Reactions of Arenes | 芳烃的反应

📚 Reactions of Arenes | 芳烃的反应

Arenes (aromatic hydrocarbons) such as benzene are fundamentally different from alkenes in their chemical behaviour. While both contain multiple C–C bonds, arenes undergo electrophilic substitution rather than addition, and their characteristic stability governs all their reactions. This article reviews the full set of arene reactions required for Cambridge A-Level Chemistry, including nitration, halogenation, Friedel–Crafts alkylation and acylation, sulfonation, side-chain oxidation, hydrogenation, and the directing effects of substituents.

芳烃（芳香烃）如苯，其化学行为与烯烃有本质区别。尽管两者都含有多重 C–C 键，但芳烃发生的是亲电取代而非亲电加成；这一特殊的稳定性决定了芳烃的所有反应。本文系统梳理剑桥 A-Level 化学所要求的芳烃反应，包括硝化、卤化、傅-克烷基化与酰基化、磺化、侧链氧化、氢化以及取代基的定位效应。

1. Structure and Stability of Benzene | 苯的结构与稳定性

Benzene (C₆H₆) is the simplest arene. Its molecule consists of a planar hexagonal ring of six carbon atoms, each bonded to one hydrogen atom. Every carbon atom is sp² hybridised, giving a bond angle of 120°. Each carbon also possesses a perpendicular p orbital, and the six p orbitals overlap sideways to form a delocalised π-electron system extending above and below the plane of the ring.

苯（C₆H₆）是最简单的芳烃。其分子由六个碳原子组成的平面六元环构成，每个碳原子各连一个氢原子。每个碳原子均为 sp² 杂化，键角为 120°。每个碳原子还拥有一个垂直于环平面的 p 轨道，六个 p 轨道侧面重叠，在环平面的上方和下方形成离域 π 电子体系。

This delocalisation confers exceptional thermodynamic stability on the benzene ring. The delocalisation energy is around 150 kJ mol⁻¹; consequently, benzene is far less reactive than an alkene toward addition reactions such as bromination or hydrogenation.

这种离域作用赋予苯环极高的热力学稳定性。离域能约为 150 kJ mol⁻¹；因此，苯对溴化、加氢等加成反应的反应活性远低于烯烃。

Key evidence comes from enthalpy of hydrogenation. The measured value for benzene is only −208 kJ mol⁻¹, far less exothermic than the predicted −360 kJ mol⁻¹ for a hypothetical cyclohexa-1,3,5-triene with three isolated C=C bonds. The difference of about 152 kJ mol⁻¹ is the resonance stabilisation energy of the aromatic ring.

关键证据来自加氢焓。苯的实验测定值为 −208 kJ mol⁻¹，远低于假想环己-1,3,5-三烯（含三个孤立 C=C 键）的预测值 −360 kJ mol⁻¹。二者相差约 152 kJ mol⁻¹，这正是芳香环的共振稳定化能。

2. Why Electrophilic Substitution Rather Than Addition | 为什么是亲电取代而非加成

The aromatic ring is electron-rich because of its π system, so it attracts electrophiles. However, benzene seldom undergoes addition because addition would destroy the stable delocalised π system and give a non-aromatic product of higher energy. In electrophilic substitution, by contrast, an electrophile replaces a hydrogen atom and the aromatic sextet is fully regenerated.

芳香环因其 π 体系而富电子，能吸引亲电试剂。然而，苯极少发生加成反应，因为加成会破坏稳定的离域 π 体系，生成能量更高的非芳香产物。相比之下，在亲电取代反应中，一个亲电试剂取代一个氢原子，芳香六电子体系得以完全恢复。

This explains observable behaviour: benzene does not decolourise bromine water at room temperature, whereas alkenes do so instantly. Benzene requires a catalyst (e.g. FeBr₃) and heat for bromination. Substitution has a lower activation energy than addition because the product retains aromatic stabilisation.

这解释了实际现象：苯在室温下不能使溴水褪色，而烯烃能立即褪色。苯发生溴化需要催化剂（如 FeBr₃）和加热。取代反应的活化能低于加成反应，因为产物保留了芳香稳定化。

3. General Mechanism of Electrophilic Substitution | 亲电取代的一般机理

The mechanism occurs in two steps. Step 1: the electrophile E⁺ is attacked by the π electrons of the ring, forming a positively charged arenium ion (σ-complex) in which the positive charge is delocalised over three ring carbon atoms. Step 2: a proton (H⁺) is lost from the sp³ carbon bearing the electrophile, and the aromatic ring is restored. The departing proton is usually removed by the counter-ion of the electrophile acting as a base.

该机理分两步进行。第一步：环的 π 电子进攻亲电试剂 E⁺，生成带正电荷的芳烃离子（σ-络合物），正电荷离域分布于三个环碳原子上。第二步：携带亲电试剂的 sp³ 碳失去一个质子（H⁺），芳香环得以恢复。离去的质子通常由亲电试剂的抗衡阴离子作为碱夺取。

The formation of the arenium ion is the rate-determining step. The stability of this intermediate determines the overall rate of reaction, and it also explains why substitu

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