Carboxylic Acids | 羧酸考点精讲

📚 Carboxylic Acids | 羧酸考点精讲

Carboxylic acids are a fundamental homologous series in organic chemistry, characterised by the carboxyl functional group –COOH. They appear widely in nature, from the acetic acid in vinegar to the long-chain fatty acids in lipids. For IB and WJEC chemistry students, a thorough understanding of their structure, nomenclature, physical properties, acidity, and chemical reactions is essential. This article provides a concise yet comprehensive revision guide, pairing key concepts in both English and Chinese to support bilingual learners. All content is aligned with the core and higher level requirements of these specifications.

羧酸是有机化学中一个基础的同系列，其特征是羧基官能团 –COOH。它们广泛存在于自然界中，从食醋中的乙酸到脂质中的长链脂肪酸。对于 IB 和 WJEC 化学的学生来说，彻底理解羧酸的结构、命名、物理性质、酸性以及化学反应至关重要。本文提供了一份简明而全面的复习指南，以中英双语配对关键概念，支持双语学习者。所有内容均符合这些课程大纲的核心和高阶要求。

1. Introduction to Carboxylic Acids | 羧酸简介

Carboxylic acids contain the carboxyl group, –COOH, which consists of a carbonyl (C=O) and a hydroxyl (–OH) attached to the same carbon atom. The functional group is often written as –CO₂H in condensed form. The carbon atom of the carboxyl group is sp² hybridised, giving a planar geometry around that carbon. The acidic hydrogen is the one attached to the oxygen of the hydroxyl group. The general formula for a monocarboxylic acid is R–COOH, where R can be an alkyl or aryl group.

羧酸含有羧基 –COOH，它由连接在同一个碳原子上的羰基 (C=O) 和羟基 (–OH) 组成。该官能团在缩合式中常写作 –CO₂H。羧基的碳原子为 sp² 杂化，使得该碳周围呈平面几何形状。酸性氢是与羟基氧相连的那个氢。一元羧酸的通式为 R–COOH，其中 R 可以是烷基或芳基。

The simplest carboxylic acid is methanoic acid, HCOOH, commonly known as formic acid, found in ant venom. Ethanoic acid, CH₃COOH, is the main component of vinegar. Benzoic acid, C₆H₅COOH, is the simplest aromatic carboxylic acid and is used as a food preservative.

最简单的羧酸是甲酸 HCOOH，俗称蚁酸，存在于蚂蚁毒液中。乙酸 CH₃COOH 是食醋的主要成分。苯甲酸 C₆H₅COOH 是最简单的芳香族羧酸，用作食品防腐剂。

2. Nomenclature of Carboxylic Acids | 羧酸的命名

IUPAC names for carboxylic acids are derived by replacing the terminal ‘e’ of the corresponding alkane with ‘oic acid’. For example, methane becomes methanoic acid, ethane becomes ethanoic acid, and propane becomes propanoic acid. The carboxyl carbon is always assigned number 1 in the chain. Substituents are numbered accordingly, giving the lowest possible numbers to the branches. If two carboxyl groups are present, the suffix is ‘dioic acid’, such as ethanedioic acid, HOOC–COOH, commonly known as oxalic acid.

羧酸的 IUPAC 命名是通过将相应烷烃的词尾 ‘e’ 替换为 ‘oic acid’ 来推导的。例如，甲烷变为 methanoic acid，乙烷变为 ethanoic acid，丙烷变为 propanoic acid。羧基碳总是被指定为链中的第 1 位。取代基相应编号，使支链获得尽可能小的编号。如果存在两个羧基，词尾为 ‘dioic acid’，例如乙二酸 HOOC–COOH，俗称草酸。

Aromatic carboxylic acids are usually named as derivatives of benzoic acid, with substituents numbered relative to the –COOH group (position 1). Examples include 4-methylbenzoic acid and 2-chlorobenzoic acid. Many carboxylic acids have common names preserved by IUPAC, such as acetic acid (ethanoic acid) and butyric acid (butanoic acid).

