📚 Benzene and its Compounds | 苯及其化合物
Benzene is the simplest aromatic hydrocarbon and the foundational building block of an entire family of compounds known as arenes. Understanding benzene's unusual structure and reactivity is essential for Cambridge A-Level Chemistry, as it ties together concepts of bonding, energetics, and mechanism in a single elegant molecule.
苯是最简单的芳香烃，也是整个芳烃化合物家族的基础构件。理解苯独特的结构和反应活性对剑桥A-Level化学至关重要，它将成键、能量学和反应机理的概念汇集于一个简洁优美的分子之中。
This revision article moves from the historical Kekulé model, through the experimental evidence that replaced it, to the full electrophilic substitution mechanisms of nitration, halogenation and Friedel-Crafts reactions. It then extends the same ideas to substituted arenes such as methylbenzene and phenol, and finishes with the exam technique that turns this knowledge into marks.
本文从历史上的凯库勒模型出发，经过取代它的实验证据，讲到达成完整书写水平的硝化、卤代与傅-克反应亲电取代机理；随后把同样的思路扩展到甲基苯、苯酚等取代芳烃，最后给出把知识转化为分数的考试技巧。
1. The Structure of Benzene | 苯的结构
The empirical formula of benzene is CH, with molecular formula C₆H₆ and relative molecular mass 78. German chemist Friedrich August Kekulé proposed in 1865 that benzene is a six-membered ring with alternating single and double carbon-carbon bonds, like a 'cyclohexa-1,3,5-triene'.
苯的实验式为CH，分子式为C₆H₆，相对分子质量为78。德国化学家弗里德里希·奥古斯特·凯库勒于1865年提出苯是一个六元环，碳碳单键与双键交替排列，如同"环己-1,3,5-三烯"。
The Kekulé structure, however, cannot explain why benzene fails to undergo typical alkene reactions such as bromine water decolourisation. Modern evidence shows that all six C-C bonds are identical; the six p-orbitals overlap sideways to form a ring of delocalised π-electrons above and below the plane of the carbon atoms.
然而，凯库勒结构无法解释苯为何不发生烯烃的典型反应（如使溴水褪色）。现代证据表明六个C-C键完全相同；六个p轨道侧面重叠，在碳原子平面的上方和下方形成一个离域π电子环。
In the accepted delocalised model, each carbon atom is bonded to two other carbons and one hydrogen atom, so it is sp² hybridised with bond angles of 120°. The remaining unhybridised p-orbital on every carbon lies perpendicular to the ring plane, and adjacent p-orbitals overlap sideways to give one continuous π-system containing six delocalised electrons shared by all six carbons.
在公认的离域模型中，每个碳原子与另外两个碳原子及一个氢原子成键，因此为sp²杂化，键角120°。每个碳上剩余的一个未杂化p轨道垂直于环平面，相邻p轨道侧面重叠，形成一个连续的π体系，六个离域电子由全部六个碳原子共享。
C₆H₆: six sp² carbons, six p-orbitals, one delocalised π-system
C₆H₆：六个sp²碳原子、六个p轨道、一个离域π体系
The molecule is therefore a flat, regular hexagon. The delocalised electrons are drawn either as a circle inside the hexagon or as two shaded lobes above and below the ring; both conventions represent the same physical picture, and examiners accept either.
因此该分子是一个平面正六边形。离域电子既可以画成环内的圆圈，也可以画成环上、环下两片阴影区；两种表示法描述同一物理图像，考官两者都接受。
2. Evidence for the Delocalised Model | 离域模型的证据
Three pieces of experimental evidence support the delocalised model over Kekulé's alternating double-bond structure.
有三项实验证据支持离域模型而非凯库勒的交替双键结构。
Enthalpy of hydrogenation: nds in a hypothetical cyclohexa-1,3,5-triene would release about −360 kJ mol⁻¹, but benzene releases only −208 kJ mol⁻¹. Benzene is 152 kJ mol⁻¹ more stable than the Kekulé structure predicts.
