📚 AQA A-Level Chemistry 3.10 Aromatic Chemistry | AQA A-Level 化学 3.10 芳香化学
Aromatic chemistry is one of the most important organic topics in AQA A-Level Chemistry. It explains why benzene behaves so differently from alkenes and introduces the mechanism of electrophilic substitution, which is used again and again in organic synthesis.
芳香化学是 AQA A-Level 化学中最重要的有机专题之一。它解释了为什么苯的性质与烯烃如此不同，并引入了亲电取代反应机理，该机理在有机合成中反复出现。
1. Introduction to Aromatic Chemistry | 芳香化学简介
Aromatic chemistry focuses on benzene and its derivatives, which are called arenes. The term 'aromatic' originally referred to pleasant smells, but in modern chemistry it describes a special stability arising from a delocalised π electron system.
芳香化学研究苯及其衍生物，即芳烃。'芳香'一词最初指宜人的气味，但在现代化学中，它描述由离域 π 电子体系产生的特殊稳定性。
Benzene is the simplest aromatic hydrocarbon and has the molecular formula C₆H₆. Its reactions are dominated by substitution rather than addition, which is a key difference from alkenes such as ethene.
苯是最简单的芳香烃，分子式为 C₆H₆。它的反应以取代为主，而不是加成，这是它与乙烯等烯烃的一个关键区别。
2. Benzene: Molecular Formula and Kekulé Structure | 苯的分子式与凯库勒结构
Benzene has the molecular formula C₆H₆ and is a planar cyclic molecule. Each carbon atom uses sp² hybrid orbitals to form three σ bonds, giving bond angles of 120° around the ring.
苯的分子式为 C₆H₆，是一个平面环状分子。每个碳原子使用 sp² 杂化轨道形成三个 σ 键，环上键角为 120°。
The Kekulé structure proposed a six-membered ring with alternating single and double bonds, often written as cyclohexa-1,3,5-triene. However, this model does not explain the true stability and reactivity of benzene.
凯库勒结构提出六元环具有交替的单键和双键，通常写为环己-1,3,5-三烯。然而，该模型无法解释苯的真实稳定性和反应活性。
In benzene, each carbon still has one unhybridised p orbital perpendicular to the ring plane. These p orbitals overlap sideways to form a continuous π system.
在苯中，每个碳原子仍有一个未杂化的 p 轨道垂直于环平面。这些 p 轨道侧向重叠，形成连续的 π 体系。
3. The Delocalised Model | 离域模型
The delocalised model states that the six p orbitals in benzene overlap sideways to create a continuous π cloud above and below the plane of the ring. The six π electrons are spread over all six carbon atoms.
离域模型认为，苯中的六个 p 轨道侧向重叠，在环平面上下形成连续的 π 云。六个 π 电子分布在所有六个碳原子上。
This delocalisation gives benzene extra stability, often called delocalisation energy or resonance stabilisation energy. The electron density is evenly distributed, which makes all carbon–carbon bonds identical.
这种离域作用使苯具有额外的稳定性，通常称为离域能或共振稳定化能。电子密度均匀分布，因此所有碳碳键完全相同。
All C–C bond lengths in benzene are about 0.139 nm, which is intermediate between a C– le bond (0.134 nm). Benzene is often drawn as a hexagon with a circle inside to represent the delocalised electrons.
苯中所有 C–C 键长约为 0.139 nm，介于 C–C 单键（0.154 nm）和 C=C 双键（0.134 nm）之间。苯通常画成六边形内加一个圆圈，以表示离域电子。
4. Evidence Against the Kekulé Model | 反对凯库勒模型的证据
Three main pieces of evidence reject the Kekulé omine. If it contained three C=C bonds, it would decolourise bromine water like an alkene.
三个主要证据否定了具有交替单双键的凯库勒结构。首先，苯与溴发生取代反应而非加成反应。如果苯含有三个 C=C 双键，它就会像烯烃一样使溴水褪色。
Second, all carbon–carbon bond lengths in benzene are equal. A molecule with alternating single and double bonds would show two different bond lengths.
第二，苯中所有碳碳键长完全相等。具有交替单双键的分子会显示出两种不同的键长。
Third, the enthalpy change of hydrogenation is less exothermic than expected. Cyclohexene releases about 120 kJ mol⁻¹ when hydrogenated. If benzene had three separate double bonds, hydrogenation would release about 360 kJ mol⁻¹. In reality, benzene releases only about 208 kJ mol⁻¹, so it is about 152 kJ mol⁻¹ more stable than the Kekulé structure predicts.
第三，加氢反应的焓变放热比预期少。环己烯加氢释放约 120 kJ mol⁻¹。如果苯有三个独立的双键，加氢将释放约 360 kJ mol⁻¹。实际上苯仅释放约 208 kJ mol⁻¹，因此它比凯库勒结构预测的稳定约 152 kJ mol⁻¹。
5. Naming Aromatic Compounds | 芳香化合物的命名
Monosubstituted benzene derivatives are named by placing the substituent name before benzene, for example methylbenzene, chlorobenzene and nitrobenzene. Some have accepted common names, such as toluene for methylbenzene.
单取代苯衍生物的命名方法是将取代基名称放在苯之前，例如甲苯（甲基苯）、氯苯和硝基苯。有些具有通用的俗名，例如甲苯就是甲基苯的俗名。
When a benzene ring is a substituent attached to a longer chain, it is called a phenyl group and is written as C₆H₅–.
当苯环作为取代基连接到较长碳链上时，它被称为苯基，写作 C₆H₅–。
Disubstituted rings use the prefixes ortho- (1,2-), meta- (1,3-) and para- (1,4-). For example, 1,3-dimethylbenzene is called meta-xylene.
