📚 Carboxylic Acids: Key Revision Points for IB & AQA Chemistry | 羧酸考点精讲
Carboxylic acids are organic compounds containing the carboxyl functional group, –COOH. They are weak acids and feature prominently in both IB and AQA Chemistry specifications. This article consolidates the essential exam points, from structure and nomenclature to typical reactions, helping you master this topic.
羧酸是含有羧基官能团 (–COOH) 的有机化合物，属于弱酸，在IB和AQA化学大纲中占有重要地位。本文整合了从结构、命名到典型反应的核心考点，帮助大家彻底掌握这一主题。
1. Introduction to Carboxylic Acids | 羧酸简介
Carboxylic acids have the general formula R–COOH, where R can be hydrogen, alkyl, or aryl. The simplest member is methanoic acid (HCOOH), commonly known as formic acid, found in ant venom. Ethanoic acid (CH₃COOH) is the main component of vinegar. These compounds are widely used in the production of polymers, pharmaceuticals, and food preservatives, and they play a central role in organic synthesis.
羧酸的通式为 R–COOH，R 可以是氢、烷基或芳基。最简单的成员是甲酸 (HCOOH)，俗称蚁酸，存在于蚂蚁毒液中。乙酸 (CH₃COOH) 是食醋的主要成分。这些化合物广泛用于聚合物、药物和食品防腐剂的生产，在有机合成中占据核心地位。
2. Structure and the Carboxyl Functional Group | 结构与羧基官能团
The carboxyl group consists of a carbonyl (C=O) and a hydroxyl (–OH) attached to the same sp² hybridised carbon. The carbon–oxygen bonds are not equal: the C=O is shorter than the C–OH due to resonance. The lone pair on the hydroxyl oxygen delocalises into the π system of the carbonyl, giving the C–O bond partial double‑bond character and stabilising the carboxylate ion formed after deprotonation. The bond angles around the carboxyl carbon are approximately 120°, making the group planar.
羧基由羰基 (C=O) 和羟基 (–OH) 连接在同一个 sp² 杂化碳上构成。由于共振，碳氧键并不等同：C=O 键比 C–OH 键短。羟基氧上的孤对电子离域进羰基的 π 体系，使 C–O 键具有部分双键性质，并稳定去质子后形成的羧酸根离子。羧基碳周围的键角约为 120°，整个基团呈平面形。
3. Nomenclature of Carboxylic Acids | 羧酸的命名
In IUPAC nomenclature, the parent chain is the longest carbon chain containing the –COOH group. The suffix ‘-oic acid’ replaces the ‘-e’ of the corresponding alkane, and the carboxyl carbon is always assigned position 1. For dicarboxylic acids, the suffix ‘-dioic acid’ is used. Common examples include: CH₃CH₂COOH (propanoic acid), HOOC–COOH (ethanedioic acid), and C₆H₅COOH (benzoic acid). When substituents are present, their positions are indicated relative to the carboxyl carbon, e.g., 2‑hydroxypropanoic acid (lactic acid) and 3‑bromobutanoic acid.
IUPAC 命名中，母链是含有 –COOH 的最长碳链。后缀 ‘–oic acid’ 取代相应烷烃的 ‘–e’，羧基碳永远定为 1 号位。对于二元羧酸，使用后缀 ‘–dioic acid’。常见实例：CH₃CH₂COOH（丙酸），HOOC–COOH（乙二酸），C₆H₅COOH（苯甲酸）。当存在取代基时，位置相对羧基碳编号，如 2‑羟基丙酸（乳酸）和 3‑溴丁酸。
4. Physical Properties: Boiling Points and Solubility | 物理性质：沸点与溶解度
Carboxylic acids exhibit significantly higher boiling points than alcohols of similar molecular mass because they can form strong hydrogen‑bonded dimers. In the liquid state and even in the vapour phase, two molecules associate through a pair of hydrogen bonds between their carboxyl groups. Short‑chain acids (up to butanoic acid) are completely miscible with water due to hydrogen bonding with water molecules. As the non‑polar hydrocarbon chain lengthens, solubility decreases sharply; long‑chain fatty acids are virtually insoluble.
