Carboxylic Acids | 羧酸考点精讲

📚 Carboxylic Acids | 羧酸考点精讲

Carboxylic acids are organic compounds containing the carboxyl functional group –COOH. They appear widely in nature and industry, from vinegar (ethanoic acid) to fatty acids and polymers. In OCR A-Level Chemistry, this topic covers structure, acidity, preparation, and key reactions including esterification and acyl chloride formation.

羧酸是含有羧基官能团 –COOH 的有机化合物。它们广泛存在于自然界和工业中，从食醋（乙酸）到脂肪酸和聚合物。在 OCR A-Level 化学中，本主题涵盖结构、酸性、制备及关键反应，包括酯化和酰氯的形成。

1. The Carboxyl Functional Group | 羧基官能团

The carboxyl group, –COOH, is a combination of a carbonyl (C=O) and a hydroxyl (–OH) on the same carbon. The carbon is sp² hybridised, giving a planar geometry with bond angles of approximately 120°.

羧基 –COOH 是羰基 (C=O) 和羟基 (–OH) 连接在同一个碳上的组合。该碳原子为 sp² 杂化，形成平面几何结构，键角约为 120°。

Delocalisation occurs between the lone pair on the oxygen of the –OH group and the π system of the C=O bond. This resonance stabilises the carboxylate anion after deprotonation and makes the O–H bond more polar.

–OH 基团氧上的孤对电子与 C=O 键的 π 体系发生离域。这种共振作用使去质子化后的羧酸根阴离子更加稳定，并使 O–H 键极性更强。

2. Nomenclature of Carboxylic Acids | 羧酸的命名

Under IUPAC rules, carboxylic acids are named by identifying the longest carbon chain containing the –COOH group. The ‘e’ at the end of the corresponding alkane name is replaced with ‘oic acid’. The carboxyl carbon is always position 1.

根据 IUPAC 规则，命名羧酸时需找出含 –COOH 基团的最长碳链。将对应烷烃名称末尾的 ‘e’ 替换为 ‘oic acid’。羧基碳总是位号 1。

Examples:

示例：

HCOOH – methanoic acid | 甲酸
CH₃COOH – ethanoic acid | 乙酸
CH₃CH₂COOH – propanoic acid | 丙酸
HOOC–COOH – ethanedioic acid (oxalic acid) | 乙二酸（草酸）

When other substituents are present, numbering starts from the carboxyl carbon. For instance, 2-hydroxypropanoic acid (lactic acid).

当存在其他取代基时，编号从羧基碳开始。例如 2-羟基丙酸（乳酸）。

3. Physical Properties: Boiling Points and Solubility | 物理性质：沸点与溶解度

Carboxylic acids have significantly higher boiling points than alcohols of comparable molecular mass. This arises from strong intermolecular hydrogen bonding. Two carboxylic acid molecules can hydrogen bond to form a dimer, effectively doubling the apparent molecular mass.

羧酸的沸点远高于分子量相近的醇。这是因为强烈的分子间氢键作用。两个羧酸分子可通过氢键形成二聚体，有效使表观分子量加倍。

Short-chain carboxylic acids (up to four carbons) are completely miscible with water owing to their ability to hydrogen bond with water molecules. Solubility falls as the hydrophobic alkyl chain lengthens.

短链羧酸（至多四个碳）因能与水分子形成氢键而完全与水混溶。随着疏水烷基链增长，溶解度下降。

4. Acidity of Carboxylic Acids | 羧酸的酸性

Carboxylic acids are weak acids. In aqueous solution they partially dissociate, establishing an equilibrium: CH₃COOH ⇌ CH₃COO⁻ + H⁺. The acid dissociation constant Kₐ is typically of the order 10⁻⁵ mol dm⁻³, giving pKₐ values around 4.8 for ethanoic acid.

羧酸是弱酸。在水溶液中它们部分电离，建立平衡：CH₃COOH ⇌ CH₃COO⁻ + H⁺。酸解离常数 Kₐ 通常约为 10⁻⁵ mol dm⁻³，乙酸的 pKₐ 值约 4.8。

The relatively high acidity compared to alcohols is explained by the resonance stabilisation of the carboxylate anion. The negative charge is delocalised over two oxygen atoms, lowering the energy of the conjugate base.

与醇相比酸性较强，原因在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，降低了共轭碱的能量。

Electron-withdrawing substituents near the carboxyl group (e.g. chloroethanoic acid) increase acidity by stabilising the anion inductively. Electron-donating groups decrease acidity.

