Edexcel A Level Chemistry: Carboxylic Acids (Topic 18.3) | Edexcel A-Level 化学：羧酸（主题 18.3）

📚 Edexcel A Level Chemistry: Carboxylic Acids (Topic 18.3) | Edexcel A-Level 化学：羧酸（主题 18.3）

Carboxylic acids are a homologous series of organic compounds containing the carboxyl functional group, -COOH. This topic focuses on their structure, naming, physical properties, acidity, characteristic reactions, and methods of preparation. Mastery of carboxylic acid chemistry is essential for Edexcel A Level Chemistry Paper 2 and for understanding organic synthesis routes.

羧酸是含有羧基官能团 -COOH 的同系有机物。本主题重点讲解羧酸的结构、命名、物理性质、酸性、特征反应以及制备方法。掌握羧酸化学对 Edexcel A-Level 化学 Paper 2 考试以及理解有机合成路线至关重要。

1. The Carboxyl Functional Group | 羧基官能团

The carboxyl group, -COOH, combines a carbonyl group C=O with a hydroxyl group -OH attached to the same carbon. The general formula for a monocarboxylic acid is RCOOH, where R is H or an alkyl/aryl group. The carboxyl carbon is sp² hybridised, giving a trigonal planar arrangement around that carbon.

羧基 -COOH 由羰基 C=O 和连接在同一碳原子上的羟基 -OH 组成。一元羧酸的通式为 RCOOH，其中 R 可以是氢原子或烷基/芳基。羧基碳为 sp² 杂化，因此羧基碳周围呈平面三角形结构。

In the carboxylate ion RCOO⁻, the negative charge is delocalised over both oxygen atoms. This delocalisation makes the two C-O bonds equivalent in length and greatly stabilises the carboxylate ion, which explains why carboxylic acids release H⁺ more readily than alcohols.

在羧酸根离子 RCOO⁻ 中，负电荷离域分布在两个氧原子上。这种离域使两个 C-O 键长度相等，并极大地稳定了羧酸根离子，这就是羧酸比醇更容易释放 H⁺ 的原因。

2. Nomenclature of Carboxylic Acids | 羧酸的命名

In IUPAC nomenclature, the longest carbon chain containing the -COOH group is selected, and the ending -e of the parent alkane is replaced by -oic acid. The carboxyl carbon is always assigned position 1, so no number is needed for the acid group. Common examples include HCOOH methanoic acid, CH₃COOH ethanoic acid, and CH₃CH₂COOH propanoic acid.

在 IUPAC 命名中，选择含 -COOH 的最长碳链，将母体烷烃词尾 -e 替换为 -oic acid。羧基碳始终编号为 1，因此酸基不需要号码。常见例子包括 HCOOH 甲酸、CH₃COOH 乙酸和 CH₃CH₂COOH 丙酸。

• HCOOH — methanoic acid | 甲酸
• CH₃COOH — ethanoic acid | 乙酸
• CH₃CH₂COOH — propanoic acid | 丙酸
• C₆H₅COOH — benzoic acid | 苯甲酸

When substituent groups are present, the carbon chain is numbered from the carboxyl carbon. For example, CH₃CH(CH₃)COOH is named 2-methylpropanoic acid, and CH₃CH(OH)COOH is 2-hydroxypropanoic acid.

当取代基存在时，碳链从羧基碳开始编号。例如，CH₃CH(CH₃)COOH 命名为 2-甲基丙酸，CH₃CH(OH)COOH 命名为 2-羟基丙酸。

3. Physical Properties and Hydrogen Bonding | 物理性质与氢键

Carboxylic acids have relatively high melting and boiling points because they form intermolecular hydrogen bonds. In the liquid and vapour phases, ethanoic acid molecules pair up as dimers through two O-H···O hydrogen bonds. This dimerisation effectively doubles the molecular mass and raises the boiling point.

羧酸具有较高的熔点和沸点，因为它们能形成分子间氢键。在液态和气态中，乙酸分子通过两个 O-H···O 氢键形成二聚体。这种二聚化实际上使相对分子质量翻倍，从而提高了沸点。

Short-chain carboxylic acids are soluble in water because they can form hydrogen bonds with water molecules. However,

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