芳香族羧酸通常作为苯甲酸的衍生物命名，取代基相对于 –COOH 基团（1 位）编号。例子包括 4-甲基苯甲酸和 2-氯苯甲酸。许多羧酸保留了 IUPAC 认可的俗名，如乙酸 (ethanoic acid) 和丁酸 (butanoic acid)。

3. Physical Properties | 物理性质

Carboxylic acids exhibit relatively high boiling points compared to alcohols and aldehydes of similar molar mass. This is because they can form strong intermolecular hydrogen bonds. In the pure liquid and solid states, most carboxylic acids exist as cyclic dimers, where two molecules are held together by two hydrogen bonds between their carboxyl groups. This effectively doubles the molecular mass of the moving unit, requiring more energy to vaporise.

与摩尔质量相近的醇和醛相比，羧酸表现出较高的沸点。这是因为它们能形成强的分子间氢键。在纯液态和固态中，大多数羧酸以环状二聚体形式存在，两个分子通过它们羧基之间的两个氢键连接在一起。这实际上使运动单元的分子量加倍，需要更多的能量才能汽化。

The first four aliphatic carboxylic acids (methanoic to butanoic) are miscible with water in all proportions due to hydrogen bonding between the carboxyl group and water molecules. As the non-polar hydrocarbon chain lengthens, solubility decreases. Benzoic acid is slightly soluble in cold water but more soluble in hot water. Carboxylic acids also dissolve in organic solvents.

前四种脂肪族羧酸（甲酸至丁酸）可以与水以任意比例混溶，这是因为羧基与水分子之间形成氢键。随着非极性烃链增长，溶解度下降。苯甲酸在冷水中微溶，但在热水中溶解度较大。羧酸也可溶于有机溶剂。

4. Acidity of Carboxylic Acids | 羧酸的酸性

Carboxylic acids are weak acids, meaning they partially dissociate in water to form carboxylate ions (R–COO⁻) and hydronium ions (H₃O⁺). The equilibrium lies well to the left. Typical pKa values for aliphatic carboxylic acids are around 4.8, making them far more acidic than alcohols (pKa ~16) and phenols (pKa ~10). The greater acidity is due to the resonance stabilisation of the carboxylate anion, which delocalises the negative charge over two oxygen atoms, making the conjugate base more stable.

羧酸是弱酸，这意味着它们在水溶液中部分电离，生成羧酸根离子 (R–COO⁻) 和水合氢离子 (H₃O⁺)。平衡强烈偏向左侧。脂肪族羧酸的典型 pKa 值约为 4.8，使其酸性远强于醇（pKa ~16）和酚（pKa ~10）。酸性较强是由于羧酸根阴离子的共振稳定作用，负电荷离域到两个氧原子上，使得共轭碱更稳定。

Electron-withdrawing substituents near the carboxyl group increase acidity by stabilising the negative charge through inductive effects. For example, chloroethanoic acid (ClCH₂COOH, pKa = 2.86) is stronger than ethanoic acid. Trichloroethanoic acid is a relatively strong acid. Conversely, electron-donating alkyl groups slightly decrease acidity.

羧基附近的吸电子取代基通过诱导效应稳定负电荷，从而增加酸性。例如，氯乙酸 (ClCH₂COOH, pKa = 2.86) 比乙酸强。三氯乙酸是一种相对强的酸。反之，给电子烷基会略微降低酸性。

The table below compares acid strength in context:

Compound	Formula	Approximate pKa
Ethanol	CH₃CH₂OH	~16
Phenol	C₆H₅OH	~10
Ethanoic acid	CH₃COOH	4.76
Chloroethanoic acid	ClCH₂COOH	2.86

5. Preparation of Carboxylic Acids | 羧酸的制备

Carboxylic acids can be prepared in the laboratory by several methods. The most common is the oxidation of primary alcohols or aldehydes using acidified potassium dichromate(VI) (K₂Cr₂O₇/H₂SO₄) under reflux. The alcohol is first oxidised to the aldehyde, which is further oxidised to the carboxylic acid. Care must be taken to prevent loss of the aldehyde through distillation if the aldehyde is the target; for carboxylic acid synthesis, full oxidation under reflux is used.

羧酸可以通过多种方法在实验室制备。最常见的是用酸化重铬酸钾 (K₂Cr₂O₇/H₂SO₄) 在回流下氧化伯醇或醛。醇首先被氧化成醛，醛进一步被氧化成羧酸。如果目标产物是醛，必须小心通过蒸馏防止醛损失；对于合成羧酸，则采用回流充分氧化。

Another important method is the hydrolysis of nitriles (R–CN) under acidic or basic conditions. Acidic hydrolysis yields the carboxylic acid directly. Basic hydrolysis produces the carboxylate salt, which must be acidified to obtain the free acid. This reaction is useful for extending the carbon chain by one carbon atom. Additionally, carboxylic acids can be synthesised by the carbonation of Grignard reagents: RMgX + CO₂ → RCOOMgX, followed by acidic work-up to give RCOOH.