羧酸的沸点显著高于相对分子质量相近的醇，因为它们能形成强氢键二聚体。在液态乃至气相中，两个分子通过羧基之间的一对氢键缔合。短链羧酸（最多丁酸）由于能与水分子形成氢键，可完全混溶于水。随着非极性碳氢链增长，溶解度急剧下降；长链脂肪酸几乎不溶于水。
5. Acidity of Carboxylic Acids: The Key Concept | 羧酸的酸性：核心概念
Carboxylic acids dissociate partially in water, establishing an equilibrium that yields a carboxylate ion and a hydronium ion. For ethanoic acid, the equation is:
CH₃COOH + H₂O ⇌ CH₃COO⁻ + H₃O⁺
The pKa values typically range from 3 to 5, making them much stronger acids than alcohols (pKa ~16) or phenols (pKa ~10). The pronounced acidity arises because the carboxylate ion is resonance‑stabilised. The negative charge is delocalised equally over the two oxygen atoms, which lowers the energy of the conjugate base and shifts the equilibrium to the right relative to alcohols, where no such stabilisation exists.
羧酸在水中部分电离，建立平衡生成羧酸根离子和水合氢离子。以乙酸为例：
CH₃COOH + H₂O ⇌ CH₃COO⁻ + H₃O⁺
典型 pKa 值在 3 到 5 之间，远强于醇 (pKa ~16) 和酚 (pKa ~10)。酸性显著增强的原因在于羧酸根离子具有共振稳定作用。负电荷均等地离域在两个氧原子上，降低了共轭碱的能量，与无此稳定作用的醇相比，平衡更偏向右侧。
6. Factors Affecting Acid Strength: Inductive Effects | 影响酸性强度的因素：诱导效应
Electron‑withdrawing substituents attached to the α‑carbon enhance acid strength by pulling electron density away from the carboxylate group, thereby stabilising the negative charge. The following table illustrates the effect for a series of substituted ethanoic acids:
| Acid | pKa at 25°C |
|---|---|
| CH₃COOH | 4.76 |
| ClCH₂COOH | 2.86 |
| Cl₂CHCOOH | 1.29 |
| CCl₃COOH | 0.65 |
As the number of electronegative chlorine atoms increases, the pKa decreases, indicating a stronger acid. By contrast, alkyl groups are electron‑releasing and slightly weaken acidity; propanoic acid (pKa 4.87) is marginally weaker than ethanoic acid. IB and AQA exams frequently ask you to explain these trends using inductive effects.
吸电子取代基连在 α‑碳上时，将电子密度从羧酸根引开，稳定负电荷，从而增强酸性。下表展示一系列取代乙酸的效应：随着电负性氯原子数量增加，pKa 降低，酸性增强。相反，烷基是给电子基团，会略微减弱酸性；丙酸 (pKa 4.87) 比乙酸稍弱。IB 和 AQA 考试常要求用电感效应解释这些趋势。
7. Preparation Methods for Carboxylic Acids | 羧酸的制备方法
Several reliable synthetic routes lead to carboxylic acids, all of which are required knowledge:
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Oxidation of primary alcohols or aldehydes: Reflux with acidified K₂Cr₂O₇ or alkaline KMnO₄. CH₃CH₂OH + 2[O] → CH₃COOH + H₂O.
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Hydrolysis of esters: Reflux with dilute HCl or NaOH. CH₃COOC₂H₅ + H₂O ⇌ CH₃COOH + C₂H₅OH.
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Hydrolysis of nitriles: Heat with dilute acid or alkali; the nitrile (R–C≡N) adds two water molecules to give RCOOH and ammonia.
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Side‑chain oxidation of alkylbenzenes: Hot alkaline KMnO₄ converts a methyl group attached to a benzene ring into a carboxyl group, yielding benzoic acid from methylbenzene.
制备羧酸的可靠合成路线有多种，均为必考知识：
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氧化伯醇或醛：用酸化 K₂Cr₂O₇ 或碱性 KMnO₄ 回流。CH₃CH₂OH + 2[O] → CH₃COOH + H₂O。
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水解酯：用稀 HCl 或 NaOH 回流。CH₃COOC₂H₅ + H₂O ⇌ CH₃COOH + C₂H₅OH。
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水解腈：用稀酸或稀碱加热，腈 (R–C≡N) 加合两分子水生成 RCOOH 和氨。
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