Free radical substitution is a type of substitution reaction in which a free radical — an atom or molecule with an unpaired electron — replaces a hydrogen atom in an alkane. The reaction typically requires ultraviolet (UV) light or heat to initiate, and the overall equation for methane and chlorine is shown below.

自由基取代反应是一种取代反应，其中自由基（具有未成对电子的原子或分子）取代烷烃中的一个氢原子。该反应通常需要紫外线或加热引发，甲烷与氯气的总反应方程式如下所示。

CH₄ + Cl₂ → CH₃Cl + HCl

This reaction does not occur in the dark at room temperature, because the Cl–Cl bond is relatively strong and requires energy input to break homolytically. The reaction is a classic example of a photochemical reaction, and it proceeds through three distinct stages: initiation, propagation, and termination.

该反应在室温黑暗中不会发生，因为 Cl–Cl 键相对较强，需要输入能量才能发生均裂。该反应是光化学反应的一个经典例子，并通过三个不同阶段进行：链引发、链增长和链终止。

2. The Role of UV Light and Homolytic Bond Breaking | 紫外光的作用与均裂

For the reaction to begin, the chlorine molecule must absorb energy from UV light. This energy causes the Cl–Cl bond to break homolytically, meaning each chlorine atom receives one of the shared electrons and forms a chlorine free radical. The bond dissociation energy of Cl–Cl is approximately 242 kJ mol⁻¹, which corresponds to light in the UV region.

反应要开始，氯气分子必须吸收紫外光能量。该能量使 Cl–Cl 键发生均裂，即每个氯原子获得一个共享电子，形成一个氯自由基。Cl–Cl 键的键解离能约为 242 kJ mol⁻¹，对应紫外区域的光。

Cl₂ → 2Cl•

It is important to note that homolytic fission produces radicals with unpaired electrons, which are highly reactive species. The chlorine radical is represented with a single dot (Cl•) to indicate the unpaired electron.

需要注意的是，均裂产生具有未成对电子的自由基，这是高活性物种。氯自由基用单点（Cl•）表示，以指示未成对电子。

3. Initiation Step | 链引发阶段

The initiation step is the very first stage of the mechanism, in which free radicals are generated. For the chlorination of methane, the initiation step involves the homolytic cleavage of the chlorine molecule under UV light.

链引发是机理的第一阶段，在此阶段产生自由基。对于甲烷的氯代反应，链引发步骤涉及氯气分子在紫外光下的均裂。

Key features of the initiation step:

Requires UV light or high temperature (approximately 300 °C for some reactions).
Only one molecule of chlorine produces two chlorine radicals.
No product is formed during this step alone.
The energy input must be sufficient to overcome the bond dissociation energy.

需要紫外光或高温（某些反应约需 300 °C）。
一个氯气分子产生两个氯自由基。
仅此阶段不形成产物。
能量输入必须足以克服键解离能。

4. Propagation Steps | 链增长阶段

The propagation steps are the repeating cycle that builds the final product. In the chlorination of methane, there are two propagation steps. In the first step, a chlorine radical abstracts a hydrogen atom from methane, forming hydrogen chloride and a methyl radical (CH₃•).

链增长阶段是构建最终产物的重复循环。在甲烷氯代反应中，有两个链增长步骤。第一步中，氯自由基从甲烷中夺取一个氢原子，形成氯化氢和甲基自由基（CH₃•）。

Cl• + CH₄ → HCl + CH₃•

In the second propagation step, the methyl radical reacts with a chlorine molecule to produce chloromethane and a new chlorine radical. This new chlorine radical can then participate in another cycle, hence the term “chain reaction”.

第二步中，甲基自由基与氯气分子反应生成氯甲烷和新的氯自由基。这个新的氯自由基可以参与下一个循环，因此称为”链反应”。

CH₃• + Cl₂ → CH₃Cl + Cl•

The two propagation steps together form a cycle. Each cycle consumes one methane molecule and one chlorine molecule, producing one chloromethane molecule and one HCl molecule, while regenerating the chlorine radical. In principle, a single chlorine radical can convert many methane molecules to chloromethane.

两个链增长步骤共同构成一个循环。每个循环消耗一个甲烷分子和一个氯气分子，产生一个氯甲烷分子和一个 HCl 分子，同时再生成氯自由基。理论上，一个氯自由基可以将许多甲烷分子转化为氯甲烷。

5. Termination Steps | 链终止阶段

Termination steps occur when two free radicals collide and combine to form a stable molecule. Since free radicals are highly reactive and present in low concentrations, termination is relatively rare, but it eventually brings the chain reaction to an end. Several termination products are possible in the chlorination of methane.

链终止阶段发生在两个自由基碰撞并结合形成稳定分子时。由于自由基高活性且浓度低，终止相对罕见，但最终会使链反应结束。在甲烷氯代反应中，可能有多种终止产物。

Possible termination reactions:

Cl• + Cl• → Cl₂ (re-formation of chlorine)
CH₃• + Cl• → CH₃Cl (formation of chloromethane)
CH₃• + CH₃• → C₂H₆ (formation of ethane)

Cl• + Cl• → Cl₂（重新形成氯气）
CH₃• + Cl• → CH₃Cl（形成氯甲烷）
CH₃• + CH₃• → C₂H₆（形成乙烷）

The formation of ethane (C₂H₆) is particularly important because it shows that the mechanism is indeed radical-based; the coupling of two methyl radicals cannot be explained by any ionic mechanism. In exam questions, the presence of ethane as a minor by-product is often used as evidence for a free radical mechanism.

