Organic Reactions – Mechanisms | 有机反应——机理

📚 Organic Reactions – Mechanisms | 有机反应——机理

Understanding organic reaction mechanisms is essential in A-Level Chemistry. A mechanism shows the step-by-step movement of electrons during a reaction, explaining how bonds break and form. Cambridge A-Level questions often ask you to draw curly arrows, identify intermediates, and explain why a particular product is formed. This article covers the main mechanisms you need to know: free radical substitution, electrophilic addition, nucleophilic substitution, elimination, nucleophilic addition, electrophilic substitution, and addition-elimination hydrolysis.

理解有机反应机理是 A-Level 化学的核心内容。机理展示了反应过程中电子逐步移动的方式，从而解释化学键如何断裂和生成。剑桥 A-Level 考试经常要求你画出弯箭头、判断中间体，并解释为什么生成某种特定产物。本文涵盖你需要掌握的主要机理：自由基取代、亲电加成、亲核取代、消除反应、亲核加成、亲电取代以及加成-消除水解反应。

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1. Why Mechanisms Matter | 机理为何重要

A reaction mechanism answers three questions: which bonds break, which bonds form, and how electrons move. Chemists use mechanisms to predict products, design synthesis routes, and control reaction conditions. In exams, drawing a mechanism correctly usually earns more marks than simply writing the overall equation, because it proves you understand electron flow rather than just memorising reactants and products.

反应机理回答三个问题：哪些键断裂、哪些键生成以及电子如何移动。化学家利用机理预测产物、设计合成路线并控制反应条件。在考试中，正确画出机理通常比只写总反应方程式得分更高，因为这证明你真正理解了电子流动，而不只是记住反应物和产物。

For Cambridge A-Level, you should be familiar with bond polarity, electronegativity, lone pairs, and carbocation stability. These concepts are used repeatedly in all organic mechanisms. Make sure you can identify nucleophiles, electrophiles, radicals, and leaving groups before you start drawing mechanisms.

对于剑桥 A-Level，你应当熟悉键的极性、电负性、孤对电子和碳正离子稳定性。这些概念在所有有机机理中反复使用。在开始画机理之前，请务必能够识别亲核试剂、亲电试剂、自由基和离去基团。

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

A curly arrow shows the movement of an electron pair. The arrow must start from a source of electrons, such as a lone pair, a negative charge, or a covalent bond, and point toward an electron-deficient atom or centre. A double-headed curly arrow means two electrons move; a single-headed fish-hook arrow means one electron moves, as in free radical reactions.

弯箭头表示一对电子的移动。箭头必须从电子源出发，例如孤对电子、负电荷或共价键，并指向缺电子的原子或中心。双头弯箭头表示两个电子移动，单头鱼钩箭头表示一个电子移动，常见于自由基反应。

Do not draw curly arrows starting from a positive charge or from an atom without electrons. Common mistakes include pointing the arrow in the wrong direction or forgetting to show charges on intermediates. Always check that your intermediate has the correct number of bonds and formal charges, especially after heterolytic bond fission.

不要从正电荷或没有电子的原子上画弯箭头。常见错误包括箭头方向画反或忘记标出中间体的电荷。特别是在异裂断键之后，务必检查中间体是否具有正确的成键数和形式电荷。

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3. Bond Fission: Homolytic vs Heterolytic | 键的断裂：均裂与异裂

Homolytic fission occurs when a covalent bond breaks and each atom receives one electron from the shared pair. This produces two free radicals. It requires energy from ultraviolet light or high temperature, as in the chlorination of methane. Heterolytic fission occurs when one atom receives both electrons from the shared pair, producing a positive and a negative ion; this is common in polar reactions involving electrophiles and nucleophiles.

均裂发生在共价键断裂时每个原子各得到一对共享电子中的一个电子，生成两个自由基。均裂需要紫外线或高温提供能量，例如甲烷的氯化反应。异裂则是一个原子得到这对共享电子的两个电子，生成正离子和负离子，这在涉及亲电试剂和亲核试剂的极性反应中很常见。

Recognising the type of fission is important because free radical mechanisms use single-headed curly arrows, while ionic mechanisms use double-headed curly arrows. The conditions of a reaction often tell you which type of fission is taking place; ultraviolet light points to homolytic fission, while polar solvents and charged reagents suggest heterolytic fission.