另一个重要方法是腈 (R–CN) 在酸性或碱性条件下的水解。酸性水解直接得到羧酸。碱性水解生成羧酸盐，必须酸化才能获得游离酸。该反应可用于将碳链延长一个碳原子。此外，羧酸还可以通过格氏试剂的羧化反应合成：RMgX + CO₂ → RCOOMgX，然后通过酸性后处理得到 RCOOH。

6. Reactions of Carboxylic Acids: Salt Formation | 反应：成盐

As acids, carboxylic acids react readily with bases to form salts. With sodium hydroxide, ethanoic acid gives sodium ethanoate and water: CH₃COOH + NaOH → CH₃COONa + H₂O. The reaction is neutralisation, and the ionic salt is usually soluble in water. Carboxylic acids are strong enough to react with carbonates and hydrogencarbonates, producing carbon dioxide gas, water, and the corresponding carboxylate salt. This effervescence is a useful test for the carboxyl group.

作为酸，羧酸很容易与碱反应生成盐。与氢氧化钠反应，乙酸生成乙酸钠和水：CH₃COOH + NaOH → CH₃COONa + H₂O。该反应是中和反应，离子盐通常可溶于水。羧酸的酸性足够强，能与碳酸盐和碳酸氢盐反应，生成二氧化碳气体、水和相应的羧酸盐。这种冒泡现象是检验羧基的有用方法。

The reaction with sodium carbonate can be written as: 2RCOOH + Na₂CO₃ → 2RCOONa + CO₂ + H₂O. With sodium hydrogencarbonate: RCOOH + NaHCO₃ → RCOONa + CO₂ + H₂O. Because phenols and alcohols do not produce CO₂ with NaHCO₃, this reaction distinguishes carboxylic acids from them.

与碳酸钠的反应可写为：2RCOOH + Na₂CO₃ → 2RCOONa + CO₂ + H₂O。与碳酸氢钠的反应：RCOOH + NaHCO₃ → RCOONa + CO₂ + H₂O。由于酚和醇不能与 NaHCO₃ 反应生成 CO₂，这个反应可以区分羧酸与它们。

7. Esterification | 酯化反应

Carboxylic acids react with alcohols in the presence of a strong acid catalyst (usually concentrated sulfuric acid) to form esters and water. This is known as Fischer esterification, and it is a reversible condensation reaction. The general equation is: RCOOH + R’OH ⇌ RCOOR’ + H₂O. For example, ethanoic acid and ethanol produce ethyl ethanoate, a common solvent with a fruity smell.

羧酸在强酸催化剂（通常是浓硫酸）存在下与醇反应，生成酯和水。这称为费歇尔酯化，是一个可逆的缩合反应。通用方程式为：RCOOH + R’OH ⇌ RCOOR’ + H₂O。例如，乙酸和乙醇生成乙酸乙酯，这是一种具有水果香味的常见溶剂。

The equilibrium can be shifted to favour ester formation by using an excess of one reactant or by removing the water or ester as it forms. The reaction mechanism involves protonation of the carbonyl oxygen, nucleophilic attack by the alcohol, proton transfer, and elimination of water. Isotopic labelling with oxygen-18 shows that the acyl–oxygen bond from the carboxylic acid is broken, not the alkyl–oxygen bond of the alcohol.

可以通过使用过量的一种反应物，或者在酯或水生成时将其移走，使平衡向有利于酯生成的方向移动。反应机理包括羰基氧的质子化、醇的亲核进攻、质子转移以及水的消除。氧-18 同位素标记显示，断裂的是来自羧酸的酰基–氧键，而不是醇的烷基–氧键。

8. Reduction of Carboxylic Acids | 羧酸的还原

Carboxylic acids are resistant to catalytic hydrogenation, but they can be reduced to primary alcohols using the powerful reducing agent lithium aluminium hydride, LiAlH₄, in dry ether. The reaction proceeds: RCOOH + 4[H] → RCH₂OH + H₂O (overall using LiAlH₄ followed by aqueous acidic work-up). For instance, ethanoic acid is reduced to ethanol. Sodium borohydride (NaBH₄) is not strong enough to reduce carboxylic acids.

