Carboxylic Acids | 羧酸考点精讲

📚 Carboxylic Acids | 羧酸考点精讲

Carboxylic acids are a homologous series of organic compounds that contain the functional group –COOH. They are weak acids and appear in many everyday substances, such as vinegar and citrus fruits. In GCSE AQA Chemistry, you need to understand their structure, naming, physical and chemical properties, and some key reactions including esterification.

羧酸是同系物的一类有机化合物，其官能团为 –COOH。它们是弱酸，出现在醋和柑橘类水果等许多日常物质中。在 GCSE AQA 化学中，你需要了解羧酸的结构、命名、物理和化学性质，以及包括酯化在内的一些关键反应。

1. Introduction to Carboxylic Acids | 羧酸简介

Carboxylic acids form a homologous series with the general formula CₙH₂ₙ₊₁COOH (or CₙH₂ₙO₂ for n ≥ 1). The first four members are methanoic acid (HCOOH), ethanoic acid (CH₃COOH), propanoic acid (C₂H₅COOH), and butanoic acid (C₃H₇COOH). They are widely used as preservatives, flavourings, and in the production of esters and polymers.

羧酸构成一个同系物，通式为 CₙH₂ₙ₊₁COOH（或 n≥1 时 CₙH₂ₙO₂）。前四种成员是甲酸 (HCOOH)、乙酸 (CH₃COOH)、丙酸 (C₂H₅COOH) 和丁酸 (C₃H₇COOH)。它们广泛用作防腐剂、调味剂，并用于酯和聚合物的生产。

2. Functional Group: The Carboxyl Group | 官能团：羧基

The functional group of carboxylic acids is the carboxyl group, –COOH. It consists of a carbonyl group (C=O) and a hydroxyl group (–OH) attached to the same carbon atom. This combination gives carboxylic acids their acidic properties because the O–H bond can break to release a hydrogen ion (H⁺).

羧酸的官能团是羧基 (–COOH)。它由连接在同一碳原子上的羰基 (C=O) 和羟基 (–OH) 组成。这种组合赋予了羧酸酸性，因为 O–H 键可以断裂释放出氢离子 (H⁺)。

3. Naming Carboxylic Acids | 羧酸的命名

Carboxylic acids are named by replacing the final ‘e’ of the corresponding alkane with ‘oic acid’. The carbon in the –COOH group is always counted as carbon 1. For example, CH₃COOH is ethanoic acid (from ethane), and HCOOH is methanoic acid (from methane). Branched-chain acids follow similar IUPAC rules, numbering the longest chain that contains the carboxyl group.

羧酸命名时，将对应烷烃末尾的“烷”改为“酸”。–COOH 基团中的碳始终标记为 1 号碳。例如，CH₃COOH 是乙酸（来自乙烷），HCOOH 是甲酸（来自甲烷）。支链酸遵循类似的 IUPAC 规则，对含羧基的最长碳链进行编号。

4. Physical Properties | 物理性质

Short-chain carboxylic acids (up to four carbons) are soluble in water due to hydrogen bonding between the –COOH group and water molecules. As the hydrocarbon chain lengthens, solubility decreases. They have higher boiling points than alcohols of similar relative molecular mass because carboxylic acid molecules can form dimers via two hydrogen bonds.

短链羧酸（不超过四个碳）可溶于水，这是因为 –COOH 基团与水分子之间形成氢键。随着烃链增长，溶解度降低。它们的沸点比相对分子质量相近的醇更高，因为羧酸分子可通过两个氢键形成二聚体。

5. Acidity of Carboxylic Acids | 羧酸的酸性

Carboxylic acids are weak acids, meaning they only partially ionise in aqueous solution. For example, ethanoic acid establishes an equilibrium: CH₃COOH ⇌ CH₃COO⁻ + H⁺. The presence of the carbonyl group next to the –OH makes the O–H bond more polar, favouring the release of H⁺ compared with alcohols.

羧酸是弱酸，这意味着它们在水溶液中只能部分电离。例如，乙酸存在平衡：CH₃COOH ⇌ CH₃COO⁻ + H⁺。与羟基相邻的羰基使 O–H 键极性更强，与醇相比更有利于释放 H⁺。

6. Reactions with Metals | 与金属的反应

Carboxylic acids react with reactive metals (such as magnesium, zinc, or iron) to form a salt and hydrogen gas, just like typical acids. For example, ethanoic acid + magnesium → magnesium ethanoate + hydrogen: 2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂.

羧酸与活泼金属（如镁、锌或铁）反应生成盐和氢气，与典型酸一样。例如，乙酸 + 镁 → 乙酸镁 + 氢气：2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂。

7. Reactions with Bases and Carbonates | 与碱和碳酸盐的反应

Carboxylic acids undergo neutralisation with metal oxides, hydroxides, or carbonates. With carbonates, they produce a salt, water, and carbon dioxide gas. For instance, ethanoic acid + sodium carbonate → sodium ethanoate + water + carbon dioxide: 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂. This effervescence confirms the presence of an acid.

羧酸与金属氧化物、氢氧化物或碳酸盐发生中和反应。与碳酸盐反应生成盐、水和二氧化碳气体。例如，乙酸 + 碳酸钠 → 乙酸钠 + 水 + 二氧化碳：2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂。这种冒泡现象可证实酸的存在。

8. Esterification | 酯化反应

Carboxylic acids react with alcohols in the presence of an acid catalyst (usually concentrated sulfuric acid) to form esters and water. This is a reversible condensation reaction. For example, ethanoic acid + ethanol ⇌ ethyl ethanoate + water: CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O. Esters have characteristic fruity smells and are used in flavourings and perfumes.

羧酸与醇在酸催化剂（通常是浓硫酸）存在下反应生成酯和水。这是一个可逆的缩合反应。例如，乙酸 + 乙醇 ⇌ 乙酸乙酯 + 水：CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O。酯具有特征性的水果香味，用于调味剂和香水。

9. Preparation: Oxidation of Ethanol | 制备：乙醇的氧化

Ethanoic acid can be prepared in the laboratory by oxidising ethanol. Mild oxidising agents, such as acidified potassium dichromate(VI), turn ethanol first into ethanal and then into ethanoic acid. The reaction is often carried out with heating under reflux to ensure complete oxidation: C₂H₅OH + 2[O] → CH₃COOH + H₂O. This is a key link between the alcohols and carboxylic acids in organic reaction pathways.

在实验室中，可通过氧化乙醇制备乙酸。使用温和氧化剂，如酸化重铬酸钾(VI)，先将乙醇转化为乙醛，再转化为乙酸。该反应通常需要在回流下加热以确保完全氧化：C₂H₅OH + 2[O] → CH₃COOH + H₂O。这是有机反应路线中醇与羧酸之间的关键联系。

10. Common Carboxylic Acids and Their Uses | 常见羧酸及其用途

Methanoic acid (formic acid) is found in ant and nettle stings and is used in leather tanning. Ethanoic acid (acetic acid) is the acid in vinegar and is used as a food preservative and in the chemical industry. Propanoic acid and butanoic acid are important in ester manufacture, while longer-chain fatty acids are essential components of soaps and detergents.

甲酸（蚁酸）存在于蚂蚁和荨麻的刺中，用于皮革鞣制。乙酸（醋酸）是食醋中的酸，用作食品防腐剂并用于化学工业。丙酸和丁酸在酯的生产中很重要，而长链脂肪酸是肥皂和洗涤剂的关键成分。

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Carboxylic Acids | 羧酸 考点精讲