A-Level Chemistry: Reaction Mechanisms Core Revision | A-Level 化学：反应机理考点精讲

📚 A-Level Chemistry: Reaction Mechanisms Core Revision | A-Level 化学：反应机理考点精讲

Understanding reaction mechanisms is fundamental in A-Level Chemistry. Mechanisms illustrate the step-by-step sequence of bond breaking and bond making at the molecular level, helping us predict products, explain reaction rates, and design synthetic routes. Mastering the use of curly arrows to show electron movement is essential for success in exams.

理解反应机理是 A-Level 化学的基础。机理揭示了分子水平上键的断裂与生成的分步过程，帮助我们预测产物、解释反应速率并设计合成路线。掌握使用弯箭头表示电子转移是考试成功的关键。

1. What Are Reaction Mechanisms? | 什么是反应机理？

A reaction mechanism is a detailed description of the individual elementary steps that make up an overall chemical reaction. Each step involves bond breaking, bond making, or rearrangement of atoms and electrons. The overall balanced equation does not reveal which bonds break first or which intermediates form, but a mechanism does.

反应机理是对构成总化学反应各个基元步骤的详细描述。每一步都涉及键的断裂、生成或原子与电子的重排。总配平的方程式无法揭示哪些键先断裂或形成了什么中间体，但机理可以。

An elementary step is a single molecular event, such as a collision between two particles leading to product. The molecularity (unimolecular or bimolecular) determines the rate law for that step. Importantly, the slowest elementary step, called the rate-determining step, governs the overall rate of the reaction.

一个基元步骤是单个分子事件，比如两个粒子碰撞生成产物。反应的分子数（单分子或双分子）决定了该步骤的速率方程。最慢的基元步骤称为决速步，它控制着整个反应的速率。

For exam success, you must learn to recognise common mechanistic patterns: nucleophilic substitution, electrophilic addition, free radical substitution, and nucleophilic addition. Each follows characteristic electron movements that we represent with curly arrows.

为了在考试中取得成功，你必须学会辨识常见的机理模式：亲核取代、亲电加成、自由基取代和亲核加成。每种反应都有其特定的电子转移方式，我们用弯箭头来表示。

2. Curly Arrows: The Language of Mechanisms | 弯箭头：机理的语言

A curly arrow (↷) shows the movement of an electron pair during a reaction step. The arrow starts from an electron-rich site — a lone pair, a negative charge, or a π bond — and points towards an electron-deficient atom, such as a carbocation or a partially positive carbon. The arrow head indicates where the electrons end up, either forming a new bond or becoming a lone pair.

弯箭头（↷）表示一步反应中电子对的转移。箭头从电子富集处（孤对电子、负电荷或 π 键）出发，指向缺电子的原子，如碳正离子或带有部分正电荷的碳。箭头指向电子最终的去向——要么形成新键，要么变成孤对电子。

A double-barbed arrow (→) represents the movement of an electron pair, which is typical in polar reactions. A single-barbed (fishhook) arrow (⇀) is used for the movement of a single electron in radical mechanisms. At A-Level, you will mainly use the double-barbed curly arrow, but you must recognise the fishhook arrow in free radical substitution.

双钩箭头（→）表示电子对转移，常见于极性反应。单钩（鱼钩）箭头（⇀）用于自由基机理中单个电子的转移。在 A-Level 阶段，你主要使用双钩弯箭头，但在自由基取代中必须能识别单钩箭头。

Always draw curly arrows starting exactly from the electron source (lone pair or bond) and pointing precisely to the atom receiving the electrons. Never start an arrow from a positive charge or place the head in empty space. Correct arrow placement is a common mark in exam questions.

绘制弯箭头时，一定要让箭头精确地从电子源（孤对电子或化学键）出发，指向接受电子的原子。切勿从正电荷开始画箭头，也不要将箭头指向空白处。正确的箭头位置是考试中常见的得分点。

3. Heterolytic vs. Homolytic Fission | 异裂与均裂

Bond breaking is the first event in many mechanisms. In heterolytic fission, both electrons from the covalent bond move to one atom, generating a cation and an anion. This occurs in polar reactions such as SN1 or electrophilic addition. For example, in the ionisation of (CH₃)₃C–Br, the electrons go to Br, forming Br⁻ and the carbocation (CH₃)₃C⁺.

键的断裂是许多机理的第一步。在异裂中，共价键上的两个电子都转移至同一个原子上，生成一个阳离子和一个阴离子。这发生在 SN1 或亲电加成等极性反应中。例如，在 (CH₃)₃C–Br 的电离中，电子全部归溴，形成 Br⁻ 和碳正离子 (CH₃)₃C⁺。

Homolytic fission involves each atom taking one electron from the bond, producing two neutral radicals. This requires energy, typically supplied by UV light. The C–Cl bond in chloromethane can undergo homolytic fission to give •CH₃ and •Cl radicals, initiating free radical substitution.

