Types of Organic Reactions | 有机反应类型

📚 Types of Organic Reactions | 有机反应类型

Organic chemistry at Cambridge International AS and A Level is organised around a relatively small number of reaction types. Being able to identify whether a reaction is substitution, addition, elimination, hydrolysis, oxidation, reduction, condensation or polymerisation is essential for predicting products, writing mechanisms and answering synthesis questions. This article explains the main types of organic reaction, their characteristic features and typical conditions, with emphasis on the language used in Cambridge exam papers.

剑桥国际 AS 和 A Level 有机化学围绕数量相对有限的几类反应展开。能够判断一个反应是取代、加成、消除、水解、氧化、还原、缩合还是聚合，对于预测产物、书写机理和解答合成题至关重要。本文解释有机反应的主要类型、特征及典型条件，并强调剑桥试卷中常见的表述方式。

1. Classifying Organic Reactions | 有机反应分类

Organic reactions are usually classified by what happens to the carbon skeleton and by the movement of electron pairs. The broad structural classes are substitution, addition, elimination, condensation, hydrolysis, oxidation and reduction, and polymerisation. Mechanistically, reactions can be homolytic (free radical) or heterolytic (ionic), and ionic reactions are further labelled electrophilic or nucleophilic depending on the attacking species.

有机反应通常根据碳骨架发生的变化以及电子对的移动方式来分类。结构大类包括取代、加成、消除、缩合、水解、氧化和还原以及聚合。从机理上看，反应可以是均裂（自由基）或异裂（离子型），离子型反应又根据进攻物种进一步分为亲电反应或亲核反应。

2. Free Radical Substitution | 自由基取代

Free radical substitution is characteristic of alkanes reacting with halogens in the presence of ultraviolet light. The mechanism has three stages: initiation, propagation and termination. Initiation produces chlorine radicals, Cl•, by homolytic fission of Cl₂. Propagation steps form the halogenoalkane and regenerate the radical. Termination occurs when two radicals combine. A common exam point is that further substitution produces a mixture of mono-, di- and poly-substituted products.

自由基取代是烷烃在紫外光存在下与卤素反应的典型类型。机理分为三个阶段：链引发、链增长和链终止。链引发通过 Cl₂ 的均裂产生氯自由基 Cl•。链增长步骤生成卤代烷并再生自由基。两个自由基结合时发生链终止。常见考点是进一步取代会生成一取代、二取代和多取代产物的混合物。

CH₄ + Cl• → •CH₃ + HCl

•CH₃ + Cl₂ → CH₃Cl + Cl•

3. Electrophilic Substitution | 亲电取代

Electrophilic substitution is the main reaction of arenes such as benzene. Unlike alkenes, benzene tends to undergo substitution rather than addition because addition would destroy the stable delocalised π system. An electrophile, for example NO₂⁺ or Cl⁺ generated with a halogen carrier, attacks the ring; the intermediate carbocation loses H⁺ to restore aromaticity. Typical reactions include nitration, halogenation, alkylation and acylation. Conditions must be learned for each: concentrated HNO₃ and H₂SO₄ for nitration, AlCl₃ or FeCl₃ for chlorination.

亲电取代是苯等芳烃的主要反应。与烯烃不同，苯倾向于发生取代而不是加成，因为加成会破坏稳定的离域 π 体系。亲电试剂，例如 NO₂⁺ 或通过卤素载体生成的 Cl⁺，进攻苯环；中间体碳正离子失去 H⁺ 恢复芳香性。典型反应包括硝化、卤代、烷基化和酰基化。必须掌握各反应的条件：硝化使用浓 HNO₃ 和浓 H₂SO₄，氯化使用 AlCl₃ 或 FeCl₃。

C₆H₆ + NO₂⁺ → C₆H₅NO₂ + H⁺

4. Nucleophilic Substitution | 亲核取代

Nucleophilic substitution occurs when a nucleophile donates an electron pair to replace a leaving group in a saturated compound. Halogenoalkanes are the most common substrates. Primary halogenoalkanes react by an Sₙ2 mechanism: the nucleophile attacks from the opposite side and the leaving group leaves in one step. Tertiary halogenoalkanes react by Sₙ1, forming a stable carbocation intermediate before the nucleophile attacks. The rate equation and product optical activity can be used to distinguish the two mechanisms; for example, Sₙ2 with an optically active substrate gives inversion of configuration.

亲核取代发生在亲核试剂提供电子对取代饱和化合物中离去基团时。卤代烷是最常见的底物。伯卤代烷按 Sₙ2 机理反应：亲核试剂从背面进攻，离去基团在同一步离去。叔卤代烷按 Sₙ1 反应，先形成稳定的碳正离子中间体，亲核试剂再进攻。速率方程和产物光学活性可用于区分这两种机理；例如，光学活性底物发生 Sₙ2 反应会得到构型翻转产物。

CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻

5. Electrophilic Addition | 亲电加成

Electrophilic addition is the characteristic reaction of alkenes with reagents such as HBr, Br₂, H₂SO₄ and H₂O. The electron-rich C=C bond attracts an electrophile. In the addition of HBr, the H⁺ adds to one carbon, producing a carbocation on the more substituted carbon; the bromide ion then attacks. Markovnikov’s rule predicts the major product: the hydrogen attaches to the carbon with more hydrogen atoms already, because the more stable carbocation is formed. For unsymmetrical alkenes, this gives a major and minor product.

亲电加成是烯烃与 HBr、Br₂、H₂SO₄ 和 H₂O 等试剂反应的典型类型。富电子的 C=C 键吸引亲电试剂。在 HBr 加成中，H⁺ 加到一个碳上，在取代较多的碳上生成碳正离子；溴离子随后进攻。马氏规则预测主要产物：氢加在已经连有更多氢原子的碳上，因为这样生成更稳定的碳正离子。对于不对称烯烃，会得到主要产物和次要产物。

CH₃CH=CH₂ + HBr → CH₃CHBrCH₃

6. Nucleophilic Addition | 亲核加成

Nucleophilic addition is typical of carbonyl compounds. Aldehydes and ketones react with HCN to form hydroxynitriles, and with NaBH₄ or LiAlH₄ to form alcohols. The polar C=O bond is attacked by a nucleophile such as CN⁻ or H⁻; the π bond breaks and the oxygen becomes negatively charged, then protonation gives the product. Aldehydes are more reactive than ketones because the alkyl groups in ketones are electron-donating and also cause steric hindrance. This reaction is important for extending carbon chains in synthesis.

亲核加成是羰基化合物的典型反应。醛和酮与 HCN 反应生成羟基腈，与 NaBH₄ 或 LiAlH₄ 反应生成醇。亲核试剂如 CN⁻ 或 H⁻ 进攻极性的 C=O 键；π 键断裂，氧带负电荷，然后质子化得到产物。醛比酮更活泼，因为酮中的烷基具有给电子作用并且会造成位阻。该

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