📚 Aromatic Compounds: Core Concepts for IB & Edexcel Chemistry | 芳香族化合物：IB与Edexcel化学核心考点精讲
Aromatic compounds, centred around the unique stability and reactivity of the benzene ring, form a cornerstone of organic chemistry in both the IB Diploma (HL) and Edexcel A Level specifications. Understanding the delocalised π-electron system, the mechanism of electrophilic substitution, and the directing effects of substituents is essential for success in examinations. This guide systematically covers the key concepts, reactions, and mechanisms you need to master.
芳香族化合物以苯环独特的稳定性和反应性为核心，构成了IB文凭（高水平）和Edexcel A Level有机化学的基石。掌握离域π电子体系、亲电取代反应机理以及取代基的定位效应是考试成功的关键。本指南系统地涵盖了你需要掌握的核心概念、反应和机理。
1. Introduction to Aromaticity | 芳香性简介
What makes a compound aromatic? According to Hückel’s rule, a planar, cyclic molecule with (4n+2) π electrons (where n is an integer) is aromatic and exhibits exceptional stability. Benzene (C₆H₆) is the archetypal aromatic compound with 6 π electrons (n=1). This aromatic stability is why benzene undergoes substitution rather than addition reactions, distinguishing it from alkenes.
什么使化合物具有芳香性？根据休克尔规则，具有(4n+2)个π电子（n为整数）的平面环状分子是芳香性的，并表现出特殊的稳定性。苯（C₆H₆）是典型的芳香族化合物，拥有6个π电子（n=1）。这种芳香稳定性解释了为什么苯进行取代而非加成反应，从而区别于烯烃。
2. Structure of Benzene | 苯的结构
Benzene is a planar hexagonal molecule with bond angles of 120°. All carbon-carbon bonds are identical in length (139 pm), intermediate between a single C–C bond (154 pm) and a double C=C bond (134 pm). The current model describes the structure as a delocalised π-electron cloud above and below the ring, formed by the sideways overlap of six p orbitals.
苯是一个平面正六边形分子，键角为120°。所有碳碳键的长度完全相同（139 pm），介于单键（154 pm）和双键（134 pm）之间。当前模型将结构描述为环上下方的离域π电子云，由六个p轨道侧向重叠形成。
3. Evidence for the Delocalised Model | 离域模型的证据
Thermochemical evidence: Hydrogenation of benzene is less exothermic than expected for a triene. Hydrogenation of cyclohexene releases -120 kJ mol⁻¹ per double bond, so a hypothetical ‘cyclohexatriene’ would be expected to give -360 kJ mol⁻¹. However, actual hydrogenation of benzene releases only -208 kJ mol⁻¹, revealing a stabilisation energy of about 152 kJ mol⁻¹. Moreover, benzene does not decolourise bromine water without a catalyst, and all C–C bond lengths are equal, as revealed by X-ray diffraction.
热化学证据：苯的氢化比三烯预期的放热要少。环己烯每个双键的氢化热为 -120 kJ mol⁻¹，因此假想的“环己三烯”预期会放出 -360 kJ mol⁻¹。然而，苯的实际氢化热仅为 -208 kJ mol⁻¹，揭示了约152 kJ mol⁻¹的稳定化能。此外，没有催化剂时苯不能使溴水褪色，且X射线衍射显示所有C–C键长相等。
C₆H₆(l) + 3H₂(g) → C₆H₁₂(l) ΔH = -208 kJ mol⁻¹
4. Nomenclature of Aromatic Compounds | 芳香族化合物的命名
Monosubstituted benzenes are named with the substituent prefix followed by ‘benzene’, e.g., methylbenzene, chlorobenzene, nitrobenzene. Some common names are accepted: phenol (hydroxybenzene), aniline (aminobenzene), benzoic acid. Disubstituted benzenes use the prefixes ortho- (1,2-), meta- (1,3-), para- (1,4-) or numbers to indicate positions.