均裂则是每个原子各取一个电子，产生两个中性自由基。这需要能量，通常由紫外光提供。氯甲烷中的 C–Cl 键可发生均裂，生成 •CH₃ 和 •Cl 自由基，从而引发自由基取代反应。

The type of fission dictates the whole mechanism: heterolytic → ionic intermediates (carbocations, anions), homolytic → radical intermediates. Being able to identify which fission is operating from reaction conditions is a key skill tested in exams.

裂解类型决定了整个机理的走向：异裂 → 离子型中间体（碳正离子、负离子），均裂 → 自由基中间体。能够根据反应条件判断发生了哪种裂解，是考试中考查的关键技能。

4. Nucleophilic Substitution: SN1 Mechanism | 亲核取代：SN1 机理

The SN1 mechanism stands for Substitution, Nucleophilic, Unimolecular. It occurs in two steps. First, the leaving group departs, taking the bonding electrons, to form a planar carbocation intermediate. This step is slow and rate-determining. Second, the nucleophile attacks the carbocation rapidly from either face, leading to a mixture of retention and inversion products — a racemic mixture if the carbon is chiral.

SN1 机理代表亲核取代、单分子过程。它分两步进行：首先，离去基团带着一对电子离开，生成平面型碳正离子中间体，此步骤是慢的决速步；随后，亲核试剂从平面两侧快速进攻碳正离子，得到构型保留和翻转的混合物——如果碳是手性的，则会生成外消旋混合物。

Rate = k[substrate]; the nucleophile’s concentration does not appear in the rate law. Tertiary substrates favour SN1 because the resulting carbocation is stabilised by alkyl inductive effects. Weak nucleophiles and polar protic solvents also promote the SN1 pathway.

速率 = k[底物]；亲核试剂的浓度不出现在速率方程中。叔碳底物倾向于 SN1，因为生成的碳正离子可通过烷基诱导效应稳定。弱亲核试剂和极性质子溶剂也有利于 SN1 路径。

A typical example is the hydrolysis of (CH₃)₃CBr in aqueous NaOH. The mechanism: (CH₃)₃C–Br → (CH₃)₃C⁺ + Br⁻ (slow), then (CH₃)₃C⁺ + OH⁻ → (CH₃)₃COH (fast). The intermediate carbocation can be shown in brackets with a positive charge.

一个典型例子是 (CH₃)₃CBr 在 NaOH 水溶液中的水解。机理：(CH₃)₃C–Br → (CH₃)₃C⁺ + Br⁻（慢），然后 (CH₃)₃C⁺ + OH⁻ → (CH₃)₃COH（快）。中间体碳正离子可用带正电荷的括号表示。

5. Nucleophilic Substitution: SN2 Mechanism | 亲核取代：SN2 机理

The SN2 mechanism (Bimolecular Nucleophilic Substitution) is a concerted process: bond making and bond breaking occur simultaneously in a single step. The nucleophile attacks the electrophilic carbon from the back side, opposite the leaving group, resulting in inversion of configuration (Walden inversion). The transition state has a trigonal bipyramidal geometry with the nucleophile and leaving group partially bonded.

SN2 机理（双分子亲核取代）是协同过程：键的生成与断裂同时发生在一基元步骤中。亲核试剂从离去基团的背面进攻缺电子碳，导致构型翻转（瓦尔登翻转）。过渡态为三角双锥几何构型，亲核试剂和离去基团均部分键连。

Rate = k[substrate][nucleophile]; both concentrations influence the rate. SN2 is favoured by primary substrates, strong nucleophiles, and polar aprotic solvents. Steric hindrance around the electrophilic carbon disfavours SN2, which is why tertiary substrates undergo SN1 instead.

速率 = k[底物][亲核试剂]；两者的浓度都影响速率。SN2 受伯碳底物、强亲核试剂和极性非质子溶剂促进。亲电碳周围的位阻不利于 SN2，因此叔碳底物倾向于走 SN1 途径。

Example: CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻. Here the mechanism is drawn with a curly arrow from the OH⁻ lone pair to the carbon, and simultaneously a curly arrow from the C–Br bond to Br, showing the breaking bond. The transition state is often drawn with dotted lines.

例子：CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻。画机理时，用一个弯箭头从 OH⁻ 的孤对电子指向碳，同时用一个弯箭头从 C–Br 键指向 Br，表示键的断裂。渡态通常用虚线表示部分形成的键。

6. Comparing SN1 and SN2 Reactions | SN1 与 SN2 反应的比较

Exam questions frequently ask you to distinguish between SN1 and SN2 or justify which mechanism operates under given conditions. The table below summarises the essential differences you need to know.