氢化焓：假想的环己-1,3,5-三烯中三个孤立C=C键氢化应放出约−360 kJ mol⁻¹，而苯只放出−208 kJ mol⁻¹。苯比凯库勒结构预测的稳定性高出152 kJ mol⁻¹。
X-ray diffraction and bond lengths: ouble bond (134 pm). Alternating single and double bonds would give two different lengths.
X射线衍射与键长：苯中六个C-C键长完全相同，约139 pm，介于C-C单键（154 pm）与C=C双键（134 pm）之间。若单双键交替，应出现两种不同键长。
Isomer count and chemical behaviour: only one 1,2-disubstituted benzene exists, whereas the Kekulé structure predicts two isomers; benzene also does not decolourise bromine water and does not undergo addition with HBr under normal conditions.
异构体数目与化学行为：1,2-二取代苯只有一种，而凯库勒结构预测应有两种异构体；此外苯不能使溴水褪色，在通常条件下也不与HBr发生加成反应。
C₆H₆ + 3H₂ → C₆H₁₂ ΔH = −208 kJ mol⁻¹
C₆H₆ + 3H₂ → C₆H₁₂ ΔH = −208 kJ mol⁻¹（实际值）
Delocalisation energy = 360 − 208 = 152 kJ mol⁻¹
离域能 = 360 − 208 = 152 kJ mol⁻¹
The 152 kJ mol⁻¹ difference is called the delocalisation energy (or resonance energy). It is the extra stability benzene gains by spreading six electrons over six nuclei instead of confining them to three localised π-bonds.
这152 kJ mol⁻¹的差值称为离域能（或称共振能）。它是苯把六个电子分散在六个原子核周围、而不是局限于三个定域π键所获得的额外稳定性。
3. Shape, Bonding and Physical Properties | 形状、成键与物理性质
Because every carbon is sp² hybridised, benzene is planar with bond angles of exactly 120° and a perfectly regular hexagonal shape. The carbon skeleton is held together by six σ-bonds, and the delocalised π-system is responsible for the characteristic ultraviolet absorption of aromatic compounds.
由于每个碳都是sp²杂化，苯是平面分子，键角恰为120°，呈完全规则的正六边形。碳骨架由六个σ键连接，而离域π体系则造成芳香族化合物特征性的紫外吸收。
Benzene is a non-polar molecule, so it is insoluble in water but miscible with organic solvents such as ethanol and cyclohexane. It is a volatile liquid at room temperature with a boiling point of about 80 °C and a melting point of about 5.5 °C, and it is toxic and carcinogenic, so it must be handled in a fume cupboard.
苯是非极性分子，因此不溶于水，但可与乙醇、环己烷等有机溶剂互溶。室温下它是挥发性液体，沸点约80 °C，熔点约5.5 °C；苯有毒且致癌，必须在通风橱中操作。
Benzene burns in air with a very smoky, luminous flame because its carbon-to-hydrogen ratio (1:1) is much higher than that of alkanes and alkenes, so incomplete combustion produces large amounts of soot.
苯在空气中燃烧时火焰明亮并带浓烟，因为其碳氢比（1:1）远高于烷烃和烯烃，不完全燃烧会生成大量炭黑。
2C₆H₆ + 15O₂ → 12CO₂ + 6H₂O
2C₆H₆ + 15O₂ → 12CO₂ + 6H₂O
4. Why Substitution Rather than Addition | 为何取代而非加成
Alkenes readily undergo electrophilic addition because the reaction converts a weaker π-bond into two stronger σ-bonds. In benzene, the delocalised π-system must first be broken, and that costs about 152 kJ mol⁻¹ of stabilisation energy, which is not recovered by simple addition.
