IB & Edexcel Chemistry: Reaction Mechanisms – Key Points Review | IB & Edexcel 化学：反应机理 考点精讲

📚 IB & Edexcel Chemistry: Reaction Mechanisms – Key Points Review | IB & Edexcel 化学：反应机理 考点精讲

Understanding reaction mechanisms is a cornerstone of both IB and Edexcel A-Level Chemistry. A mechanism describes the step-by-step sequence of bond breaking and forming, illustrating the movement of electrons. Mastering these pathways not only helps in predicting products but also in explaining rate equations and stereochemistry, which are frequently examined.

理解反应机理是 IB 和 Edexcel A-Level 化学的核心。机理描述了化学键断裂和生成的逐步过程，展示了电子的移动。掌握这些途径不仅有助于预测产物，还能解释速率方程和立体化学，这些都是常考内容。

1. The Language of Mechanisms: Curly Arrows and Bond Movement | 机理的语言：弯箭头与键的移动

In mechanism diagrams, a full curly arrow ( → ) depicts the movement of an electron pair during heterolytic bond fission or covalent bond formation. A half-headed or fishhook arrow ( ↷ ) indicates the movement of a single electron, characteristic of homolytic fission in free-radical reactions. Always draw arrows from the electron-rich species (nucleophile or electron source) to the electron-deficient centre (electrophile). In heterolytic fission, a double-headed arrow starts from a bond and ends on the atom receiving the electron pair, generating a cation and an anion.

在机理图中，完整的弯箭头（ → ）表示异裂过程中电子对的移动或共价键的形成。半箭头或称鱼钩箭头（ ↷ ）表示单个电子的移动，是自由基反应中均裂的特征。箭头总是从富电子物种（亲核试剂或电子源）指向缺电子中心（亲电试剂）。在异裂中，双电子箭头从键起始，指向接收电子对的原子，生成阳离子和阴离子。

2. Free-Radical Substitution: Halogenation of Alkanes | 自由基取代：烷烃的卤代

The free-radical substitution of methane with chlorine proceeds via three stages: initiation, propagation, and termination. Initiation: Cl₂ undergoes homolytic fission upon UV light, producing two chlorine radicals (Cl•). Propagation: Cl• abstracts a hydrogen from CH₄ to form HCl and a methyl radical (•CH₃); then •CH₃ reacts with Cl₂ to give CH₃Cl and regenerate Cl•. This chain reaction continues. Termination: two radicals combine, e.g., Cl• + Cl• → Cl₂, •CH₃ + Cl• → CH₃Cl, or •CH₃ + •CH₃ → C₂H₆. Mixtures of products arise from further substitution, a limitation in synthesis.

甲烷与氯的自由基取代反应通过引发、传递和终止三个阶段进行。引发：Cl₂在紫外光下均裂，产生两个氯自由基（Cl•）。传递：Cl•从CH₄夺取一个氢原子，生成HCl和甲基自由基（•CH₃）；随后•CH₃与Cl₂反应生成CH₃Cl并再生Cl•。这个链式反应持续进行。终止：两个自由基结合，例如Cl• + Cl• → Cl₂、•CH₃ + Cl• → CH₃Cl或•CH₃ + •CH₃ → C₂H₆。进一步取代会产生混合物，这是合成中的限制。

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

Alkenes undergo electrophilic addition because the π-bond is electron-rich. In the addition of HBr to ethene, the mechanism has two steps. First, the π-electrons attack the partially positive hydrogen of HBr, forming a C–H bond and generating a carbocation intermediate, while the H–Br bond breaks heterolytically to give a bromide ion. In the second step, the bromide ion donates its electron pair to the carbocation, forming the C–Br bond. Markovnikov’s rule states that the hydrogen adds to the carbon with more hydrogen atoms already, because the more substituted carbocation is more stable (tertiary > secondary > primary). For unsymmetrical alkenes, the major product arises from the more stable carbocation. With bromine water (Br₂), a cyclic bromonium ion intermediate explains the anti-addition stereochemistry.

烯烃由于π键富电子而发生亲电加成。在HBr与乙烯加成中，机理分两步。首先，π电子进攻HBr中部分带正电的氢，形成C–H键并生成碳正离子中间体，同时H–Br键异裂给出溴离子。第二步，溴离子将电子对给予碳正离子，形成C–Br键。马氏规则指出氢加到含氢较多的碳上，因为更取代的碳正离子更稳定（叔碳 > 仲碳 > 伯碳）。对于不对称烯烃，主产物来自更稳定的碳正离子。与溴水（Br₂）反应时，环状溴鎓离子中间体解释了反式加成立体化学。

4. Nucleophilic Substitution: SN1 vs SN2 | 亲核取代：SN1 对比 SN2

Nucleophilic substitution occurs when a nucleophile replaces a leaving group on a saturated carbon. The two limiting mechanisms are SN1 and SN2. SN2 is a one-step, bimolecular process: the nucleophile attacks the carbon from the opposite side of the leaving group, leading to inversion of configuration (Walden inversion). The rate equation is rate = k[RX][Nu⁻]. It is favoured by primary haloalkanes, strong nucleophiles, and aprotic solvents. SN1 is a two-step, unimolecular mechanism: first, the leaving group departs, forming a planar carbocation; then the nucleophile attacks from either face, giving racemisation. The rate depends only on [RX], rate = k[RX]. It is favoured by tertiary haloalkanes, weak nucleophiles, and protic solvents, which stabilise the carbocation and leaving group. Stereochemistry and kinetics are key examinable differences.