识别断裂类型很重要，因为自由基机理使用单头弯箭头，而离子机理使用双头弯箭头。反应条件通常能提示你发生的是哪种断裂：紫外线条件指向均裂，而极性溶剂和带电试剂则倾向于异裂。

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4. Free Radical Substitution | 自由基取代

The reaction between methane and chlorine in ultraviolet light is a classic free radical substitution. The overall equation is CH₄ + Cl₂ → CH₃Cl + HCl, but the reaction proceeds through three stages: initiation, propagation, and termination. In initiation, ultraviolet light breaks a Cl–Cl bond homolytically: Cl₂ → 2Cl•.

甲烷与氯气在紫外线照射下的反应是典型的自由基取代反应。总反应方程式为 CH₄ + Cl₂ → CH₃Cl + HCl，但反应分三个阶段进行：引发、传递和终止。在引发阶段，紫外线使 Cl–Cl 键均裂：Cl₂ → 2Cl•。

In propagation, a chlorine radical attacks methane to form hydrogen chloride and a methyl radical: Cl• + CH₄ → HCl + CH₃•. The methyl radical then reacts with a chlorine molecule: CH₃• + Cl₂ → CH₃Cl + Cl•. This chlorine radical can continue the chain reaction. Termination occurs when two radicals combine, for example Cl• + Cl• → Cl₂, CH₃• + Cl• → CH₃Cl, or CH₃• + CH₃• → CH₃CH₃.

在传递阶段，氯自由基进攻甲烷，生成氯化氢和甲基自由基：Cl• + CH₄ → HCl + CH₃•。甲基自由基接着与氯分子反应：CH₃• + Cl₂ → CH₃Cl + Cl•。这个氯自由基可以继续链式反应。终止阶段发生在两个自由基结合时，例如 Cl• + Cl• → Cl₂、CH₃• + Cl• → CH₃Cl 或 CH₃• + CH₃• → CH₃CH₃。

In excess methane, chloromethane is the main product, but if chlorine is in excess, further substitution can produce dichloromethane, trichloromethane, and tetrachloromethane. In an exam, mention that free radical substitution often gives a mixture of products and that chain termination steps are needed to complete the mechanism.

在甲烷过量的条件下，主要产物是一氯甲烷；但如果氯气过量，继续取代会生成二氯甲烷、三氯甲烷和四氯甲烷。考试时应指出自由基取代往往得到混合物，并且机理中必须写出链终止步骤。

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5. Electrophilic Addition | 亲电加成

Alkenes react with electrophiles such as hydrogen bromide, bromine, and concentrated sulfuric acid by electrophilic addition. The C=C double bond has high electron density, so it attracts electrophiles. In the reaction of ethene with hydrogen bromide, the polar HBr molecule provides the electrophile H⁺: CH₂=CH₂ + HBr → CH₃CH₂Br.

烯烃与亲电试剂如溴化氢、溴和浓硫酸发生亲电加成反应。C=C 双键具有较高的电子密度，因此能吸引亲电试剂。在乙烯与溴化氢的反应中，极性的 HBr 分子提供亲电试剂 H⁺：CH₂=CH₂ + HBr → CH₃CH₂Br。

The mechanism has two steps. First, the H⁺ attacks the double bond and attaches to one carbon, while the other carbon becomes a carbocation: CH₂=CH₂ + H⁺ → CH₃CH₂⁺. Second, the bromide ion attacks the carbocation to form bromoethane: CH₃CH₂⁺ + Br⁻ → CH₃CH₂Br. The second step is fast and completes the addition.

机理分为两步。首先，H⁺ 进攻双键并连接到一个碳原子上，另一个碳原子形成碳正离子：CH₂=CH₂ + H⁺ → CH₃CH₂⁺。然后，溴离子进攻碳正离子生成溴乙烷：CH₃CH₂⁺ + Br⁻ → CH₃CH₂Br。第二步很快，完成加成。

For unsymmetrical alkenes such as propene, the major product follows Markovnikov’s rule: hydrogen adds to the carbon with more hydrogens, while the halogen adds to the carbon with fewer hydrogens. This is because the more stable carbocation intermediate is formed preferentially. Propene + HBr gives mainly CH₃CHBrCH₃ via the secondary carbocation CH₃CH⁺CH₃, not the primary carbocation CH₃CH₂CH₂⁺.

