📚 Carboxylic Acids: Structure, Acidity, Esters and Acyl Chlorides | 羧酸：结构、酸性、酯与酰氯
Carboxylic acids are a homologous series of organic compounds containing the carboxyl functional group, –COOH. In Edexcel A-Level Chemistry Topic 18.5 you must understand their structure, physical properties, acidity and reactions, including esterification and the formation of acyl chlorides. This revision guide covers every one of those points and adds the supporting detail that examiners look for in extended written answers, together with the equations and numerical data you should be able to quote from memory.
羧酸是一类含有羧基官能团（–COOH）的同系有机化合物。在爱德思 A-Level 化学 Topic 18.5 中，你需要理解它们的结构、物理性质、酸性以及反应，包括酯化和生成酰氯。本复习指南覆盖上述全部考点，并补充考官在长篇书面作答中期待看到的支撑细节，以及你应当能够凭记忆写出的方程式与数据。
1. Structure of the Carboxyl Group | 羧基的结构
The carboxyl group –COOH consists of a carbonyl group C=O and a hydroxyl group – oup is planar and the bond angles around it are close to 120°. Both the C=O bond and the O– r, which leaves the hydroxyl hydrogen partly positive and relatively easy to remove as H⁺.
羧基 –COOH 由羰基 C=O 和羟基 –OH 连接在同一个碳原子上组成。羧基中的碳为 sp² 杂化，因此羧基呈平面结构，该碳周围的键角接近 120°。C=O 键与 O–H 键极性都很强，使羟基上的氢带部分正电荷，相对容易以 H⁺ 形式离去。
When the O–H proton is lost, the resulting carboxylate ion, RCOO⁻, is stabilised by delocalisation: the negative charge and the two carbon–oxygen bonds are shared equally between the two oxygen atoms. This resonance stabilisation of the conjugate base is the main reason why carboxylic acids are far more acidic than alcohols.
当 O–H 上的质子失去后，生成的羧酸根离子 RCOO⁻ 通过电子离域得到稳定：负电荷与两个碳氧键平均分布在两个氧原子之间。共轭碱的这种共振稳定作用，是羧酸酸性远强于醇的主要原因。
- Carbonyl C=O plus hydroxyl –OH on the same carbon | 同一个碳上同时连有羰基 C=O 和羟基 –OH
- sp² hybridised, planar, bond angles ≈ 120° | sp² 杂化、平面结构、键角约 120°
- Conjugate base RCOO⁻ stabilised by delocalisation | 共轭碱 RCOO⁻ 通过离域作用被稳定
2. Nomenclature and Common Examples | 命名与常见实例
Aliphatic carboxylic acids are named by replacing the final -e of the parent alkane with -oic acid. The carboxyl carbon is always counted as carbon 1, so the carbon chain is numbered from the –COOH end. Substituents are then located in the usual way, for example 2-hydroxypropanoic acid (lactic acid).
脂肪族羧酸的命名方法是将母体烷烃词尾的 -e 改为 -oic acid。羧基碳始终编为 1 号碳，因此碳链从 –COOH 一端开始编号。取代基再按常规方式定位，例如 2-羟基丙酸（乳酸）。
| Name | Formula | 中文名 |
|---|---|---|
| Methanoic acid | HCOOH | 甲酸 |
| Ethanoic acid | CH₃COOH | 乙酸 |
| Propanoic acid | CH₃CH₂COOH | 丙酸 |
| Butanoic acid | CH₃CH₂CH₂COOH | 丁酸 |
| Benzoic acid | C₆H₅COOH | 苯甲酸 |
Aromatic carboxylic acids also exist: benzoic acid is the simplest example, with the –COOH group attached directly to a benzene ring. Dicarboxylic acids contain two carboxyl groups, for example ethanedioic acid (oxalic acid), HOOC–COOH, and hexanedioic acid (adipic acid), used in nylon manufacture.
芳香族羧酸也存在：苯甲酸是最简单的例子，其 –COOH 直接连接在苯环上。二元羧酸含有两个羧基，例如乙二酸（草酸）HOOC–COOH，以及用于制造尼龙的己二酸。
3. Physical Properties: Boiling Point and Solubility | 物理性质：沸点与溶解性
Carboxylic acids have higher boiling points than alcohols, aldehydes and ketones of similar molar mass. This is because each carboxylic acid molecule can form two hydrogen bonds per carboxyl group, allowing the formation of hydrogen-bonded dimers. Boiling therefore requires the two hydrogen bonds of the dimer to be broken, which takes considerably more energy.