当亲核试剂取代饱和碳上的离去基团时发生亲核取代。两种极限机理是SN1和SN2。SN2是单步双分子过程：亲核试剂从离去基团的反侧进攻碳，导致构型翻转（瓦尔登翻转）。速率方程为 rate = k[RX][Nu⁻]。伯卤代烷、强亲核试剂和非质子溶剂有利于SN2。SN1是两步单分子机理：首先离去基团离去，形成平面碳正离子；然后亲核试剂从平面任一侧进攻，导致外消旋化。速率仅取决于[RX]，rate = k[RX]。叔卤代烷、弱亲核试剂和质子溶剂有利于SN1，因为溶剂能稳定碳正离子和离去基团。立体化学和动力学是常考的差异。

5. Elimination Reactions: E1 and E2 | 消除反应：E1 与 E2

Elimination reactions produce alkenes by removing atoms or groups from adjacent carbons. The E2 mechanism is a one-step, bimolecular process where a strong base abstracts a β-hydrogen at the same time as the leaving group departs, forming a π-bond. The rate law is rate = k[RX][base]. It requires anti-periplanar geometry (H and leaving group must be anti-coplanar). Saytzeff’s rule predicts the more substituted alkene as the major product because it is more stable. The E1 mechanism proceeds via a carbocation intermediate: first, the leaving group leaves (rate-determining step), then a base removes a β-hydrogen to form the alkene. Rate = k[RX]. E1 competes with SN1, and both give mixtures. Bulky bases (e.g., t-BuO⁻) favour Hofmann elimination (less substituted alkene) in E2.

消除反应通过脱去相邻碳上的原子或基团生成烯烃。E2机理是单步双分子过程，强碱夺取β-氢的同时离去基团离去，形成π键。速率律为 rate = k[RX][base]。要求反式共平面（氢与离去基团必须反式共平面）。Saytzeff规则预测取代更多的烯烃是主产物，因其更稳定。E1机理经过碳正离子中间体：首先离去基团离去（决速步），然后碱夺取β-氢生成烯烃。速率 = k[RX]。E1与SN1竞争，二者均给出混合物。大位阻碱（如t-BuO⁻）在E2中倾向于Hofmann消除（取代少的烯烃）。

6. Electrophilic Aromatic Substitution | 亲电芳香取代

Benzene undergoes electrophilic substitution rather than addition, preserving its aromatic stability. The general mechanism involves generation of a strong electrophile (E⁺), then attack by the π-system to form a resonance-stabilised arenium ion (Wheland intermediate), followed by loss of a proton to restore aromaticity. Typical reactions: nitration (NO₂⁺ from HNO₃/H₂SO₄), halogenation (Cl⁺ or Br⁺ with FeX₃ catalyst), Friedel–Crafts alkylation (R⁺ from RCl and AlCl₃) and acylation (RC⁺O). The rate-determining step is formation of the arenium ion. Substituted benzenes direct further substitution: activating groups (e.g., –OH, –NH₂) are ortho/para-directing and increase reactivity; deactivating groups (e.g., –NO₂, –COOH) are meta-directing and decrease reactivity, except halogens which are deactivating but ortho/para-directing.

苯发生亲电取代而非加成，以保持芳香稳定性。一般机理涉及生成强亲电试剂（E⁺），然后被π体系进攻形成共振稳定的芳正离子（Wheland中间体），随后失去质子恢复芳香性。典型反应：硝化（来自HNO₃/H₂SO₄的NO₂⁺）、卤代（使用FeX₃催化剂的Cl⁺或Br⁺）、傅-克烷基化（来自RCl和AlCl₃的R⁺）和酰基化（RC⁺O）。决速步是芳正离子的形成。取代苯指导进一步取代：活化基团（如–OH、–NH₂）是邻/对位定位基，增加反应活性；钝化基团（如–NO₂、–COOH）是间位定位基，降低活性，但卤素除外，它们是钝化基团却是邻/对位定位基。

7. Addition–Elimination (Nucleophilic Acyl Substitution) | 加成–消除（亲核酰基取代）

Carboxylic acid derivatives (acyl chlorides, acid anhydrides, esters, amides) react via addition–elimination. The mechanism begins with nucleophilic attack at the electrophilic carbonyl carbon, forming a tetrahedral intermediate. In the second step, the intermediate collapses, eliminating the leaving group (Cl⁻, RCOO⁻, RO⁻, NH₂⁻) and regenerating the C=O double bond. The leaving group ability follows: Cl⁻ > RCOO⁻ > RO⁻ > NH₂⁻. Thus, acyl chlorides are the most reactive, while amides are the least. This mechanism is crucial for understanding esterification, hydrolysis, and polyamide/polyester formation, which appear in both IB and Edexcel syllabi.

羧酸衍生物（酰氯、酸酐、酯、酰胺）通过加成–消除机理反应。机理始于亲核试剂进攻缺电子的羰基碳，形成四面体中间体。第二步中间体瓦解，离去基团离开（Cl⁻、RCOO⁻、RO⁻、NH₂⁻），重新生成C=O双键。离去能力顺序为：Cl⁻ > RCOO⁻ > RO⁻ > NH₂⁻。因此酰氯最活泼，酰胺最不活泼。该机理对于理解酯化、水解以及聚酰胺/聚酯的形成至关重要，这些内容均出现在IB和Edexcel大纲中。

8. Free-Radical Polymerisation | 自由基聚合

Addition polymerisation of alkenes often proceeds via a free-radical mechanism. Initiation: an initiator (e.g., benzoyl peroxide) decomposes to give radicals, which add to the alkene monomer forming a new radical. Propagation: the radical adds to many

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