乙烷（C₂H₆）的形成尤其重要，因为它表明该机理确实是基于自由基的；两个甲基自由基的偶联无法用任何离子机理来解释。在考试中，乙烷作为少量副产物的存在常被用作自由基机理的证据。

6. Complete Mechanism for Methane and Chlorine | 甲烷与氯气的完整机理

The entire mechanism can be summarised in the following three stages. This is the exact format you should use when writing the mechanism in an IB exam.

整个机理可以概括为以下三个阶段。这是你在 IB 考试中书写机理时应使用的确切格式。

Initiation: Cl₂ ⇌ 2Cl• (UV light)

Propagation: Cl• + CH₄ → HCl + CH₃•

Propagation: CH₃• + Cl₂ → CH₃Cl + Cl•

Termination: CH₃• + Cl• → CH₃Cl, etc.

Note that in some textbooks the initiation is written as Cl₂ → 2Cl•, while others use the notation Cl₂ ⇌ 2Cl• to indicate that chlorine radicals can recombine. In IB exams, both forms are generally accepted, but you must always show the curly arrow or half-arrow notation for homolytic fission if drawing the mechanism.

注意，一些教材将链引发写作 Cl₂ → 2Cl•，而其他教材使用 Cl₂ ⇌ 2Cl• 表示氯自由基可以重新组合。在 IB 考试中两种形式一般都可以接受，但如果画机理图，必须用鱼钩箭头（半箭头）表示均裂。

7. Halogen Reactivity: F₂, Cl₂, Br₂, I₂ | 卤素反应活性：F₂、Cl₂、Br₂、I₂

Not all halogens react with alkanes under the same conditions. The reactivity of halogens in free radical substitution decreases down the group: fluorine is the most reactive, while iodine is essentially unreactive.

并非所有卤素都能在相同条件下与烷烃反应。卤素在自由基取代反应中的活性随族向下而降低：氟最活泼，而碘基本上不反应。

Halogen	Reactivity	Notes
F₂	Extremely vigorous	Explosive; difficult to control
Cl₂	Moderate	Requires UV light; most commonly studied
Br₂	Slow	Requires strong UV or heat
I₂	Very slow / negligible	Thermodynamically unfavourable; I–H bond is weak

卤素	反应活性	备注
F₂	极其剧烈	爆炸性；难以控制
Cl₂	中等	需要紫外光；最常研究
Br₂	慢	需要强紫外光或加热
I₂	非常慢 / 可忽略	热力学不利；I–H 键较弱

The trend in reactivity is related to the bond dissociation energy of the X–X bond. Fluorine has a surprisingly weak F–F bond due to lone pair repulsions, making it extremely reactive. Iodine has a weak I–I bond, but the I–H bond formed in the propagation step is also weak, making the overall reaction endothermic and unfavourable.

活性趋势与 X–X 键的键解离能有关。氟的 F–F 键因孤对电子排斥而出奇地弱，因此氟极其活泼。碘的 I–I 键虽弱，但链增长中形成的 I–H 键也较弱，使整个反应吸热而不利。

8. Selectivity: Why Chlorination Is Less Selective Than Bromination | 选择性：为什么氯代不如溴代具有选择性

When an alkane has more than one type of hydrogen atom, substitution can occur at different positions, leading to a mixture of products. Chlorination tends to produce a mixture of all possible products, whereas bromination is much more selective and preferentially substitutes the hydrogen on the carbon that forms the most stable radical.

当烷烃具有不止一种类型的氢原子时，取代可以发生在不同位置，导致产物混合物。氯代倾向于产生所有可能产物的混合物，而溴代选择性更强，优先取代能形成最稳定自由基的碳上的氢。

Radical stability follows the order:

自由基稳定性顺序为：

tertiary (3°) > secondary (2°) > primary (1°) > methyl (CH₃)

叔碳（3°）> 仲碳（2°）> 伯碳（1°）> 甲基（CH₃）

The reason for this stability order is hyperconjugation: alkyl groups donate electron density to the radical centre, stabilising the unpaired electron. Bromine radicals are less reactive than chlorine radicals, so they are more discriminating and react preferentially with the position that forms the most stable radical. Chlorine radicals are so reactive that they abstract hydrogen atoms almost indiscriminately.

稳定性的原因是超共轭效应：烷基向自由基中心提供电子密度，稳定未成对电子。溴自由基比氯自由基反应性低，因此更具区分性，优先与形成最稳定自由基的位置反应。氯自由基过于活泼，几乎不加区分地夺取氢原子。

9. Energy Profile Diagram | 能量剖面图

The overall reaction for methane chlorination is exothermic, but the first propagation step — hydrogen abstraction by Cl• — is slightly exothermic (approximately -4 kJ mol⁻¹) because the H–Cl bond formed (431 kJ mol⁻¹) is slightly stronger than the C–H bond broken (435 kJ mol⁻¹). The second propagation step is significantly exothermic.