对于不对称烯烃如丙烯，主要产物遵循马氏规则：氢加到含氢较多的碳上，卤素加到含氢较少的碳上。这是因为反应优先生成更稳定的碳正离子中间体。丙烯与 HBr 反应主要生成 CH₃CHBrCH₃，经由二级碳正离子 CH₃CH⁺CH₃，而不是一级碳正离子 CH₃CH₂CH₂⁺。

Bromine addition to ethene is similar: CH₂=CH₂ + Br₂ → CH₂BrCH₂Br. The electron-rich double bond polarises the Br₂ molecule so that one bromine atom acts as the electrophile. After the first bromine attaches to the double bond, a bromonium ion or carbocation intermediate forms, and the bromide ion immediately attacks the other carbon to give 1,2-dibromoethane. This reaction decolourises bromine water and is used as a test for unsaturation.

溴与乙烯的加成类似：CH₂=CH₂ + Br₂ → CH₂BrCH₂Br。富电子的双键使 Br₂ 分子极化，其中一个溴原子充当亲电试剂。第一个溴原子连接到双键后，形成溴鎓离子或碳正离子中间体，溴离子立即进攻另一个碳原子，生成 1,2-二溴乙烷。该反应能使溴水褪色，因此常用于检验不饱和键。

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6. Nucleophilic Substitution: SN1 and SN2 | 亲核取代：SN1 与 SN2

A nucleophile is an electron pair donor that attacks an electron-deficient carbon atom. Haloalkanes are common substrates for nucleophilic substitution because the carbon–halogen bond is polar and the halogen can leave as a halide ion. The two main mechanisms are SN2 and SN1, and the choice depends mainly on the structure of the haloalkane.

亲核试剂是电子对给体，可进攻缺电子的碳原子。卤代烷是亲核取代的常见底物，因为碳–卤键极性较强，卤素可以作为卤离子离去。两种主要机理是 SN2 和 SN1，选择哪种主要取决于卤代烷的结构。

SN2 is a one-step bimolecular mechanism: the nucleophile attacks from the opposite side of the leaving group, forming a transition state, and the leaving group departs simultaneously. Primary haloalkanes favour SN2 because there is less steric hindrance. For example, OH⁻ + CH₃CH₂Br → CH₃CH₂OH + Br⁻. The rate equation is rate = k[haloalkane][nucleophile].

SN2 是一步双分子机理：亲核试剂从离去基团的相反一侧进攻，形成过渡态，同时离去基团离开。伯卤代烷因位阻较小而有利于 SN2 反应。例如 OH⁻ + CH₃CH₂Br → CH₃CH₂OH + Br⁻。其速率方程为 rate = k[卤代烷][亲核试剂]。

SN1 is a two-step unimolecular mechanism. The slow step is the heterolytic fission of the carbon–halogen bond to form a carbocation intermediate. A tertiary haloalkane such as (CH₃)₃CBr forms a stable tertiary carbocation: (CH₃)₃CBr → (CH₃)₃C⁺ + Br⁻. In the fast second step, the nucleophile attacks the carbocation: (CH₃)₃C⁺ + OH⁻ → (CH₃)₃COH. The rate equation is rate = k[haloalkane].

SN1 是两步单分子机理。慢步骤是碳–卤键异裂生成碳正离子中间体。叔卤代烷如 (CH₃)₃CBr 能生成稳定的叔碳正离子：(CH₃)₃CBr → (CH₃)₃C⁺ + Br⁻。在快速的第二步中，亲核试剂进攻碳正离子：(CH₃)₃C⁺ + OH⁻ → (CH₃)₃COH。其速率方程为 rate = k[卤代烷]。

When writing a nucleophilic substitution mechanism, show the nucleophile’s lone pair, the curly arrow from the nucleophile to the carbon atom, and the curly arrow from the C–X bond to the halogen. For SN1, always show the carbocation intermediate and its positive charge. Leaving group ability also matters; weaker bases such as I⁻ are better leaving groups than F⁻.

书写亲核取代机理时，要画出亲核试剂的孤对电子、从亲核试剂指向碳原子的弯箭头，以及从 C

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