考试经常要求区分 SN1 和 SN2，或解释在给定条件下哪个机理起作用。下表总结了你需要掌握的关键差异。

Feature 特征	SN1 SN1 机理	SN2 SN2 机理
Kinetics / 动力学	Rate = k[substrate] 一级反应	Rate = k[substrate][Nu] 二级反应
Steps / 步骤	Two steps, carbocation intermediate 两步，有碳正离子中间体	One concerted step 一步协同
Stereochemistry / 立体化学	Racemisation (planar intermediate) 外消旋化（平面型中间体）	Inversion of configuration 构型完全翻转
Substrate preference / 底物偏好	3° > 2° (1° and methyl rarely) 叔碳 > 仲碳	Methyl > 1° > 2° (3° no reaction) 甲基 > 伯碳 > 仲碳
Nucleophile / 亲核试剂	Weak nucleophile (e.g. H₂O, ROH) 弱亲核试剂	Strong nucleophile (e.g. OH⁻, CN⁻) 强亲核试剂
Solvent / 溶剂	Polar protic (e.g. water, alcohols) 极性质子溶剂	Polar aprotic (e.g. acetone, DMSO) 极性非质子溶剂

Remember: primary halogenoalkanes react by SN2, while tertiary halogenoalkanes react by SN1. Secondary substrates can go either way depending on the nucleophile and solvent — a favourite exam twist.

记住：伯卤代烷按 SN2 反应，叔卤代烷按 SN1 反应。仲卤代烷则取决于亲核试剂和溶剂条件，这常常成为考试中的变体。

7. Electrophilic Addition to Alkenes | 烯烃的亲电加成

Alkenes undergo electrophilic addition because the electron-rich π bond can attack an electrophile. The typical example is the reaction of ethene with bromine. The mechanism: the π electrons attack one end of the polarised Br–Br molecule, forming a cyclic bromonium ion and releasing Br⁻. Then Br⁻ attacks the bromonium ion from the opposite side to give trans addition product.

烯烃因富电子 π 键可进攻亲电试剂而发生亲电加成。典型例子是乙烯与溴的反应。机理：π 电子进攻被极化的 Br–Br 分子一端，形成环状溴鎓离子并释放出 Br⁻；然后 Br⁻ 从背面进攻该鎓离子，得到反式加成产物。

With hydrogen halides like HBr, the first step is protonation of the alkene to form the most stable carbocation (Markovnikov’s rule). Then the halide ion adds to the carbocation. Markovnikov addition means the hydrogen adds to the carbon with more hydrogens already, while the halide ends up on the more substituted carbon because the intermediate carbocation is more stable there.

与 HBr 等卤化氢反应时，第一步是烯烃质子化生成更稳定的碳正离子（马氏规则），然后卤离子加成到碳正离子上。马氏加成意味着氢加到了本来氢较多的碳上，而卤原子最终连接在取代较多的碳上，因为该处碳正离子更稳定。

When drawing the mechanism, use a curly arrow from the π bond to the electrophile, and if a halide ion is released simultaneously, show the breaking of the Br–Br bond with the electrons moving to the leaving Br. Always include charges on intermediates.

绘制机理时，要用弯箭头从 π 键指向亲电试剂；若同时释放卤离子，则用第二个弯箭头表示 Br–Br 键的断裂，电子移向离去的溴。中间体上一定要标出电荷。

8. Free Radical Substitution (Alkanes) | 自由基取代（烷烃）

Alkanes react with halogens in the presence of UV light through a free radical chain mechanism. The classic example is the chlorination of methane: CH₄ + Cl₂ → CH₃Cl + HCl. The mechanism has three stages: initiation, propagation, and termination.

烷烃在紫外光下与卤素发生自由基链式反应。经典例子是甲烷的氯化：CH₄ + Cl₂ → CH₃Cl + HCl。机理分三个阶段：链引发、链增长和链终止。

Initiation: Cl₂ → 2 Cl• (homolytic fission, single-barbed arrows). Propagation: Cl• + CH₄ → HCl + •CH₃, then •CH₃ + Cl₂ → CH₃Cl + Cl•. These two steps repeat, sustaining the chain. Termination occurs when two radicals combine: Cl• + Cl• → Cl₂, •CH₃ + •CH₃ → C₂H₆, or Cl• + •CH₃ → CH₃Cl.