羧酸的沸点高于相对分子质量相近的醇、醛和酮。这是因为每个羧酸分子可通过每个羧基形成两个氢键，从而形成氢键二聚体。沸腾时必须破坏二聚体中的两个氢键，所需能量明显更大。
ed dimer of ethanoic acid can be represented as two molecules held together by two C=O···H– ter molecules, but solubility decreases as the non-polar hydrocarbon chain becomes longer.
乙酸的氢键二聚体可以表示为两个分子通过两个 C=O···H–O 相互作用连接在一起。碳链不超过约四个碳原子的短链羧酸易溶于水，因为它们能与水分子形成氢键；但随着非极性烃链增长，溶解度会下降。
- Dimer formation raises boiling point | 二聚体形成使沸点升高
- Two hydrogen bonds per molecule in the dimer | 二聚体中每个分子参与两个氢键
- Solubility falls as the alkyl chain lengthens | 烃链越长，溶解度越低
- Lower members have sharp, pungent smells | 低级成员具有刺鼻气味
4. Acidity of Carboxylic Acids | 羧酸的酸性
d and its ions. Ethanoic acid has pKa ≈ 4.76 at 298 K, so in a 0.1 mol dm⁻³ solution only about 1% of the molecules are ionised, giving a pH of roughly 2.9.
羧酸是弱酸。在水溶液中它们只部分电离，在未电离的酸分子与其离子之间建立平衡。乙酸在 298 K 时 pKa ≈ 4.76，因此在 0.1 mol dm⁻³ 溶液中只有约 1% 的分子电离，pH 约为 2.9。
−log₁₀ Ka. A larger Ka, or a smaller pKa, means a stronger acid. Carboxylic acids are stronger acids than alcohols and phenols, but weaker than the mineral acids.
酸解离常数定义为 Ka = [H⁺][A⁻] / [HA]，且 pKa = −log₁₀ Ka。Ka 越大或 pKa 越小，酸性越强。羧酸的酸性强于醇和酚，但弱于无机强酸。
CH₃COOH(aq) ⇌ CH₃COO⁻(aq) + H⁺(aq)
| Acid | 酸 | pKa | Reason | 原因 |
|---|---|---|
| Ethanol | 乙醇 | ≈ 15.9 | Charge localised on one O | 负电荷定域在一个氧上 |
| Phenol | 苯酚 | ≈ 10.0 | Some delocalisation into the ring | 负电荷部分离域到苯环 |
| Ethanoic acid | 乙酸 | ≈ 4.76 | Charge delocalised over two O | 负电荷离域到两个氧上 |
| Chloroethanoic acid | 氯乙酸 | ≈ 2.86 | Cl withdraws electron density | 氯吸电子 |
5. Factors Affecting Acid Strength | 影响酸强度的因素
Anything that stabilises the carboxylate ion increases the acidity. Electron-withdrawing groups such as –Cl, –Br, –NO₂ and –CN pull electron density away from the carboxyl group by the inductive effect, spreading out the negative charge and making the anion more stable, so Ka increases and pKa decreases.
任何能稳定羧酸根离子的因素都会提高酸性。–Cl、–Br、–NO₂、–CN 等吸电子基团通过诱导效应把电子密度从羧基拉走，使负电荷更加分散、阴离子更稳定，因此 Ka 增大、pKa 减小。
The effect weakens rapidly with distance from the carboxyl group. In the chlorobutanoic acids the acid strength order is 2-chloro > 3-chloro > 4-chloro, because the inductive effect has to travel through more sigma bonds. Alkyl groups have the opposite, electron-pushing effect, so methanoic acid is slightly stronger than ethanoic acid.
该效应随距羧基的距离增大而迅速减弱。在氯代丁酸中，酸性顺序为 2-氯 > 3-氯 > 4-氯，因为诱导效应需要通过更多 σ 键传递。烷基的作用相反，会推电子，所以甲酸的酸性略强于乙酸。
- More electron-withdrawing groups, stronger acid | 吸电子基团越多，酸性越强
- CF₃COOH > CHCl₂COOH > CH₂ClCOOH > CH₃COOH | 三氟乙酸 > 二氯乙酸 > 氯乙酸 > 乙酸
- Effect decreases with distance | 效应随距离减弱
- Alkyl groups weaken the acid slightly | 烷基使酸性略微减弱
6. Reactions with Metals, Bases and Carbonates | 与金属、碱和碳酸盐的反应
Carboxylic acids show all the typical reactions of an acid, because they release H⁺ in aqueous solution. They react with
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