单取代苯的命名使用取代基前缀加“苯”，如甲基苯、氯苯、硝基苯。一些俗名也是可以接受的：苯酚（羟基苯）、苯胺（氨基苯）、苯甲酸。二取代苯使用前缀邻-（1,2-）、间-（1,3-）、对-（1,4-）或数字表示位置。
5. Electrophilic Substitution Overview | 亲电取代反应概述
The most characteristic reaction of benzene is electrophilic aromatic substitution (EAS). The delocalised π system attracts electrophiles (E⁺). The general mechanism involves: (1) generation of the electrophile; (2) attack of the electrophile on the ring to form a carbocation intermediate (arenium ion, stabilized by resonance); (3) loss of a proton (H⁺) to restore aromaticity. The overall reaction replaces a hydrogen by an electrophile.
苯最典型的反应是亲电芳香取代（EAS）。离域π体系吸引亲电试剂（E⁺）。一般机理包括：(1) 生成亲电试剂；(2) 亲电试剂攻击苯环形成碳正离子中间体（芳正离子，通过共振稳定）；(3) 失去一个质子（H⁺）以恢复芳香性。总反应是用亲电试剂取代一个氢。
6. Nitration of Benzene | 苯的硝化反应
Benzene reacts with a nitrating mixture of concentrated nitric acid and concentrated sulfuric acid at 50–60 °C to form nitrobenzene. The electrophile is the nitronium ion (NO₂⁺), generated by protonation and dehydration of nitric acid: HNO₃ + 2H₂SO₄ ⇌ NO₂⁺ + H₃O⁺ + 2HSO₄⁻. The reaction is used to introduce the –NO₂ group, which can later be reduced to –NH₂.
苯与浓硝酸和浓硫酸组成的硝化混合物在50–60°C下反应，生成硝基苯。亲电试剂是硝鎓离子（NO₂⁺），由硝酸的质子化和脱水产生：HNO₃ + 2H₂SO₄ ⇌ NO₂⁺ + H₃O⁺ + 2HSO₄⁻。该反应用于引入–NO₂基团，随后可将其还原为–NH₂。
HNO₃ + 2H₂SO₄ ⇌ NO₂⁺ + H₃O⁺ + 2HSO₄⁻
7. Halogenation of Benzene | 苯的卤代反应
Benzene reacts with chlorine or bromine in the presence of a halogen carrier catalyst (e.g., AlCl₃, AlBr₃, FeBr₃) to form chlorobenzene or bromobenzene. The catalyst generates a more powerful electrophile: Br₂ + FeBr₃ → Br⁺ + FeBr₄⁻. The reaction does not occur without the catalyst, reflecting benzene’s stability. Fluorination is too vigorous, and iodination requires a stronger oxidising agent.
苯在卤素载体催化剂（如 AlCl₃、AlBr₃、FeBr₃）存在下与氯或溴反应，生成氯苯或溴苯。催化剂生成更强的亲电试剂：Br₂ + FeBr₃ → Br⁺ + FeBr₄⁻。没有催化剂反应不会发生，反映了苯的稳定性。氟化过于剧烈，碘化则需更强的氧化剂。
Br₂ + FeBr₃ → Br⁺ + FeBr₄⁻
8. Friedel-Crafts Alkylation & Acylation | 傅-克烷基化和酰基化反应
Friedel-Crafts alkylation introduces an alkyl group using a haloalkane and a Lewis acid catalyst (AlCl₃). The electrophile is a carbocation (R⁺). A limitation is polyalkylation and carbocation rearrangements. Friedel-Crafts acylation uses an acyl chloride and AlCl₃ to produce an acylbenzene (ketone). The electrophile is an acylium ion (R–C≡O⁺). Acylation does not suffer from rearrangements and stops after one substitution, thereby being synthetically more useful.