靠近羧基的吸电子取代基（例如氯乙酸）通过诱导效应稳定阴离子，从而增强酸性。给电子基团则降低酸性。

5. Salt Formation with Bases and Carbonates | 与碱和碳酸盐成盐

Carboxylic acids react with bases such as sodium hydroxide to form the corresponding carboxylate salt and water: CH₃COOH + NaOH → CH₃COONa + H₂O.

羧酸与氢氧化钠等碱反应，生成相应的羧酸盐和水：CH₃COOH + NaOH → CH₃COONa + H₂O。

With metal carbonates or hydrogencarbonates, they produce a salt, carbon dioxide, and water. Effervescence is observed: 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + CO₂ + H₂O.

与金属碳酸盐或碳酸氢盐反应，生成盐、二氧化碳和水，观察到起泡：2CH₃COOH + Na₂CO₃ → 2CH₃COONa + CO₂ + H₂O。

This reaction is used as a test to distinguish carboxylic acids from other acidic compounds like phenols, which do not react with carbonates.

这一反应用作区分羧酸与其他酸性化合物（如酚，不与碳酸盐反应）的检验。

6. Preparation of Carboxylic Acids | 羧酸的制备

A standard laboratory preparation is the oxidation of primary alcohols or aldehydes. Refluxing ethanol with acidified potassium dichromate(VI) yields ethanoic acid. The oxidising agent changes from orange to green (Cr³⁺).

标准的实验室制备方法是氧化伯醇或醛。乙醇与酸化的重铬酸钾(VI)回流加热生成乙酸。氧化剂由橙色变为绿色（Cr³⁺）。

Hydrolysis of nitriles is another route. Heating a nitrile with dilute hydrochloric acid produces the corresponding carboxylic acid and ammonium chloride: CH₃CN + 2H₂O + HCl → CH₃COOH + NH₄Cl.

腈的水解是另一条途径。将腈与稀盐酸加热，得到对应的羧酸和氯化铵：CH₃CN + 2H₂O + HCl → CH₃COOH + NH₄Cl。

Alkaline hydrolysis followed by acidification can also be used. This method allows chain length extension by one carbon atom from a halogenoalkane via nitrile synthesis.

也可先用碱水解再酸化。该方法通过从卤代烷合成腈，使碳链增长一个碳原子。

7. Esterification Reaction | 酯化反应

Carboxylic acids react with alcohols in the presence of a strong acid catalyst (commonly concentrated sulfuric acid) to form esters and water. This is a reversible condensation reaction, often called Fischer esterification.

羧酸与醇在强酸催化剂（常用浓硫酸）存在下反应生成酯和水。这是一个可逆的缩合反应，常称为费歇尔酯化。

General equation: RCOOH + R’OH ⇌ RCOOR’ + H₂O.

通式：RCOOH + R’OH ⇌ RCOOR’ + H₂O。

For example, ethanoic acid reacts with ethanol to give ethyl ethanoate, a sweet-smelling liquid used as a solvent. The mechanism involves nucleophilic addition–elimination with the alcohol acting as the nucleophile.

例如，乙酸与乙醇反应生成乙酸乙酯，一种有甜香气味的液体，常用作溶剂。该机理涉及亲核加成–消除，醇作为亲核试剂。

8. Formation of Acyl Chlorides | 酰氯的形成

Acyl chlorides are highly reactive derivatives. Carboxylic acids react with phosphorus(V) chloride (PCl₅), phosphorus trichloride oxide (POCl₃), or thionyl chloride (SOCl₂) to replace the –OH group with –Cl.

酰氯是高活性衍生物。羧酸与五氯化磷 (PCl₅)、三氯氧磷 (POCl₃) 或亚硫酰氯 (SOCl₂) 反应，将 –OH 基团替换为 –Cl。

The reaction with SOCl₂ is convenient because the by-products (SO₂ and HCl) are gases that escape, leaving the acyl chloride in high purity: CH₃COOH + SOCl₂ → CH₃COCl + SO₂ + HCl.

与 SOCl₂ 的反应较为便利，因为副产物 SO₂ 和 HCl 为气体逸出，得到高纯度的酰氯：CH₃COOH + SOCl₂ → CH₃COCl + SO₂ + HCl。

Acyl chlorides are used to prepare esters, amides, and anhydrides under mild conditions, avoiding the need for strong acid catalysts.

酰氯可用于在温和条件下制备酯、酰胺和酸酐，无需强酸催化剂。

9. Reduction of Carboxylic Acids | 羧酸的还原

Carboxylic acids can be reduced to primary alcohols using a strong reducing agent such as lithium tetrahydridoaluminate (LiAlH₄) in dry ether. This reaction requires anhydrous conditions.

