📚 Aromatic Compounds Revision for IGCSE WJEC Chemistry | IGCSE WJEC 化学：芳香族化合物 考点精讲
In this article, we break down the key concepts about aromatic compounds for your IGCSE WJEC Chemistry exam. You will learn about the unique structure of benzene, how it reacts differently from alkenes, and how to write equations for its characteristic substitution reactions.
在这篇文章中，我们为你的 IGCSE WJEC 化学考试梳理芳香族化合物的核心概念。你将学习苯的独特结构、它与烯烃反应方式的差异，以及如何书写其特征取代反应的方程式。
1. What Are Aromatic Compounds? | 什么是芳香族化合物？
Aromatic compounds are organic molecules that contain a benzene ring in their structure. The term ‘aromatic’ originally referred to their sweet smell, but in modern chemistry it describes compounds with a specific type of stable ring of delocalised electrons.
芳香族化合物是结构中含有苯环的有机分子。术语“芳香”最初指它们的甜香气，但在现代化学中它描述的是具有特定类型稳定离域电子环的化合物。
The simplest aromatic hydrocarbon is benzene, with the molecular formula C₆H₆. Many common aromatic compounds are derivatives of benzene, where one or more hydrogen atoms have been replaced by other atoms or groups.
最简单的芳香烃是苯，分子式为 C₆H₆。许多常见的芳香族化合物都是苯的衍生物，其中一个或多个氢原子被其他原子或基团取代。
2. The Structure of Benzene | 苯的结构
Benzene consists of a hexagonal ring of six carbon atoms, with each carbon bonded to one hydrogen atom. The Kekulé model proposed alternating single and double bonds (cyclohexa-1,3,5-triene), but modern evidence shows that all six carbon–carbon bonds are identical in length and strength.
苯由一个六元碳环组成，每个碳原子上连接一个氢原子。凯库勒模型提出单双键交替结构（环己-1,3,5-三烯），但现代证据表明所有六个碳–碳键都具有相同的长度和强度。
The true structure is a resonance hybrid, where the p orbitals on each carbon overlap above and below the ring plane, forming a delocalised π electron cloud. This is often represented by a hexagon with a circle inside.
真实结构是共振杂化体，每个碳上的 p 轨道在环平面上方和下方重叠，形成一个离域的 π 电子云。这常用带圆圈的六边形表示。
3. Delocalised Electrons and Stability | 离域电子与稳定性
In benzene, each carbon atom uses three of its four valence electrons to form sigma bonds with two neighbouring carbons and one hydrogen. The remaining p electron on each carbon is not fixed in a double bond; it becomes delocalised around the ring.
在苯中，每个碳原子用四个价电子中的三个与两个相邻碳和一个氢形成 σ 键。每个碳上剩余的 p 电子并不固定在双键中，而是在整个环中离域。
This delocalisation gives benzene exceptional thermodynamic stability. The enthalpy change for hydrogenation of benzene is much less exothermic than expected for a hypothetical cyclohexatriene, demonstrating that the delocalised structure is more stable.
这种离域使苯具有非凡的热力学稳定性。苯的氢化焓变远低于假想的环己三烯的预期值，证明了离域结构更为稳定。
4. Naming Aromatic Compounds | 芳香族化合物的命名
When a hydrogen on the benzene ring is replaced by another atom or group, the compound is usually named as a substituted benzene. For example, C₆H₅Cl is chlorobenzene, C₆H₅NO₂ is nitrobenzene, and C₆H₅CH₃ is methylbenzene (commonly known as toluene).
当苯环上的氢被另一原子或基团取代时，化合物通常命名为取代苯。例如 C₆H₅Cl 称为氯苯，C₆H₅NO₂ 称为硝基苯，C₆H₅CH₃ 称为甲基苯（俗称甲苯）。
If two groups are present, their positions are numbered (e.g. 1,2-dimethylbenzene). In you IGCSE course, you are most likely to encounter simple monosubstituted benzenes.
如果存在两个基团，则需要用数字标示位置（如 1,2-二甲苯）。在你的 IGCSE 课程中，你很可能只遇到简单的单取代苯。
5. Physical Properties of Benzene | 苯的物理性质
At room temperature, benzene is a colourless, volatile liquid with a characteristic aromatic odour. It is insoluble in water but miscible with most organic solvents. Benzene is highly flammable and burns with a very sooty flame due to its high carbon-to-hydrogen ratio.
在室温下，苯是一种无色、有挥发性的液体，具有特殊的芳香气味。它不溶于水，但可与大多数有机溶剂混溶。苯高度易燃，由于其碳氢比高，燃烧时会产生浓烟火焰。
Benzene is a toxic and carcinogenic substance, so it must be handled in a fume cupboard. In the lab, safer substituted benzenes are often used when possible.
苯是一种有毒且致癌的物质，因此必须在通风橱中操作。在实验室中，可能时会使用更安全的取代苯。
6. Combustion of Benzene | 苯的燃烧
Like other hydrocarbons, benzene undergoes complete combustion in excess oxygen to form carbon dioxide and water. The equation is:
与其他烃类类似，苯在过量氧气中完全燃烧生成二氧化碳和水。方程式为：
2C₆H₆ + 15O₂ → 12CO₂ + 6H₂O
Incomplete combustion produces carbon monoxide and carbon particles (soot). The characteristic black smoke from burning benzene is a key visual indicator used to distinguish it from less carbon-rich fuels.
