羧酸Carboxylic acid

系统命名用后缀 -oic acid，羧基碳编为 1 号：乙酸 ethanoic acid（俗名醋酸 acetic acid），丁酸 butanoic acid。羧酸分子之间形成两个氢键，常以二聚体存在，所以沸点比相对分子质量相近的醇还高：乙酸 118 °C，丙醇 97 °C。低级羧酸易溶于水。

Systematic names use -oic acid, with the carboxyl carbon as C-1: ethanoic acid (common name acetic acid), butanoic acid. Carboxylic acid molecules form pairs held by two hydrogen bonds, so they boil even higher than alcohols of similar Mr: ethanoic acid 118 °C, propan-1-ol 97 °C. Small carboxylic acids dissolve readily in water.

RCOOH ⇌ RCOO⁻ + H⁺。乙酸的 pKa 为 4.76，0.1 mol dm⁻³ 溶液中只有约 1% 电离。羧酸的酸性强于醇和酚，原因是羧酸根离子中的负电荷离域在两个氧原子上而得到稳定。吸电子基团使酸性增强：ClCH₂COOH（pKa 2.86）强于 CH₃COOH。

RCOOH ⇌ RCOO⁻ + H⁺. Ethanoic acid has pKa 4.76; in a 0.1 mol dm⁻³ solution only about 1% is ionised. Carboxylic acids are stronger than alcohols and phenols because the negative charge of the carboxylate ion is delocalised over two oxygens. Electron-withdrawing groups strengthen them: ClCH₂COOH (pKa 2.86) is stronger than CH₃COOH.

具有酸的通性：与活泼金属反应放出氢气，与碱中和，与碳酸盐或碳酸氢盐反应放出 CO₂（可用于区分羧酸与酚）。与醇在浓硫酸催化下酯化。被 LiAlH₄ 还原为伯醇（NaBH₄ 不能还原羧酸）。与 SOCl₂、PCl₅ 或 PCl₃ 反应生成酰氯。

They show typical acid reactions: hydrogen with reactive metals, neutralisation by bases, and CO₂ from carbonates or hydrogencarbonates (which distinguishes them from phenols). With alcohols and concentrated sulfuric acid they form esters. LiAlH₄ reduces them to primary alcohols (NaBH₄ does not). SOCl₂, PCl₅ or PCl₃ convert them to acyl chlorides.

甲酸存在于蚂蚁和荨麻中，乙酸是食醋的主要成分（约 3%–5%），柠檬酸存在于柑橘类水果中，脂肪酸是油脂的组成部分。乙酸用于制造醋酸纤维素和醋酸乙烯酯，对苯二甲酸用于生产聚酯。

Methanoic acid occurs in ants and nettles, ethanoic acid is the main component of vinegar (about 3–5%), citric acid is found in citrus fruits, and fatty acids make up fats and oils. Ethanoic acid is used to make cellulose acetate and vinyl acetate, and benzene-1,4-dicarboxylic acid to make polyesters.