二取代苯使用邻位 (1,2-)、间位 (1,3-) 和对位 (1,4-) 前缀。例如，1,3-二甲基苯称为间二甲苯。
6. General Mechanism of Electrophilic Substitution | 亲电取代反应的一般机理
Benzene reacts with electrophiles by electrophilic substitution. The delocalised ring is electron-rich and attracts electrophiles. The general mechanism has two main stages: attack by the electrophile and restoration of the aromatic ring.
苯与亲电试剂发生亲电取代反应。离域环电子云密度高，会吸引亲电试剂。一般机理包括两个主要阶段：亲电试剂进攻和芳香环恢复。
In step 1, a curly arrow is drawn from the delocalised ring to the electrophile E⁺. This forms a covalent bond and produces a carbocation intermediate, which is no longer aromatic. In step 2, the intermediate loses H⁺, and the delocalised π system is regenerated.
第一步，从离域环指向亲电试剂 E⁺ 画一个弯箭头。这形成共价键，生成碳正离子中间体，该中间体不再具有芳香性。第二步，中间体失去 H⁺，离域 π 体系得以恢复。
The electrophile is usually generated in a separate step using a catalyst. The overall process is substitution because a hydrogen atom on the ring is replaced by the electrophile.
亲电试剂通常在催化剂的帮助下通过单独一步生成。整个过程是取代反应，因为环上的一个氢原子被亲电试剂取代。
7. Nitration of Benzene | 苯的硝化反应
Nitration replaces a hydrogen atom on benzene with a nitro group, –NO₂. Benzene is heated with concentrated nitric acid and concentrated sulfuric acid at about 50–60 °C.
硝化反应将苯上的一个氢原子替换为硝基 –NO₂。苯与浓硝酸和浓硫酸在约 50–60 °C 下加热。
The electrophile is the nitronium ion, NO₂⁺. It is formed when nitric acid reacts with sulfuric acid:
亲电试剂是硝鎓离子 NO₂⁺。它由硝酸与硫酸反应生成：
HNO₃ + H₂SO₄ → NO₂⁺ + HSO₄⁻ + H₂O
Sulfuric acid acts as a catalyst because HSO₄⁻ can later regenerate H₂SO₄. The overall reaction is:
硫酸起催化剂作用，因为 HSO₄⁻ 随后可以再生成 H₂SO₄。总反应为：
C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O
Nitrobenzene is an important intermediate in the manufacture of dyes, pharmaceuticals and explosives. If the temperature is too high, further nitration may occur, giving 1,3-dinitrobenzene.
硝基苯是制造染料、药物和炸药的重要中间体。如果温度过高，可能发生进一步硝化，生成 1,3-二硝基苯。
8. Friedel-Crafts Acylation | 傅克酰基化反应
Friedel-Crafts acylation introduces an acyl group, RCO–, onto the benzene ring. An acyl chloride or acid anhydride reacts with benzene in the presence of anhydrous aluminium chloride, AlCl₃.
傅克酰基化将酰基 RCO– 引入苯环。酰氯或酸酐在无水三氯化铝 AlCl₃ 存在下与苯反应。
AlCl₃ generates the acylium ion, which is the electrophile. For example, ethanoyl chloride reacts as follows:
AlCl₃ 生成酰基正离子，即亲电试剂。例如，乙酰氯的反应如下：
CH₃COCl + AlCl₃ → CH₃CO⁺ + AlCl₄⁻
The acylium ion is then attacked by the benzene ring. The product is a phenyl ketone. For example, benzene and ethanoyl chloride give phenylethanone:
然后苯环进攻酰基正离子。产物为苯基酮。例如，苯与乙酰氯生成苯乙酮：
C₆H₆ + CH₃COCl → C₆H₅COCH₃ + HCl
Acylation is useful in synthesis because the acyl product can be reduced to an alkylbenzene. This avoids the problem of polyalkylation that often occurs in direct alkylation.
酰基化在合成中很有用，因为酰基产物可以被还原为烷基苯。这避免了直接烷基化中常见的多烷基化问题。
9. Friedel-Crafts Alkylation | 傅克烷基化反应
Friedel-Crafts alkylation introduces an alkyl group, R–, onto the benzene ring. A halogenoalkane reacts with benzene using aluminium chloride as a catalyst.
傅克烷基化将烷基 R– 引入苯环。卤代烷在氯化铝催化下与苯反应。
The electrophile is a carbocation, R⁺. For example, chloromethane forms the methyl carbocation:
亲电试剂是碳正离子 R⁺。例如，氯甲烷生成甲基碳正离子：
CH₃Cl + AlCl₃ → CH₃⁺ + AlCl₄⁻
The product is an alkylbenzene, such as methylbenzene from chloromethane. However, alkylation often gives polyalkylated products because the alkylbenzene is more reactive than benzene itself.
产物是烷基苯，例如氯甲烷生成甲苯。然而，烷基化经常产生多烷基化产物，因为烷基苯比苯本身更活泼。
The catalyst must be anhydrous because AlCl₃ reacts readily with water. For this reason, and because of polyalkylation, acylation followed by reduction is usually preferred in synthesis.
催化剂必须无水，因为 AlCl₃ 极易与水反应。由于这一点以及多烷基化问题，在合成中通常优先选择先酰基化再还原的方法。
10. Reactivity Comparison: Benzene vs Alkenes | 苯与烯烃的反应性比较
Alkenes readily undergo electrophilic addition, whereas benzene
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