羧酸可使用强还原剂如氢化铝锂 (LiAlH₄) 在干燥乙醚中被还原为伯醇。该反应需要无水条件。

For example, propanoic acid is reduced to propan-1-ol: CH₃CH₂COOH + 4[H] → CH₃CH₂CH₂OH + H₂O.

例如，丙酸被还原为丙-1-醇：CH₃CH₂COOH + 4[H] → CH₃CH₂CH₂OH + H₂O。

Sodium borohydride (NaBH₄) is not strong enough to reduce carboxylic acids; this selectivity is often used in synthesis to reduce aldehydes or ketones in the presence of carboxylic acids.

硼氢化钠 (NaBH₄) 的还原性不足以还原羧酸；在合成中常利用这种选择性，在羧酸存在下还原醛或酮。

10. Decarboxylation and Other Reactions | 脱羧及其他反应

Sodium salts of carboxylic acids undergo decarboxylation when heated with soda lime (NaOH/CaO). This reaction removes CO₂ and gives an alkane with one carbon fewer. For example, sodium ethanoate produces methane: CH₃COONa + NaOH → CH₄ + Na₂CO₃.

羧酸的钠盐与碱石灰（NaOH/CaO）共热时发生脱羧反应，脱去 CO₂ 并生成碳原子数少一个的烷烃。例如乙酸钠生成甲烷：CH₃COONa + NaOH → CH₄ + Na₂CO₃。

This reaction is useful in organic synthesis shortening a carbon chain, but offers limited synthetic control for complex molecules.

该反应用于有机合成中缩短碳链，但对复杂分子而言合成控制有限。

Carboxylic acids can also be converted into amides via acyl chlorides and ammonia, and undergo α-halogenation (Hell–Volhard–Zelinsky reaction) in the presence of phosphorus and bromine.

羧酸还可通过酰氯与氨反应转化为酰胺，并在磷和溴存在下发生 α-卤代反应（赫尔–沃尔哈德–泽林斯基反应）。

11. Identifying Carboxylic Acids: Tests and Spectroscopy | 羧酸的鉴别：检验与光谱

Common chemical tests include effervescence with sodium carbonate solution (CO₂ turns limewater milky). Carboxylic acids also liberate hydrogen gas with reactive metals like magnesium.

常用化学检验包括与碳酸钠溶液起泡（CO₂ 使石灰水变浑浊）。羧酸还可与活泼金属如镁反应放出氢气。

In infrared (IR) spectroscopy, the key absorptions are a broad O–H stretch in the range 2500–3300 cm⁻¹ (often overlapping with C–H) and a sharp C=O stretch near 1700–1725 cm⁻¹. The broad hydrogen-bonded O–H peak is diagnostic.

在红外光谱中，关键吸收峰为 2500–3300 cm⁻¹ 范围内的宽 O–H 伸缩振动峰（常与 C–H 重叠），以及 1700–1725 cm⁻¹ 附近的尖锐 C=O 伸缩振动峰。宽大的氢键 O–H 峰具有诊断意义。

In mass spectrometry, carboxylic acids show a molecular ion peak M⁺, and often fragments corresponding to loss of OH (M–17) or COOH (M–45).

在质谱中，羧酸显示分子离子峰 M⁺，常见失去 OH (M–17) 或 COOH (M–45) 的碎片。

12. Summary and Key Points for Exams | 总结与考试要点

In OCR A-Level Chemistry, emphasis is placed on the structure–property relationship of the carboxyl group, the acidity rationalised by resonance, and the interconversion between carboxylic acids and their derivatives. Students must be able to write equations for salt formation, esterification, and preparation of acyl chlorides, and explain physical properties through hydrogen bonding.

在 OCR A-Level 化学中，重点在于羧基的结构–性质关系、通过共振解释酸性，以及羧酸及其衍生物之间的相互转化。学生必须能书写成盐、酯化和制备酰氯的方程式，并通过氢键解释物理性质。

Remember: esterification is slow and needs an acid catalyst; hydrolysis of nitriles extends the carbon chain by one atom; reduction requires LiAlH₄, not NaBH₄. Always use ‘oic acid’ for naming and indicate the position of substituents from the carboxyl carbon.

记住：酯化反应较慢，需要酸催化；腈的水解使碳链增长一个原子；还原需要 LiAlH₄ 而非 NaBH₄。命名时始终使用 ‘oic acid’，并从羧基碳标示取代基位置。

Published by TutorHao | Chemistry Revision Series | aleveler.com

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Carboxylic Acids | 羧酸考点精讲