Nucleophilic Substitution Reactions | 亲核取代反应

📚 Nucleophilic Substitution Reactions | 亲核取代反应

Nucleophilic substitution is one of the most important reaction mechanisms in A-level organic chemistry. It explains how halogenoalkanes and related compounds are converted into alcohols, nitriles, amines and many other functional groups. Understanding this reaction will help you predict products, draw mechanisms and compare the reactivity of different substrates.

亲核取代是 A-level 有机化学中最重要的反应机理之一。它解释了卤代烷及相关化合物如何转化为醇、腈、胺和许多其它官能团。理解这一反应将帮助你预测产物、绘制机理并比较不同底物的反应活性。

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1. What Is Nucleophilic Substitution? | 什么是亲核取代反应？

Nucleophilic substitution is a reaction in which an electron-rich species called a nucleophile donates a pair of electrons to an electron-poor carbon atom, forming a new covalent bond while a leaving group departs with the bonding pair of electrons.

亲核取代是一种反应：富电子的物种（亲核试剂）将一对电子提供给缺电子的碳原子，形成新的共价键，同时离去基团带着成键电子对离开。

The general equation can be written as:

其通式可以写作：

R–X + :Nu⁻ → R–Nu + :X⁻

Here R is an alkyl or aryl group, X is a leaving group such as a halogen, and Nu is the nucleophile. The reaction occurs mainly at saturated carbon atoms, especially in halogenoalkanes.

其中 R 是烷基或芳基，X 是离去基团（如卤素），Nu 是亲核试剂。该反应主要发生在饱和碳原子上，尤其是卤代烷中。

Nucleophilic substitution is an example of heterolytic fission because the C–X bond breaks unevenly, with both electrons going to the leaving group.

亲核取代是异裂反应的一个例子，因为 C–X 键不均匀断裂，两个电子都归离去基团所有。

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2. Key Players: Nucleophile, Leaving Group and Substrate | 关键角色：亲核试剂、离去基团和底物

A nucleophile is a species that has a lone pair of electrons or a negative charge and can attack an electron-deficient atom. Common nucleophiles at A-level include OH⁻, CN⁻, NH₃ and H₂O.

亲核试剂是具有孤对电子或负电荷并且能进攻缺电子原子的物种。A-level 中常见的亲核试剂包括 OH⁻、CN⁻、NH₃ 和 H₂O。

Nucleophilicity depends on charge, electronegativity and solvent. A negatively charged nucleophile such as OH⁻ is generally stronger than its neutral conjugate acid H₂O.

亲核性取决于电荷、电负性和溶剂。带负电荷的亲核试剂（如 OH⁻）通常比其共轭酸 H₂O 更强。

The leaving group is the atom or group that departs with the bonding pair. Good leaving groups are stable after leaving. Halide ions, especially iodide, are excellent leaving groups because they are large and their negative charge is spread over a large volume.

离去基团是带着成键电子对离开的原子或基团。好的离去基团离开后是稳定的。卤离子，特别是碘离子，是极好的离去基团，因为它们体积大，负电荷分散在较大体积上。

The substrate is the molecule containing the leaving group. For this topic, the substrate is usually a halogenoalkane with a polar C–X bond, where the carbon atom is electrophilic because it carries a partial positive charge.

底物是含有离去基团的分子。对于本主题，底物通常是具有极性 C–X 键的卤代烷，其中碳原子带有部分正电荷，具有亲电性。

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3. Halogenoalkanes as Substrates | 卤代烷作为底物

Halogenoalkanes contain a polar carbon–halogen bond because halogen atoms are more electronegative than carbon. The carbon atom acquires a δ⁺ charge and the halogen acquires a δ⁻ charge.

卤代烷含有极性的碳–卤键，因为卤素原子比碳电负性更强。碳原子带 δ⁺ 电荷，卤素带 δ⁻ 电荷。

This polarity makes the carbon atom open to attack by nucleophiles. However, the reactivity order of halogenoalkanes is not determined by bond polarity alone; it is determined by bond enthalpy.