羧酸对催化加氢具有抵抗力，但可以使用强还原剂氢化铝锂 LiAlH₄ 在无水乙醚中被还原为伯醇。反应过程为：RCOOH + 4[H] → RCH₂OH + H₂O（使用 LiAlH₄ 然后进行酸性水溶液后处理的总体反应）。例如，乙酸被还原为乙醇。硼氢化钠 (NaBH₄) 不足以还原羧酸。

This reduction is important in organic synthesis when a carboxylic acid is the available starting material, and a primary alcohol is desired. The intermediate is an aldehyde hydrate-like species, but it is rapidly reduced further to the alcohol under the reaction conditions.

当羧酸是可用的起始原料且需要伯醇时，这种还原反应在有机合成中非常重要。中间体是一种类似于醛水合物的物质，但在反应条件下会迅速被进一步还原为醇。

9. Conversion to Acyl Chlorides | 转化为酰氯

Carboxylic acids can be converted into more reactive acid derivatives, particularly acyl chlorides. Treatment with thionyl chloride (SOCl₂), phosphorus pentachloride (PCl₅), or phosphorus trichloride (PCl₃) replaces the –OH group with a chlorine atom. The reaction with SOCl₂ is often preferred because the by-products (SO₂ and HCl) are gases that escape, driving the reaction to completion: RCOOH + SOCl₂ → RCOCl + SO₂ + HCl.

羧酸可以转化为反应性更强的酸衍生物，特别是酰氯。用亚硫酰氯 (SOCl₂)、五氯化磷 (PCl₅) 或三氯化磷 (PCl₃) 处理，可以将 –OH 基团替换为氯原子。通常首选与 SOCl₂ 的反应，因为副产物（SO₂ 和 HCl）是气体，会逸出从而推动反应完成：RCOOH + SOCl₂ → RCOCl + SO₂ + HCl。

Acyl chlorides are extremely useful in synthesis because they undergo nucleophilic addition–elimination reactions with water, alcohols, ammonia, and amines to give carboxylic acids, esters, amides, and substituted amides respectively. They are more reactive than the parent carboxylic acid due to the good leaving group ability of the chloride ion.

酰氯在合成中极其有用，因为它们能与水、醇、氨和胺发生亲核加成–消除反应，分别生成羧酸、酯、酰胺和取代酰胺。由于氯离子是一个良好的离去基团，酰氯比母体羧酸更具反应活性。

10. Decarboxylation and Derivative Overview | 脱羧反应及衍生物概述

Decarboxylation is the loss of carbon dioxide from a carboxylic acid or its salt. Simple carboxylic acids are generally resistant to decarboxylation, but heating the sodium salt with soda lime (a mixture of NaOH and CaO) can cause decarboxylation to an alkane: RCOONa + NaOH (CaO, heat) → RH + Na₂CO₃. This reaction is useful for reducing chain length by one carbon atom. Beta-keto acids (e.g., acetoacetic acid) decarboxylate readily on gentle heating because a cyclic transition state can form.

脱羧反应是指羧酸或其盐失去二氧化碳。简单羧酸通常不易脱羧，但将钠盐与碱石灰（NaOH 和 CaO 的混合物）一起加热，可以导致脱羧生成烷烃：RCOONa + NaOH (CaO, heat) → RH + Na₂CO₃。该反应用于将碳链长度缩短一个碳原子。β-酮酸（如乙酰乙酸）在温和加热时很容易脱羧，因为可以形成环状过渡态。

As a revision summary, carboxylic acids sit at the apex of the oxidation ladder for organic oxygen compounds, and they serve as the gateway to many acid derivatives: acyl chlorides, esters, amides, and anhydrides. The relative reactivity of acid derivatives decreases in the order: acyl chloride > anhydride > ester > amide. Understanding this interplay helps predict reaction outcomes and design synthetic pathways.

作为复习总结，羧酸处于有机含氧化合物氧化阶梯的顶端，并且它们是通往许多酸衍生物的门户：酰氯、酯、酰胺和酸酐。酸衍生物的相对反应活性按以下顺序递减：酰氯 > 酸酐 > 酯 > 酰胺。理解这种相互关系有助于预测反应结果和设计合成路线。

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Carboxylic Acids | 羧酸 考点精讲