链引发：Cl₂ → 2 Cl•（均裂，使用单钩箭头）。链增长：Cl• + CH₄ → HCl + •CH₃，随后 •CH₃ + Cl₂ → CH₃Cl + Cl•。这两步循环进行，维持链反应。链终止发生在两个自由基结合时：Cl• + Cl• → Cl₂，•CH₃ + •CH₃ → C₂H₆ 或 Cl• + •CH₃ → CH₃Cl。

You must be able to write propagation steps for any given alkane and halogen. The radical attacks a hydrogen atom, forming H–X and an alkyl radical; the alkyl radical then reacts with X₂ to form the halogenoalkane product. A common mistake is forgetting to show the regeneration of the halogen radical in the second propagation step.

你必须能写出任何给定烷烃和卤素的两步链增长。自由基夺取一个氢原子，生成 H–X 和烷基自由基；随后烷基自由基与 X₂ 反应生成卤代烷产物。常见错误是忘记在第二个增长步骤中再生卤原子自由基。

9. Nucleophilic Addition to Carbonyl Compounds | 羰基化合物的亲核加成

Carbonyl groups (>C=O) are polarised due to the electronegativity difference, making the carbonyl carbon electrophilic. Nucleophiles such as cyanide ions (CN⁻) or hydride donors (from NaBH₄ or LiAlH₄) attack this carbon, forming a tetrahedral intermediate. This is the core mechanism for aldehydes and ketones.

羰基（>C=O）因电负性差异而极化，使羰基碳具有亲电性。亲核试剂如氰根离子（CN⁻）或氢负供体（来自 NaBH₄ 或 LiAlH₄）进攻该碳，形成四面体中间体。这是醛酮反应的核心机理。

In the addition of HCN to propanone, the CN⁻ ion attacks the planar carbonyl, sending the π electrons onto oxygen to form an alkoxide ion; subsequent protonation by HCN or H⁺ gives the hydroxynitrile product. The mechanism requires a curly arrow from CN⁻ to C, and a curly arrow from the C=O bond to oxygen. Then a final arrow from the O⁻ to H⁺.

在 HCN 与丙酮的加成中，CN⁻ 进攻平面型羰基，把 π 电子推至氧形成醇盐负离子；随后被 HCN 或 H⁺ 质子化得到羟基腈产物。机理中需要一个弯箭头从 CN⁻ 指向碳，一个从 C=O 键指向氧；最后再用一个箭头从 O⁻ 指向 H⁺。

For reduction with NaBH₄, the nucleophile is effectively H⁻. The mechanism is similar: H⁻ attacks the carbonyl carbon, pushing electrons onto oxygen, then the O⁻ picks up a proton from water or alcohol solvent. This is a two-step nucleophilic addition, widely assessed.

对于 NaBH₄ 还原，亲核试剂实际上是 H⁻。机理相似：H⁻ 进攻羰基碳，将电子推给氧，然后 O⁻ 从水或醇溶剂中获得质子。这是一个两步的亲核加成，经常被考察。

10. Stability of Carbocations and Induction Effects | 碳正离子稳定性与诱导效应

Carbocations are key intermediates in SN1, electrophilic addition, and rearrangements. Their stability order is: (CH₃)₃C⁺ (3°) > (CH₃)₂CH⁺ (2°) > CH₃CH₂⁺ (1°) > CH₃⁺. This is mainly explained by the positive inductive effect (+I) of alkyl groups, which push electron density towards the positively charged carbon, dispersing the charge.

碳正离子是 SN1、亲电加成和重排的关键中间体。稳定性顺序为：(CH₃)₃C⁺（叔）> (CH₃)₂CH⁺（仲）> CH₃CH₂⁺（伯）> CH₃⁺。这主要用烷基的给电子诱导效应（+I）解释：烷基将电子推向带正电荷的碳，分散电荷。

Hyperconjugation also contributes: the overlap of σ bonds (C–H or C–C) with the empty p orbital of the carbocation stabilises it. More adjacent C–H/C–C bonds mean greater hyperconjugation, hence tertiary carbocations are most stable.

超共轭效应也有贡献：σ 键（C–H 或 C–C）与碳正离子的空 p 轨道交盖产生稳定作用。相邻 C–H/C–C 键越多，超共轭效应越强，所以叔碳正离子最稳定。

Understanding carbocation stability helps predict Markovnikov addition and explain why SN1 rates increase with substrate substitution. A common exam pitfall is failing to recognise that primary carbocations are too unstable to form; thus SN1 is not viable for primary halogenoalkanes.

理解碳正离子稳定性有助于预测马氏加成产物，并解释为什么 SN1 速率随底物取代度增高而加快。常见考试失分点是没认识到伯碳正离子过于不稳定而不会生成

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A-Level Chemistry: Reaction Mechanisms Core Revision | A-Level 化学：反应机理 考点精讲