这种极性使碳原子容易受到亲核试剂的进攻。然而，卤代烷的反应活性顺序并不只由键的极性决定，而是由键焓决定。

Down the halogen group, the carbon–halogen bond becomes weaker because the halogen atom becomes larger and the overlap between orbitals decreases. Therefore the reactivity order is:

沿卤素族向下，碳–卤键变弱，因为卤素原子变大，轨道重叠减少。因此反应活性顺序为：

R–I > R–Br > R–Cl > R–F

The C–I bond is the weakest, so iodoalkanes undergo nucleophilic substitution fastest. Fluoroalkanes are very unreactive because the C–F bond is strong and difficult to break.

C–I 键最弱，因此碘代烷进行亲核取代最快。氟代烷非常不活泼，因为 C–F 键很强，难以断裂。

In exam questions, you should compare bond enthalpies when explaining reactivity trends rather than only mentioning electronegativity.

在考试题中，解释反应活性趋势时应比较键焓，而不仅仅是电负性。

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4. The SN2 Mechanism: Concerted Bimolecular Substitution | SN2 机理：协同双分子取代

SN2 stands for substitution, nucleophilic, bimolecular. It is a one-step process in which the nucleophile attacks the carbon atom from the side opposite the leaving group, and bond formation and bond breaking occur at the same time.

SN2 代表双分子亲核取代。它是一步过程：亲核试剂从离去基团的反面进攻碳原子，成键和断键同时发生。

The rate equation for an SN2 reaction is:

SN2 反应的速率方程为：

Rate = k[R–X][Nu⁻]

Both the substrate and the nucleophile are involved in the rate-determining step, so the reaction is second order overall and bimolecular.

底物和亲核试剂都参与速率决定步骤，因此反应总级数为二级，是双分子反应。

The transition state has the nucleophile and the leaving group partially bonded to the same carbon atom. The carbon centre is penta-coordinate in a roughly trigonal bipyramidal arrangement.

过渡态中，亲核试剂和离去基团与同一个碳原子部分成键。碳中心是五配位的，大致呈三角双锥排列。

A typical example is the reaction of bromoethane with hydroxide ion to form ethanol:

一个典型例子是溴乙烷与氢氧根离子反应生成乙醇：

CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻

The hydroxide ion attacks from the back side of the C–Br bond, and the bromide ion leaves. This back-side attack causes an inversion of the tetrahedral arrangement around carbon, which is a characteristic feature of SN2.

氢氧根离子从 C–Br 键背面进攻，溴离子离去。这种背面进攻导致碳周围四面体排列的翻转，这是 SN2 的典型特征。

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5. The SN1 Mechanism: Stepwise Unimolecular Substitution | SN1 机理：分步单分子取代

SN1 stands for substitution, nucleophilic, unimolecular. It is a two-step mechanism. In the first, slow step the leaving group departs to form a carbocation intermediate. In the second, fast step the nucleophile attacks the planar carbocation.

SN1 代表单分子亲核取代。它是两步机理。第一步慢步骤中离去基团离开，形成碳正离子中间体。第二步快步骤中亲核试剂进攻平面碳正离子。

The rate equation for an SN1 reaction is:

SN1 反应的速率方程为：

Rate = k[R–X]

Only the substrate is involved in the rate-determining step, so the reaction is first order overall and does not depend on the concentration of the nucleophile.

只有底物参与速率决定步骤，因此反应总级数为一级，与亲核试剂的浓度无关。

The carbocation is the key intermediate. Its stability follows the order:

碳正离子是关键中间体。其稳定性顺序为：

tertiary > secondary > primary > methyl

Alkyl groups release electron density through the +I inductive effect, stabilising the positively charged carbon. Therefore tertiary halogenoalkanes favour SN1.

烷基通过 +I 诱导效应释放电子密度，稳定带正电的碳。因此叔卤代烷倾向于 SN1。

A typical example is the reaction of 2-bromo-2-methylpropane with water:

一个典型例子是

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