不完全燃烧会产生一氧化碳和碳颗粒（烟灰）。燃烧苯时产生的黑色浓烟是区分它与其他含碳量较低燃料的关键视觉指标。
7. Reaction Overview: Benzene Prefers Substitution | 反应概述：苯倾向于取代反应
Because of the stabilising delocalised ring of electrons, benzene does not easily undergo addition reactions like alkenes do. Addition would disrupt the delocalised system, making it energetically unfavourable.
由于离域电子环的稳定作用，苯不易像烯烃那样发生加成反应。加成会破坏离域体系，这在能量上是不利的。
Instead, benzene typically undergoes electrophilic substitution reactions. In these reactions, an atom or group replaces one of the hydrogen atoms on the ring while preserving the aromatic stability.
相反，苯通常发生亲电取代反应。在这些反应中，一个原子或基团取代环上的一个氢原子，同时保留芳香稳定性。
8. Halogenation of Benzene | 苯的卤化反应
Benzene reacts with chlorine or bromine in the presence of a halogen carrier catalyst, such as anhydrous aluminium chloride (AlCl₃) or iron(III) chloride (FeCl₃). The reaction produces chlorobenzene or bromobenzene and a hydrogen halide gas.
苯在卤素载体催化剂（如无水氯化铝 AlCl₃ 或氯化铁 FeCl₃）存在下与氯气或溴反应。反应生成氯苯或溴苯和卤化氢气体。
The general equation for chlorination is:
氯化的通用方程式为：
C₆H₆ + Cl₂ → C₆H₅Cl + HCl (AlCl₃ catalyst)
Note that this is not an addition reaction; the benzene ring retains its delocalised structure. The catalyst polarises the halogen molecule, generating an electrophile that attacks the ring.
注意这不是加成反应；苯环保留了其离域结构。催化剂使卤素分子极化，产生亲电试剂攻击苯环。
9. Nitration of Benzene | 苯的硝化反应
Nitration is another important electrophilic substitution. Benzene is heated with a mixture of concentrated nitric acid and concentrated sulfuric acid at around 50–60 °C. This generates the electrophile NO₂⁺ (nitronium ion).
硝化反应是另一个重要的亲电取代反应。苯与浓硝酸和浓硫酸的混合物在约 50–60 °C 下加热，生成亲电试剂 NO₂⁺（硝鎓离子）。
The balanced equation for nitration is:
硝化反应的配平方程式为：
C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O (H₂SO₄ catalyst, 50–60 °C)
If the temperature rises too high, further substitution can occur, producing dinitrobenzene. Careful temperature control is therefore essential in the laboratory.
如果温度过高，会发生进一步取代，生成二硝基苯。因此在实验室中小心控制温度至关重要。
10. Comparing Benzene and Alkenes | 苯与烯烃的比较
Alkenes contain a localised C=C double bond and readily undergo addition reactions, such as the decolorisation of bromine water and hydrogenation. Benzene, with its delocalised ring, resists addition under the same conditions.
烯烃含有定域的 C=C 双键，容易发生加成反应，如使溴水褪色和加氢。而苯具有离域环，在相同条件下抵抗加成反应。
The table below summarises the key differences:
下表总结了关键区别：
| Property | Benzene | Ethene (typical alkene) |
|---|---|---|
| C–C bond type | Delocalised; all equal in length | Localised C=C and C–C |
| Reaction with bromine water | No decolorisation without catalyst; orange colour may be extracted | Decolorises instantly (addition) |
| Typical reaction | Electrophilic substitution | Electrophilic addition |
11. Distinguishing Benzene from Alkenes in the Lab | 实验室中区分苯和烯烃
Add a few drops of bromine water to a sample of the unknown liquid in a test tube. If the bromine water remains orange after shaking and stands, but the organic layer may become orange due to extraction, the sample is likely benzene.
向试管中的未知液体样品中滴加几滴溴水。若振荡静置后溴水层仍为橙色，而有机层可能因萃取变为橙色，则样品很可能是苯。
If the bromine water is decolorised immediately, the sample contains a C=C double bond and is an alkene or an unsaturated compound. Benzene does not undergo this addition reaction under normal laboratory conditions.
如果溴水立即褪色，则样品含有 C=C 双键，是烯烃或不饱和化合物。在正常实验室条件下，苯不会发生这种加成反应。
12. Summary of Key Reactions and Concepts | 关键反应与概念总结
You should be confident in writing equations for the combustion, halogenation, and nitration of benzene, and in explaining why benzene undergoes substitution rather than addition. Remember the importance of catalysts and temperature control, and the use of the delocalised model to explain aromatic stability.
你应熟练掌握苯的燃烧、卤化和硝化反应的方程式书写，并能解释苯为什么发生取代反应而不是加成反应。记住催化剂和温度控制的重要性，以及用离域模型解释芳香稳定性。
Revising these points thoroughly will prepare you well for exam questions on aromatic compounds. Good luck!
彻底复习这些要点将使你从容应对有关芳香族化合物的考试题目。祝你好运！
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