Aromatic Compounds | 芳香族化合物

📚 Aromatic Compounds | 芳香族化合物

Aromatic compounds are a fascinating class of organic molecules that contain one or more benzene rings. They are central to the production of dyes, pharmaceuticals, plastics, and explosives. Understanding their unique stability, substitution reactions, and the influence of substituents is a core part of A-Level Chemistry. This article covers the key concepts required for the CCEA specification, from the structure of benzene to the reactions of phenol and phenylamine.

芳香族化合物是一类含有苯环的有机分子，广泛用于染料、药物、塑料和炸药的生产。理解它们独特的稳定性、取代反应以及取代基的影响是A-Level化学的核心内容。本文覆盖CCEA考试大纲的关键概念，从苯的结构到苯酚和苯胺的反应。

1. Introduction to Aromatic Chemistry | 芳香族化学简介

The term ‘aromatic’ originally referred to fragrant compounds, but now it describes any planar, cyclic molecule with a delocalised π‑electron system that follows Hückel’s rule (4n+2 π electrons). Benzene, C₆H₆, is the simplest aromatic compound and serves as the parent structure for countless derivatives.

“芳香族”一词最初指有香味的化合物，但现在描述任何具有离域π电子体系并遵循休克尔规则（4n+2个π电子）的平面环状分子。苯（C₆H₆）是最简单的芳香族化合物，是无数衍生物的母体结构。

Benzene was first isolated by Michael Faraday in 1825. The Kekulé structure (a six‑membered ring with alternating single and double bonds) was an early model, but physical evidence showed all carbon–carbon bonds are identical.

苯由法拉第于1825年首次分离。凯库勒结构（单双键交替的六元环）是早期模型，但物理证据显示所有碳碳键是等同的。

2. Naming Aromatic Compounds | 芳香族化合物的命名

Simple monosubstituted benzenes are named by adding the substituent name as a prefix to ‘benzene’, e.g. chlorobenzene, nitrobenzene, methylbenzene (toluene). Some common names are accepted by IUPAC, such as phenol (hydroxybenzene) and aniline (phenylamine).

简单的单取代苯用取代基名称作为前缀加上“苯”命名，如氯苯、硝基苯、甲苯。一些俗名被IUPAC接受，如苯酚（羟基苯）和苯胺（氨基苯）。

For disubstituted benzenes, the relative positions are indicated by the prefixes 1,2- (ortho, o‑), 1,3‑ (meta, m‑), or 1,4‑ (para, p‑). If the substituents are different, they are listed alphabetically, and numbering gives the lowest set of locants.

对于双取代苯，相对位置用前缀1,2‑（邻位）、1,3‑（间位）或1,4‑（对位）表示。如果取代基不同，按字母顺序列出，编号使位次最小。

3. Structure and Stability of Benzene | 苯的结构与稳定性

Benzene is a planar, regular hexagon with bond angles of 120°. Each carbon atom is sp² hybridised and forms three σ bonds: two to adjacent carbons and one to hydrogen. The unhybridised p orbital on each carbon overlaps sideways to form a continuous π cloud above and below the ring — a delocalised system of 6 π electrons.

苯是一个平面正六边形，键角120°。每个碳原子为sp²杂化，形成三个σ键：两个与相邻碳原子，一个与氢原子。每个碳上未杂化的p轨道侧面重叠，形成环上下连续的π电子云——一个6个π电子的离域体系。

The delocalisation energy of benzene (its extra stability) is about 150 kJ mol⁻¹ compared with the hypothetical cyclohexatriene. Evidence includes: all C–C bond lengths are 0.139 nm (between C–C and C=C), benzene undergoes substitution rather than addition, and its enthalpy of hydrogenation is less exothermic than predicted for three isolated double bonds.

苯的离域能（额外稳定性）相比假想的环己三烯约为150 kJ mol⁻¹。证据包括：所有C–C键长均为0.139 nm（介于单双键之间），苯发生取代而非加成，氢化焓低于三个孤立双键的预期放热量。

4. Electrophilic Substitution Mechanism | 亲电取代反应机理

Aromatic compounds typically react by electrophilic substitution — an electrophile attacks the electron‑rich π‑system, replacing a hydrogen atom. The general mechanism involves two steps: formation of the electrophile, then attack and regeneration of the aromatic ring.

芳香族化合物的典型反应是亲电取代——亲电试剂进攻富电子的π体系，取代氢原子。一般机理包括两步：生成亲电试剂，然后进攻并恢复芳香环。

In the first step, the π‑electrons attack the electrophile (⁺E), forming a delocalised carbocation intermediate called a σ‑complex or Wheland intermediate. This step is rate‑determining. In the second step, the intermediate loses a proton (H⁺), and the aromatic system is restored. The overall reaction is substitution of H by E.

第一步，π电子进攻亲电试剂（⁺E），形成一个离域的碳正离子中间体，称为σ络合物或韦兰德中间体，此步是决速步。第二步，中间体失去一个质子，芳香体系恢复。总反应是H被E取代。

The electrophile varies: NO₂⁺ for nitration, X⁺ (or polarised X₂ with catalyst) for halogenation, R⁺ or RCO⁺ for Friedel–Crafts. Catalysts like AlCl₃, FeBr₃, or H₂SO₄ are used to generate strong electrophiles.

