📚 Reaction Mechanisms in AS Chemistry | AS化学中的反应机理
Understanding reaction mechanisms is a central theme in the Edexcel International AS Chemistry curriculum, especially in Unit 2 (CH02). A mechanism describes the stepwise bond‑breaking and bond‑making events, showing exactly how reactants transform into products. This article explores the essential concepts of homolytic and heterolytic fission, free radical substitution, electrophilic addition, and nucleophilic substitution, equipping you with the knowledge to tackle mechanism‑based questions in your exam.
理解反应机理是爱德思国际 AS 化学课程的核心主题，尤其在第二单元（CH02）中。机理描述了逐步的断键和成键过程，展示了反应物如何转变为产物。本文探讨均裂与异裂、自由基取代、亲电加成和亲核取代等基本概念，帮助您掌握解答考试中机理相关问题的知识。
1. What is a Reaction Mechanism? | 什么是反应机理？
A reaction mechanism is a detailed account of how a chemical reaction proceeds at the molecular level. It describes the sequence of elementary steps, each involving a small number of species, that sum to the overall balanced equation. Curly arrows are used to show the movement of electron pairs in these steps.
反应机理是对化学反应在分子水平上如何进行的详细描述。它描述了一系列基元步骤，每步涉及少数物种，总和即为总配平方程式。弯箭头用于表示这些步骤中电子对的移动。
The mechanism may include intermediates that are formed and consumed, and transition states at energy maxima. Intermediates such as free radicals or carbocations have a finite lifetime and can sometimes be detected, while transition states are momentary configurations at the top of an energy barrier.
机理可能包括形成并被消耗的中间体，以及处于能量最高点的过渡态。中间体如自由基或碳正离子具有有限寿命，有时可被检测到，而过渡态则是能量峰顶的瞬时构型。
2. Bond Fission: Homolytic and Heterolytic | 键的断裂：均裂与异裂
Bond breaking, or fission, can occur in two ways. Homolytic fission involves the equal splitting of a covalent bond, with each atom taking one electron from the shared pair, resulting in two free radicals. These radicals are neutral species with an unpaired electron, represented by a single dot (e.g., Cl•).
键断裂（裂解）有两种方式。均裂指共价键平均断裂，每个原子从共用电子对中获取一个电子，生成两个自由基。自由基是具有未成对电子的中性物种，用单点表示（例如 Cl•）。
(positive) and an anion (negative). The atom that receives both electrons becomes electron‑rich and carries a negative charge, while the other becomes electron‑deficient with a positive charge. For example, C−Br → C⁺ + Br⁻.
异裂则是不均匀的键断裂，键的两个电子都归属于一个原子，产生一个阳离子和一个阴离子。得到两个电子的原子变得富电子并带负电荷，另一个原子缺电子并带正电荷。例如，C−Br → C⁺ + Br⁻。
3. Reactive Intermediates | 活性中间体
Reaction intermediates are high‑energy species formed during a mechanism. The main types encountered at AS level are free radicals (e.g., •CH₃), carbocations (R₃C⁺), and carbanions (R₃C⁻). Carbocations are planar, sp² hybridised, with an empty p orbital, and are stabilised by electron‑donating alkyl groups through the inductive effect. The stability order is tertiary > secondary > primary > methyl.
反应中间体是机理过程中形成的高能物种。AS 阶段遇到的主要类型有自由基（如 •CH₃）、碳正离子（R₃C⁺）和碳负离子（R₃C⁻）。碳正离子为平面型 sp² 杂化，具有空 p 轨道，并通过给电子烷基的诱导效应稳定。稳定性顺序为叔碳正离子 > 仲碳正离子 > 伯碳正离子 > 甲基碳正离子。
Free radicals are also stabilised by alkyl groups; a tertiary radical is more stable than a secondary or primary radical. The stability of intermediates explains why certain products predominate in reactions involving unsymmetrical reactants.
