Benzene and Its Compounds | 苯及其化合物

📚 Benzene and Its Compounds | 苯及其化合物

Benzene is an aromatic hydrocarbon with the molecular formula C₆H₆. It is the simplest member of a class of compounds known as arenes, which are characterised by a delocalised π electron system. The study of benzene and its derivatives is fundamental in A-Level Chemistry, as it bridges organic synthesis, reaction mechanisms, and industrial applications.

苯是一种芳香烃，分子式为 C₆H₆。它是最简单的芳烃类化合物，其典型特征是具有离域 π 电子体系。苯及其衍生物的学习在 A-Level 化学中至关重要，连接了有机合成、反应机理和工业应用。

1. Structure of Benzene | 苯的结构

The molecular formula C₆H₆ suggests a high degree of unsaturation. Early chemists proposed the Kekulé structure, a ring of six carbon atoms with alternating single and double bonds. However, this model fails to explain several experimental observations.

分子式 C₆H₆ 表明其高度不饱和。早期化学家提出了凯库勒结构，即六个碳原子环上单双键交替排列。但这一模型无法解释多个实验观察结果。

Modern understanding describes benzene as a planar, hexagonal molecule where all carbon-carbon bonds are identical, with a bond length intermediate between a single and a double bond. Each carbon atom uses sp² hybridisation, forming σ bonds to two carbons and one hydrogen. The remaining unhybridised p orbital on each carbon overlaps sideways to create a delocalised π electron cloud above and below the ring.

现代理解描述苯为平面正六边形分子，所有碳-碳键完全相同，键长介于单键和双键之间。每个碳原子采用 sp² 杂化，与两个碳原子和一个氢原子形成 σ 键。每个碳上未杂化的 p 轨道侧向重叠，在环的上方和下方形成离域 π 电子云。

2. Evidence for Delocalisation | 离域结构的证据

Three key pieces of evidence support the delocalised model: bond lengths, enthalpy of hydrogenation, and resistance to addition reactions. X-ray diffraction shows all C–C bonds in benzene are 0.139 nm, equal in length and between the typical C–C single bond (0.154 nm) and C=C double bond (0.134 nm).

三项关键证据支持离域模型：键长、氢化焓以及抗拒加成反应。X 射线衍射显示苯中所有碳-碳键长度均为 0.139 nm，介于典型 C–C 单键（0.154 nm）和 C=C 双键（0.134 nm）之间。

The enthalpy of hydrogenation provides thermodynamic evidence. If benzene had three isolated double bonds, its hydrogenation enthalpy would be expected to be three times that of cyclohexene (−120 kJ mol⁻¹), i.e. −360 kJ mol⁻¹. The actual value is only −208 kJ mol⁻¹, meaning benzene is 152 kJ mol⁻¹ more stable than the hypothetical Kekulé structure. This extra stability is called the delocalisation energy or resonance energy.

氢化焓提供了热力学证据。如果苯含有三个孤立的双键，其氢化焓预计为环己烯的三倍（−120 kJ mol⁻¹），即 −360 kJ mol⁻¹。实际测量值仅为 −208 kJ mol⁻¹，这意味着苯比假想的凯库勒结构稳定 152 kJ mol⁻¹。这额外的稳定性被称为离域能或共振能。

Furthermore, benzene does not readily undergo addition reactions like alkenes. It does not decolourise bromine water under normal conditions, indicating the absence of localised double bonds.

此外，苯不像烯烃那样容易发生加成反应。它在通常条件下不能使溴水褪色，这表明没有定域的双键。

3. Electrophilic Substitution of Benzene | 苯的亲电取代反应

Due to its stable delocalised ring, benzene undergoes electrophilic substitution rather than addition. The general mechanism involves an electrophile (E⁺) attacking the π electron cloud, forming a carbocation intermediate (the Wheland intermediate or arenium ion), followed by loss of a proton to restore aromaticity.

由于其稳定的离域环，苯进行亲电取代而不是加成。一般机理是亲电试剂（E⁺）进攻 π 电子云，形成碳正离子中间体（韦兰德中间体或芳基正离子），

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