傅-克烷基化使用卤代烷烃和路易斯酸催化剂（AlCl₃）引入烷基。亲电试剂是碳正离子（R⁺）。其局限性包括多烷基化和碳正离子重排。傅-克酰基化使用酰氯和AlCl₃生成酰基苯（酮）。亲电试剂是酰基正离子（R–C≡O⁺）。酰基化不涉及重排，且只进行一次取代，因此在合成上更有用。
9. Reactivity and Directing Effects | 反应活性与定位效应
Substituents on a benzene ring influence both the rate and position of further electrophilic substitution. Activating groups (electron-donating) such as –OH, –NH₂, and alkyl groups direct new substituents to the ortho/para positions and increase reaction rate. Deactivating groups (electron-withdrawing) such as –NO₂, –COOH, and –SO₃H direct to the meta position and slow down the reaction. Halogens are deactivating but ortho/para-directing due to their opposing inductive and resonance effects. These effects are rationalised by the stability of the intermediate carbocation.
苯环上的取代基会影响进一步亲电取代的速率和位置。活化基团（给电子）如–OH、–NH₂和烷基，将新取代基导向邻对位，并加快反应速率。钝化基团（吸电子）如–NO₂、–COOH和–SO₃H，导向间位并减慢反应。卤素是钝化基团，但由于其相反的诱导和共轭效应，是邻对位定位基。这些效应可以从中间体碳正离子的稳定性来理解。
| Directing Group Type | Examples | Directing Effect | Effect on Rate |
|---|---|---|---|
| Activating (e⁻-donating) | –OH, –NH₂, –R | ortho/para | Increases |
| Deactivating (e⁻-withdrawing) | –NO₂, –COOH, –CN | meta | Decreases |
| Halogens (special case) | –Cl, –Br | ortho/para | Decreases |
10. Phenols: Acidity and Reactions | 酚：酸性与反应
Phenol (C₆H₅OH) is more acidic than alcohols because the phenoxide ion (C₆H₅O⁻) is stabilised by resonance delocalisation of the negative charge into the ring. Its pKa is about 10, compared to ~16 for alcohols. Phenol does not react with carbonates but can be deprotonated by NaOH. Phenol also undergoes electrophilic substitution much more readily than benzene; for instance, bromination with bromine water gives a white precipitate of 2,4,6-tribromophenol without a catalyst.
苯酚的酸性比醇强，因为酚氧负离子（C₆H₅O⁻）通过负电荷离域到环上而稳定。其pKa约为10，而醇约为16。苯酚不与碳酸盐反应，但可被NaOH去质子化。酚也比苯更容易进行亲电取代反应；例如，与溴水反应无需催化剂即可生成2,4,6-三溴苯酚的白色沉淀。
C₆H₅OH + NaOH → C₆H₅O⁻Na⁺ + H₂O
11. Summary of Key Reactions | 关键反应总结
Electrophilic Aromatic Substitution – A Quick Reference
- Nitration: Benzene + HNO₃/H₂SO₄, 50°C → nitrobenzene. Electrophile: NO₂⁺.
硝化：苯 + HNO₃/H₂SO₄，50°C → 硝基苯。亲电试剂：NO₂⁺。 - Chlorination: Benzene + Cl₂, AlCl₃ catalyst → chlorobenzene. Electrophile: Cl⁺.
氯代：苯 + Cl₂，AlCl₃催化剂 → 氯苯。亲电试剂：Cl⁺。 - Bromination: Benzene + Br₂, FeBr₃ catalyst → bromobenzene. Electrophile: Br⁺.
溴代：苯 + Br₂，FeBr₃催化剂 → 溴苯。亲电试剂：Br⁺。 - Friedel-Crafts Alkylation: Benzene + RCl, AlCl₃ → alkylbenzene. Electrophile: R⁺.
傅-克烷基化：苯 + RCl，AlCl₃ → 烷基苯。亲电试剂：R⁺。 - Friedel-Crafts Acylation: Benzene + RCOCl, AlCl₃ → acylbenzene. Electrophile: R–C≡O⁺.
傅-克酰基化：苯 + RCOCl，AlCl₃ → 酰基苯。亲电试剂：R–C≡O⁺。 - Sulphonation: Benzene + fuming H₂SO₄ → benzenesulphonic acid. Electrophile: SO₃.
磺化：苯 + 发烟H₂SO₄ → 苯磺酸。亲电试剂：SO₃。
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