亲电试剂因反应而异：硝化使用NO₂⁺，卤化使用X⁺（或与催化剂极化的X₂），傅‑克反应使用R⁺或RCO⁺。使用AlCl₃、FeBr₃或H₂SO₄等催化剂生成强亲电试剂。

5. Nitration of Benzene | 苯的硝化反应

Benzene reacts with a mixture of concentrated nitric acid and concentrated sulfuric acid (nitrating mixture) at 50–60 °C to form nitrobenzene. The electrophile is the nitronium ion, NO₂⁺.

苯与浓硝酸和浓硫酸的混合物（硝化混酸）在50–60℃反应生成硝基苯。亲电试剂是硝鎓离子NO₂⁺。

The generation of NO₂⁺ occurs by protonation of HNO₃ by H₂SO₄, followed by loss of water: HNO₃ + 2H₂SO₄ → NO₂⁺ + H₃O⁺ + 2HSO₄⁻. Maintaining temperature below 60 °C avoids polysubstitution.

NO₂⁺的生成通过H₂SO₄质子化HNO₃，然后失水：HNO₃ + 2H₂SO₄ → NO₂⁺ + H₃O⁺ + 2HSO₄⁻。保持温度低于60℃避免多取代。

The overall equation is: C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O. Nitrobenzene is a pale yellow oil, used as a precursor for phenylamine and dyes.

总方程式：C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O。硝基苯是淡黄色油状物，用作苯胺和染料的前体。

6. Halogenation of Benzene | 苯的卤化反应

Benzene reacts with chlorine or bromine at room temperature only in the presence of a halogen carrier catalyst (AlCl₃ or FeCl₃ for Cl₂; FeBr₃ or AlBr₃ for Br₂) to produce chlorobenzene or bromobenzene.

苯与氯或溴在室温下反应，必须有卤素载体催化剂存在（Cl₂用AlCl₃或FeCl₃；Br₂用FeBr₃或AlBr₃），生成氯苯或溴苯。

The catalyst polarises the halogen molecule, generating a more powerful electrophile: Cl₂ + AlCl₃ → Cl⁺ + AlCl₄⁻. The electrophilic chlorine attacks benzene to form the σ‑complex, and loss of H⁺ gives the halogenated product.

催化剂极化卤素分子，生成更强的亲电试剂：Cl₂ + AlCl₃ → Cl⁺ + AlCl₄⁻。亲电的氯进攻苯形成σ络合物，失去H⁺得到卤代产物。

Overall: C₆H₆ + Br₂ → C₆H₅Br + HBr (with FeBr₃). Fluorination is too vigorous, and iodination requires special oxidising conditions.

总方程式：C₆H₆ + Br₂ → C₆H₅Br + HBr（用FeBr₃）。氟化反应过剧烈，碘化需特殊氧化条件。

7. Friedel–Crafts Alkylation and Acylation | 傅‑克烷基化与酰基化反应

Friedel–Crafts alkylation introduces an alkyl group (R) onto the benzene ring using a haloalkane and anhydrous AlCl₃. The electrophile is a carbocation (R⁺) formed by: RCl + AlCl₃ → R⁺ + AlCl₄⁻. However, carbocation rearrangements can occur, leading to mixtures.

傅‑克烷基化利用卤代烷和无水AlCl₃在苯环上引入烷基。亲电试剂是碳正离子R⁺，由RCl + AlCl₃ → R⁺ + AlCl₄⁻生成。但碳正离子可能重排，导致混合物。

Friedel–Crafts acylation uses an acyl chloride (RCOCl) with AlCl₃ to form an acylium ion (RCO⁺), which is stabilised by resonance and does not rearrange. The product is a phenyl ketone. Overall: C₆H₆ + RCOCl → C₆H₅COR + HCl.

傅‑克酰基化使用酰氯（RCOCl）与AlCl₃生成酰基正离子RCO⁺，该离子因共振稳定，不会重排。产物是苯基酮。总反应：C₆H₆ + RCOCl → C₆H₅COR + HCl。

Acylation is useful for making aromatic ketones, which can then be reduced (Clemmensen or Wolff–Kishner) to alkylbenzenes, avoiding rearrangement problems.

酰基化用于制备芳香酮，继而可还原（Clemmensen或Wolff–Kishner反应）为烷基苯，避免重排问题。

8. Substituent Effects on Reactivity and Orientation | 取代基对反应活性和定位效应的影响

When a monosubstituted benzene undergoes further electrophilic substitution, the existing group influences both the rate of reaction and the position of the incoming electrophile. Substituents can be classified as activating or deactivating, and ortho/para‑directing or meta‑directing.

当单取代苯进一步发生亲电取代时，原有基团影响反应速率和新进入亲电试剂的位置。取代基可分为活化或钝化，以及邻/对位定位或间位定位。

Electron‑donating groups (e.g. –OH, –NH₂, –CH₃) activate the ring by increasing electron density, especially at the ortho and para positions. Electron‑withdrawing groups (e.g. –NO₂, –COOH, –CHO) deactivate the ring and direct to the meta position. The halogen groups are deactivating but ortho/para‑directing due to their +M and –I effects.