烯烃容易发生亲电加成，因为该反应把较弱的π键转化为两个较强的σ键。而在苯中，必须首先破坏离域π体系，这要付出约152 kJ mol⁻¹的稳定化能量代价，简单加成无法将其收回。
In electrophilic substitution, however, the aromatic ring is broken only temporarily. After the electrophile has attacked and a hydrogen ion has been lost from the same carbon, the full delocalised π-system is regenerated, so the molecule recovers its aromatic stability.
然而在亲电取代中，芳香环只是被暂时破坏。亲电试剂进攻之后，同一个碳上失去一个氢离子，完整的离域π体系随即恢复，分子重获芳香稳定性。
This is the key thermodynamic argument that A-Level answers must contain: benzene substitutes because substitution preserves aromaticity, whereas addition destroys it permanently.
这就是A-Level答案中必须包含的关键热力学论证：苯发生取代，是因为取代保住了芳香性，而加成会将其永久破坏。
5. Naming Benzene Derivatives | 苯衍生物的命名
When an alkyl group or another substituent is attached to the ring, the compound is named as a derivative of benzene. If two substituents are present, the positions are described by numbers 1,2-, 1,3- or 1,4-, or by the traditional prefixes ortho-, meta- and para-.
当烷基或其他取代基连接在苯环上时，该化合物按苯的衍生物命名。若有两个取代基，其位置用1,2-、1,3-或1,4-表示，也可用传统前缀邻（ortho-）、间（meta-）、对（para-）。
| Formula | 分子式 | Name | 名称 | Note | 说明 |
|---|---|---|
| C₆H₅CH₃ | methylbenzene | 甲基苯（甲苯） | Simplest substituted arene | 最简单的取代芳烃 |
| C₆H₅NO₂ | nitrobenzene | 硝基苯 | Product of nitration | 硝化产物 |
| C₆H₅Br | bromobenzene | 溴苯 | Requires FeBr₃ catalyst | 需FeBr₃催化 |
| C₆H₅OH | phenol | 苯酚 | -OH directly on the ring | -OH直接连在环上 |
| C₆H₅COOH | benzoic acid | 苯甲酸 | Side-chain oxidation product | 侧链氧化产物 |
The group left behind when benzene loses one hydrogen is the phenyl group, C₆H₅−, which must not be confused with the benzyl group, C₆H₅CH₂−, in which the CH₂ is outside the ring.
苯失去一个氢后留下的基团是苯基，C₆H₅−；不要与苄基C₆H₅CH₂−混淆，后者的CH₂位于环外。
6. General Mechanism of Electrophilic Substitution | 亲电取代的一般机理
All the ring reactions of benzene follow the same three-stage pattern: generation of a strong electrophile, attack of the electrophile by the π-system to give a delocalised carbocation intermediate, and finally loss of H⁺ to restore the aromatic ring.
苯的所有环上反应都遵循同一三步模式：先生成强亲电试剂；π体系进攻亲电试剂，形成离域碳正离子中间体；最后失去H⁺，恢复芳香环。
Step 1 - forming the electrophile: a catalyst or strong acid generates a species with a full or partial positive charge, written E⁺ or Eδ⁺.
第一步——生成亲电试剂：催化剂或强酸生成带完整正电荷或部分正电荷的物种，写作E⁺或Eδ⁺。
Step 2 - attack: a curly arrow from the delocalised ring (or from the centre of the ring) to the electrophile forms a new σ-bond; the ring temporarily contains only four delocalised electrons spread over five carbons, so the intermediate carries a positive charge. This is sometimes called the arenium ion.
第二步——进攻：从离域环（或环中心）指向亲电试剂的弯箭头形成新的σ键；环中暂时只剩四个离域电子分布在五个碳上，因此中间体带正电荷，常称为芳正离子中间体。
Step 3 - restoring aromaticity: a curly arrow from the C-H bond on the sp³ carbon to the C-C bond, together with a curly arrow from that bond back into the ring, eliminates H⁺ and regenerates the six-electron delocalised π-system.
