Edexcel A-Level Chemistry Topic 18: Arenes, Carbonyls & Carboxylic Acids | Edexcel A-Level 化学 18：芳烃、羰基化合物与羧酸

📚 Edexcel A-Level Chemistry Topic 18: Arenes, Carbonyls & Carboxylic Acids | Edexcel A-Level 化学 18：芳烃、羰基化合物与羧酸

This revision guide covers the key ideas in Edexcel A-Level Chemistry Topic 18, focusing on the reactivity of benzene derivatives, carbonyl compounds and carboxylic acids. You will need to apply reaction mechanisms, explain structure-property relationships, and design short organic syntheses.

本复习指南涵盖 Edexcel A-Level 化学第 18 讲的核心内容，重点包括苯衍生物、羰基化合物和羧酸的反应性。你需要运用反应机理、解释结构与性质的关系，并设计简单的有机合成路线。

1. Benzene and Kekulé | 苯与凯库勒结构

Benzene has the molecular formula C₆H₆ and a planar hexagonal ring of six carbon atoms. Each carbon uses three sp² hybrid orbitals to form two C–C σ bonds and one C–H σ bond; the remaining p orbital overlaps sideways to create a delocalised π system above and below the ring.

苯的分子式为 C₆H₆，六个碳原子形成平面正六边形环。每个碳原子使用三个 sp² 杂化轨道形成两个 C–C σ 键和一个 C–H σ 键；剩余的 p 轨道侧向重叠，在环的上下方形成离域 π 体系。

The Kekulé structure proposed alternating single and double bonds, but evidence shows all six C–C bond lengths are equal. Benzene is more stable than the Kekulé structure predicts, with a resonance energy of about 150 kJ mol⁻¹.

凯库勒结构提出单键和双键交替排列，但证据表明六个 C–C 键键长完全相等。苯比凯库勒结构预测的更稳定，共振能约为 150 kJ mol⁻¹。

Benzene does not decolourise bromine water or react readily with hydrogen halides, unlike typical alkenes. This resistance to addition reactions is a direct consequence of the delocalised π cloud.

与典型烯烃不同，苯不能使溴水褪色，也不易与卤化氢反应。这种对加成反应的抵抗力正是离域 π 电子云的直接结果。

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

Benzene undergoes electrophilic substitution rather than addition because the delocalised π system is stable. A strong electrophile is generated first, then attacks the ring; the aromaticity is restored by loss of H⁺.

苯发生亲电取代而不是加成反应，因为离域 π 体系非常稳定。反应中首先生成强亲电试剂，然后进攻苯环；失去 H⁺ 后芳香性得到恢复。

General mechanism: the electrophile E⁺ accepts an electron pair from the π cloud, forming a carbocation intermediate. A base removes H⁺ from the sp³ carbon to reform the benzene ring.

一般机理：亲电试剂 E⁺ 从 π 电子云接受一对电子，形成碳正离子中间体。碱从 sp³ 碳上移除 H⁺，重新形成苯环。

The most common electrophilic substitution reactions are summarised below.

下表总结了最常见的亲电取代反应。

In nitration the electrophile is NO₂⁺, formed by the reaction between HNO₃ and H₂SO₄. In halogenation the catalyst generates Br⁺ or Cl⁺ by polarising the halogen molecule. In Friedel-Crafts reactions the electrophile is a carbocation or an acylium ion.

硝化反应中的亲电试剂是 NO₂⁺，由 HNO₃ 与 H₂SO₄ 反应生成。卤化反应中催化剂使卤素分子极化，生成 Br⁺ 或 Cl⁺。在 Friedel-Crafts 反应中，亲电试剂是碳正离子或酰基正离子。

3. Phenol: Acidity and Reactions | 苯酚：酸性与反应

Phenol C₆H₅OH is weakly acidic because the phenoxide ion is stabilised by resonance: the negative charge on oxygen is delocalised into the ring. It reacts with NaOH but not with NaHCO₃.

苯酚 C₆H₅OH 呈弱酸性，因为酚氧负离子通过共振稳定：氧上的负电荷离域到苯环中。它能与 NaOH 反应，但不与 NaHCO₃ 反应。

The –OH group activates the ring towards electrophilic substitution. Phenol decolourises bromine water and forms 2,4,6-tribromophenol without a catalyst.

–OH 基团使苯环更容易发生亲电取代。苯酚可使溴水褪色，无需催化剂即生成 2,4,6-三溴苯酚。

Compared with ethanol, phenol is a stronger acid because the phenoxide ion is stabilised by resonance, whereas the ethoxide ion is not. Compared with carboxylic acids, phenol is much weaker, which is why it does not release CO₂ from carbonates.

与乙醇相比，苯酚的酸性更强，因为酚氧负离子通过共振稳定，而乙氧负离子不能。与羧酸相比，苯酚的酸性要弱得多，因此不能从碳酸盐中释放出 CO₂。

4. Carbonyl Compounds: Aldehydes and Ketones | 羰基化合物：醛和酮

Both aldehydes and ketones contain the carbonyl group C=O. In aldehydes at least one hydrogen is attached to the carbonyl carbon, while in ketones two alkyl or aryl groups are attached.

醛和酮都含有羰基 C=O。醛的羰基碳上至少连有一个氢，而酮的羰基碳上连有两个烷基或芳基。

The C=O bond is polar; the carbon carries a partial positive charge and is attacked by nucleophiles. Aldehydes are usually more reactive than ketones because there is less steric hindrance and less electron donation.

C=O 键是极性键；碳带部分正电荷，易受亲核试剂进攻。醛通常比酮更活泼，因为空间位阻较小且供电效应较弱。

Carbonyl compounds can be prepared by oxidation of alcohols: primary alcohols give aldehydes under distillation and carboxylic acids under reflux, while secondary alcohols give ketones under reflux.

羰基化合物可由醇氧化制得：伯醇在蒸馏条件下生成醛，在回流条件下生成羧酸；仲醇在回流条件下生成酮。

5. Nucleophilic Addition Reactions | 亲核加成反应

Hydrogen cyanide adds across the C=O bond to form hydroxynitriles. The reaction with NaCN and H₂SO₄ produces HCN in situ; the cyanide ion CN⁻ attacks the carbonyl carbon.

Published by TutorHao | A-Level Revision Series | aleveler.com

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