甲烷氯代的总反应是放热的，但第一个链增长步骤——氯自由基夺取氢——略微放热（约 -4 kJ mol⁻¹），因为形成的 H–Cl 键（431 kJ mol⁻¹）略强于断裂的 C–H 键（435 kJ mol⁻¹）。第二个链增长步骤显著放热。

In an energy profile diagram, the activation energies of the propagation steps determine the rate of reaction. For chlorination, the activation energy for hydrogen abstraction is relatively low, so the reaction is fast and non-selective. For bromination, the hydrogen abstraction step involving Br• is endothermic and has a higher activation energy, making the reaction slower and more selective.

在能量剖面图中，链增长步骤的活化能决定反应速率。对于氯代，夺氢步骤的活化能相对较低，因此反应快速且无选择性。对于溴代，涉及 Br• 的夺氢步骤吸热且活化能更高，使反应更慢且更具选择性。

10. Comparison with Ionic Mechanisms | 与离子机理的比较

It is essential to understand how free radical substitution differs from ionic reactions such as nucleophilic substitution. The key distinguishing features are summarised below.

理解自由基取代与亲核取代等离子反应的区别至关重要。主要区别如下所示。

Feature	Free Radical Substitution	Ionic (Nucleophilic) Substitution
Reactive species	Free radicals (neutral, unpaired e⁻)	Ions (charged)
Bond breaking	Homolytic	Heterolytic
Initiation	UV light / heat	No initiation needed
Solvent	Non-polar or gas phase	Polar solvent typically required
By-products	Multiple products (e.g., C₂H₆)	Usually one major product

特征	自由基取代	离子（亲核）取代
活性物种	自由基（中性，含未成对电子）	离子（带电）
键断裂方式	均裂	异裂
引发条件	紫外光 / 加热	无需引发
溶剂	非极性或气相	通常需要极性溶剂
副产物	多种产物（如 C₂H₆）	通常一个主要产物

Another common point of confusion is the difference between homolytic and heterolytic fission. Homolytic fission produces two radicals with one electron each, whereas heterolytic fission produces a cation and an anion. In exam questions, you may be asked to identify which type of fission is occurring based on the products formed.

另一个常见混淆点是均裂与异裂的区别。均裂产生各带一个电子的两个自由基，而异裂产生一个阳离子和一个阴离子。在考试题中，你可能会被要求根据产物判断发生了哪种类型的断裂。

11. Common Exam Mistakes and Tips | 常见考试错误与建议

Students often lose marks in IB exams on this topic due to small but critical errors. Below are the most frequently encountered mistakes and how to avoid them.

学生常常因微小但关键的错误在这一主题上失分。以下是最常见的错误及如何避免它们。

Mistake 1: Writing “Cl₂ → 2Cl⁻” instead of “Cl₂ → 2Cl•”. The dot is essential; a negative charge implies heterolytic fission.
Mistake 2: Using full curly arrows instead of fish-hook arrows (half-headed arrows) for homolytic fission.
Mistake 3: Forgetting to state that UV light is required for the initiation step.
Mistake 4: Confusing the two propagation steps or missing the regeneration of Cl•.
Mistake 5: Not recognizing that multiple substitution products (CH₂Cl₂, CHCl₃, CCl₄) form when chlorine is in excess.

错误1：写成 “Cl₂ → 2Cl⁻” 而不是 “Cl₂ → 2Cl•”。点至关重要；负电荷意味着异裂。
错误2：使用全箭头而不是鱼钩箭头（半箭头）表示均裂。
错误3：忘记说明链引发需要紫外光。
错误4：混淆两个链增长步骤或遗漏 Cl• 的再生。
错误5：未认识到氯过量时会形成多种取代产物（CH₂Cl₂、CHCl₃、CCl₄）。

In addition, when drawing the mechanism, always show the movement of a single electron with a half-headed curly arrow. This is a key skill tested in Paper 2 of IB Chemistry HL.

此外，画机理时始终使用半头鱼钩箭头表示单电子运动。这是 IB 化学 HL Paper 2 中考查的关键技能。

12. Summary and Key Takeaways | 总结与核心要点

Free radical substitution is a three-stage mechanism involving initiation, propagation, and termination. It is a prime example of a photochemical reaction and demonstrates the unique behaviour of radical species. Understanding this mechanism not only helps with exam questions on alkanes but also builds a foundation for understanding more complex radical reactions in polymerisation and atmospheric chemistry.

自由基取代是一个三阶段机理，包括链引发、链增长和链终止。它是光化学反应的一个典型例子，展示了自由基物种的独特行为。理解这一机理不仅有助于解答烷烃相关的考试题目，还为理解聚合反应和大气化学中更复杂的自由基反应奠定了基础。

Published by TutorHao | IB Chemistry Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）

Free Radical Substitution Mechanism Explained | 自由基取代反应机理详解