自由基同样因烷基而稳定；叔自由基比仲自由基或伯自由基更稳定。中间体的稳定性解释了为何在涉及不对称反应物的反应中某些产物占主导。
4. Curly Arrows: Showing Electron Movement | 弯箭头：表示电子移动
Curly arrows are essential for drawing mechanisms. A curly arrow starts from a source of electrons (a lone pair or a bond) and points to an atom or a bond where electrons are going. A full‑headed arrow represents the movement of an electron pair; a fish‑hook arrow (half‑headed) represents the movement of a single electron, used in radical mechanisms. Always draw arrows from electron‑rich to electron‑poor.
弯箭头在画机理时必不可少。弯箭头从电子源（孤对电子或键）出发，指向电子将要到达的原子或键。全箭头表示电子对的移动；鱼钩箭头（半箭头）表示单电子的移动，用于自由基机理。始终从富电子方向缺电子方画箭头。
For example, in the hydroxide ion attacking a halogenoalkane, a full curly arrow starts from a lone pair on :OH⁻ and points to the partially positive carbon of the C−X bond. Another arrow shows the C−X bond breaking with the electron pair going to the halogen to form X⁻.
例如，氢氧根离子进攻卤代烷时，全弯箭头从 :OH⁻ 的一对孤对电子出发，指向 C−X 键上部分带正电的碳。另一箭头表示 C−X 键断裂，电子对移向卤素形成 X⁻。
5. Free Radical Substitution of Alkanes | 烷烃的自由基取代
r₂ → RBr + HBr. The mechanism has three main stages: initiation, propagation, and termination.
烷烃在紫外光或加热下与卤素（如 Br₂）发生自由基取代反应，总反应为：RH + Br₂ → RBr + HBr。机理主要分三步：链引发、链增长和链终止。
l−Cl or Br−Br bond undergoes homolytic fission to generate two halogen radicals. Cl₂ → 2Cl•. This step requires energy from UV light.
链引发：Cl−Cl 或 Br−Br 键均裂生成两个卤素自由基。Cl₂ → 2Cl•。这一步需要紫外光提供能量。
ical abstracts a hydrogen atom from the alkane, forming HBr and an alkyl radical. CH₄ + Br• → • (2) The alkyl radical reacts with a Br₂ molecule, producing the bromoalkane and another Br• radical. •CH₃ + Br₂ → CH₃Br + Br•. These steps repeat, sustaining the chain reaction.
链增长步骤：(1) 卤素自由基从烷烃中夺取一个氢原子，形成 HBr 和一个烷基自由基。CH₄ + Br• → •CH₃ + HBr。(2) 烷基自由基与 Br₂ 分子反应，生成溴代烷和另一个 Br• 自由基。•CH₃ + Br₂ → CH₃Br + Br•。这些步骤重复进行，维持链式反应。
ermination: Two radicals combine to form a stable molecule, ending the chain. Examples: Br• + Br• → Br₂; •CH₃ + Br• → CH₃Br; •CH₃ + •CH₃ → C₂H₆.
链终止：两个自由基结合生成稳定分子，结束链反应。例如：Br• + Br• → Br₂；•CH₃ + Br• → CH₃Br；•CH₃ + •CH₃ → C₂H₆。
6. Electrophilic Addition of Alkenes | 烯烃的亲电加成
Alkenes contain a C=C double bond, which is an electron‑ H₂SO₄. The mechanism proceeds in two steps: electrophilic attack and nucleophilic attack.
烯烃含有 C=C 双键，是富电子中心。亲电加成是其典型反应。常见亲电试剂包括 HBr、Br₂ 和 H₂SO₄。机理分两步：亲电进攻和亲核进攻。
For addition of HBr to ethene: Step 1 – eaks heterolytically, forming a carbocation and Br⁻. CH₂=CH₂ + HBr → CH₃C⁺H₂ + Br⁻. Step 2 – omide ion acts as a nucleophile and attacks the carbocation, forming the product CH₃CH₂Br.
以乙烯加 HBr 为例：第 1 步 – π 键进攻 HBr 的 δ⁺ H；H−Br 异裂，形成碳正离子和 Br⁻。CH₂=CH₂ + HBr → CH₃C⁺H₂ + Br⁻。第 2 步 – 溴离子作为亲核试剂进攻碳正离子，生成产物 CH₃CH₂
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