第三步——恢复芳香性：从sp³碳上的C-H键指向C-C键的弯箭头，配合从该键回入环内的弯箭头，消除H⁺并再生六电子离域π体系。
C₆H₆ + E⁺ → [C₆H₆E]⁺ → C₆H₅E + H⁺
C₆H₆ + E⁺ → [C₆H₆E]⁺ → C₆H₅E + H⁺
When drawing the intermediate in an exam, show the five remaining carbons with a broken circle or a horseshoe-shaped partial ring, place the positive charge clearly on the intermediate, and never draw the product as an addition product with the ring broken.
在考试中画该中间体时，用断开的部分圆或马蹄形表示剩余五个碳，清楚标出中间体上的正电荷，绝不要把产物画成环被破坏的加成产物。
7. Nitration of Benzene | 苯的硝化
Benzene is nitrated by warming it with a mixture of concentrated nitric acid and concentrated sulfuric acid at 50-60 °C. The sulfuric acid acts as a catalyst and also as the source of the electrophile.
苯与浓硝酸和浓硫酸的混合酸在50-60 °C下温热即发生硝化。硫酸既作催化剂，又是亲电试剂的来源。
The nitrating mixture generates the nitronium ion, NO₂⁺, by protonating and then dehydrating nitric acid. Two molecules of sulfuric acid are needed in the balanced equation for the electrophile-forming step.
硝化混酸通过先质子化硝酸、再脱水，生成硝酰正离子NO₂⁺。生成亲电试剂的配平方程式中需要两分子硫酸。
HNO₃ + 2H₂SO₄ → NO₂⁺ + H₃O⁺ + 2HSO₄⁻
HNO₃ + 2H₂SO₄ → NO₂⁺ + H₃O⁺ + 2HSO₄⁻
The nitronium ion attacks the ring, the intermediate loses H⁺, and that proton is captured by HSO₄⁻ to regenerate sulfuric acid, which is why the acid is described as a catalyst in the overall equation.
硝酰正离子进攻苯环，中间体失去H⁺，该质子被HSO₄⁻捕获并重新生成硫酸，因此硫酸在总方程式中被描述为催化剂。
C₆H₆ + NO₂⁺ → C₆H₅NO₂ + H⁺ H⁺ + HSO₄⁻ → H₂SO₄
C₆H₆ + NO₂⁺ → C₆H₅NO₂ + H⁺ H⁺ + HSO₄⁻ → H₂SO₄
Overall: C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O
总反应：C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O
Nitrobenzene is a pale yellow, oily liquid with an almond-like smell, and it is an important intermediate in making dyes and pharmaceuticals. If the temperature rises above about 60 °C, further nitration occurs and 1,3-dinitrobenzene is also formed.
硝基苯为淡黄色油状液体，有苦杏仁气味，是制造染料和药物的重要中间体。若温度升到60 °C以上，会发生进一步硝化，同时生成1,3-二硝基苯。
8. Halogenation and Halogen Carriers | 卤代反应与卤素载体
is direct chemical evidence for delocalisation. To brominate the ring, a halogen carrier catalyst such as FeBr₃, or iron filings that generate FeBr₃ in situ, must be present.
苯在室温下不与溴水反应：检验C=C双键的试验呈阴性，这是离域结构最直接的化学证据。要使苯环溴代，必须有卤素载体催化剂，如FeBr₃，或能在原位生成FeBr₃的铁屑。
The halogen carrier polarises the halogen molecule and generates a strongly electrophilic species, Br⁺, which attacks the ring in the same way as NO₂⁺.
卤素载体使卤素分子极化，生成强亲电物种Br⁺，其进攻苯环的方式与NO₂⁺相同。
Br₂ + FeBr₃ → Br⁺ + FeBr₄⁻
Br₂ + FeBr₃ → Br⁺ + FeBr₄⁻
<p style="text-align更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图