给电子基团（如–OH、–NH₂、–CH₃）通过增加电子密度活化苯环，尤其在邻对位。吸电子基团（如–NO₂、–COOH、–CHO）钝化苯环并引导到间位。卤素基团因有+M和–I效应，虽然钝化但是邻/对位定位基。

Understanding these effects allows prediction of products in multiple substitutions, e.g. nitration of methylbenzene gives mainly 2‑ and 4‑nitromethylbenzene, while nitration of nitrobenzene requires stronger conditions and yields largely 1,3‑dinitrobenzene.

理解这些效应可以预测多取代产物，例如甲苯硝化主要生成2‑和4‑硝基甲苯，而硝基苯的进一步硝化需要更强条件，主要得到1,3‑二硝基苯。

9. Chemistry of Phenol | 苯酚的化学性质

Phenol (C₆H₅OH) is a weak acid (pKₐ ≈ 10) because the phenoxide ion is stabilised by delocalisation of the negative charge into the ring. Phenol does not react with weak bases like carbonates, but it reacts with sodium metal to produce sodium phenoxide and hydrogen, and with aqueous sodium hydroxide to give sodium phenoxide.

苯酚是弱酸（pKₐ ≈ 10），因为酚氧负离子因负电荷离域到苯环上而稳定。苯酚不与碳酸盐等弱碱反应，但与金属钠反应生成酚钠和氢气，与氢氧化钠水溶液反应生成酚钠。

Reaction with Na: 2C₆H₅OH + 2Na → 2C₆H₅O⁻Na⁺ + H₂. Reaction with NaOH: C₆H₅OH + NaOH → C₆H₅O⁻Na⁺ + H₂O.

与钠反应：2C₆H₅OH + 2Na → 2C₆H₅O⁻Na⁺ + H₂。与NaOH反应：C₆H₅OH + NaOH → C₆H₅O⁻Na⁺ + H₂O。

Phenol is highly activated towards electrophilic substitution. With bromine water at room temperature, it gives an immediate white precipitate of 2,4,6‑tribromophenol. This reaction proceeds without a catalyst and is used as a test for phenol.

苯酚对亲电取代高度活泼。与溴水在室温下立即产生2,4,6‑三溴苯酚白色沉淀。此反应无需催化剂，可用于检验苯酚。

Nitration of phenol with dilute nitric acid yields a mixture of 2‑nitrophenol and 4‑nitrophenol. Stronger conditions can give 2,4,6‑trinitrophenol (picric acid).

苯酚与稀硝酸硝化得到2‑硝基苯酚和4‑硝基苯酚的混合物。更强条件可得2,4,6‑三硝基苯酚（苦味酸）。

10. Synthesis and Reactions of Phenylamine | 苯胺的合成与反应

Phenylamine (aniline, C₆H₅NH₂) is prepared by reducing nitrobenzene with tin and concentrated hydrochloric acid under reflux, followed by addition of excess NaOH to liberate the free amine. The overall conversion: C₆H₅NO₂ + 6[H] → C₆H₅NH₂ + 2H₂O.

苯胺通过硝基苯与锡和浓盐酸回流还原制得，然后加入过量NaOH游离出胺。总转化：C₆H₅NO₂ + 6[H] → C₆H₅NH₂ + 2H₂O。

Phenylamine is a weaker base than ammonia because the lone pair on nitrogen is partially delocalised into the benzene ring. It reacts with acids to form soluble phenylammonium salts, e.g. C₆H₅NH₂ + HCl → C₆H₅NH₃⁺Cl⁻.

苯胺碱性弱于氨，因为氮上的孤对电子部分离域到苯环中。它与酸反应生成可溶性苯铵盐，如C₆H₅NH₂ + HCl → C₆H₅NH₃⁺Cl⁻。

Like phenol, the –NH₂ group strongly activates the ring. Bromine water readily reacts with phenylamine to give 2,4,6‑tribromophenylamine as a white precipitate.

与苯酚相似，–NH₂基团强烈活化苯环。溴水与苯胺迅速反应生成2,4,6‑三溴苯胺白色沉淀。

Reaction of phenylamine with nitrous acid (HNO₂, generated in situ from NaNO₂ and HCl) at 0–5 °C gives benzenediazonium chloride. This diazotisation is crucial because diazonium salts undergo coupling reactions with phenols or other amines to produce azo dyes. A typical coupling: C₆H₅N₂⁺Cl⁻ + C₆H₅OH → C₆H₅N=NC₆H₄OH + HCl, forming an orange azo compound.

苯胺与亚硝酸（由NaNO₂和HCl现场生成）在0‑5℃反应生成氯化重氮苯。该重氮化反应至关重要，因为重氮盐能与酚或其他胺偶联生成偶氮染料。典型偶联：C₆H₅N₂⁺Cl⁻ + C₆H₅OH → C₆H₅N=NC₆H₄OH + HCl，形成橙色偶氮化合物。

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Aromatic Compounds | 芳香族化合物