📚 Carboxylic Acids and Derivatives | 羧酸及其衍生物
Carboxylic acids and their derivatives form a central reaction family in AQA A-Level Chemistry Topic 3.9. They share the acyl group RCO- but differ in the atom or group attached to the carbonyl carbon. This article covers naming, physical properties, acidity, key reactions of acids, esters, acyl chlorides, acid anhydrides, amides, hydrolysis and condensation polymers.
羧酸及其衍生物是 AQA A-Level 化学第 3.9 主题的核心反应家族。它们都含有酰基 RCO-，区别在于羰基碳上连接的原子或基团不同。本文涵盖命名、物理性质、酸性、羧酸的关键反应、酯、酰氯、酸酐、酰胺、水解以及缩合聚合物。
1. Naming and Structure of Carboxylic Acids | 羧酸的命名与结构
oxyl group -OH. The general formula for a saturated monocarboxylic acid is CₙH₂ₙ₊₁COOH. The carboxyl carbon has trigonal planar geometry around the carbonyl carbon.
羧酸的官能团是 -COOH，由羰基 C=O 与羟基 -OH 直接相连组成。饱和一元羧酸的通式为 CₙH₂ₙ₊₁COOH。羧基碳原子周围呈平面三角形几何构型。
In systematic naming, the longest chain containing -COOH is identified and the suffix is -oic acid. For example, HCOOH is methanoic acid, CH₃COOH is ethanoic acid, and C₆H₅COOH is benzoic acid. Substituents are numbered starting from the carboxyl carbon as position 1.
在系统命名中，选择含 -COOH 的最长碳链，词尾为“酸”。例如 HCOOH 是甲酸，CH₃COOH 是乙酸，C₆H₅COOH 是苯甲酸。取代基编号从羧基碳开始为 1 号位。
2. Physical Properties and Solubility | 物理性质与溶解性
Carboxylic acids have higher melting and boiling points than alkanes, alcohols and aldehydes of similar molar mass. This is because two carboxylic acid molecules can form a stable dimer through two hydrogen bonds between their -COOH groups.
羧酸的熔点和沸点高于相对分子质量相近的烷烃、醇和醛。这是因为两个羧酸分子可以通过两个 -COOH 基团之间的氢键形成稳定的二聚体。
Short-chain carboxylic acids such as methanoic acid and ethanoic acid are fully miscible with water because they form hydrogen bonds with water molecules. As the non-polar hydrocarbon chain lengthens, solubility decreases sharply because the hydrophobic alkyl portion dominates.
甲酸、乙酸等短链羧酸与水完全互溶，因为它们能与水分子形成氢键。随着非极性烃链增长，溶解性急剧下降，因为疏水烷基部分占主导地位。
3. Acidity of Carboxylic Acids | 羧酸的酸性
Carboxylic acids are weak acids in aqueous solution. They partially dissociate to form a carboxylate ion and a hydronium ion. The equilibrium lies well to the left, so the pH of a 0.1 mol dm⁻³ ethanoic acid solution is around 2.9 rather than 1.0.
羧酸在水溶液中是弱酸。它们部分电离生成羧酸根离子和水合氢离子。平衡强烈偏向左侧，因此 0.1 mol dm⁻³ 乙酸溶液的 pH 约为 2.9，而不是 1.0。
RCOOH + H₂O ⇌ RCOO⁻ + H₃O⁺
The carboxylate ion is stabilised by delocalisation of the negative charge over two oxygen atoms. This makes the conjugate base much more stable than an alkoxide ion, explaining why carboxylic acids are far more acidic than alcohols.
羧酸根离子因负电荷在两个氧原子上离域而稳定。这使得其共轭碱比醇盐离子稳定得多，因此羧酸的酸性远强于醇。
Carboxylic acids react with metals, metal hydroxides, metal oxides and carbonates. The reaction with sodium carbonate produces effervescence of carbon dioxide, which is a useful test to distinguish carboxylic acids from most phenols and alcohols.
羧酸能与金属、金属氢氧化物、金属氧化物和碳酸盐反应。与碳酸钠反应产生二氧化碳气泡，这是区分羧酸与大多数酚和醇的有用检验方法。
2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂
4. Reactions of Carboxylic Acids | 羧酸的反应
Carboxylic acids undergo esterification when heated with an alcohol in the presence of a concentrated sulfuric acid catalyst. The reaction is reversible, so an excess of one reactant or removal of water is used to improve the yield of the ester.
羧酸与醇在浓硫酸催化下加热发生酯化反应。该反应可逆，因此常使用过量的一种反应物或除去水来提高酯的产率。
RCOOH + R'OH ⇌ RCOOR' + H₂O
Reduction of carboxylic acids requires a strong reducing agent such as lithium aluminium hydride, LiAlH₄, in dry ether. This converts the acid directly to a primary alcohol. Sodium borohydride is not strong enough to reduce the carboxylic acid group.
羧酸的还原需要使用强还原剂，如氢化铝锂 LiAlH₄ 在干燥乙醚中进行。该反应将羧酸直接转化为伯醇。硼氢化钠的还原能力不足以还原羧酸基团。
RCOOH + 4[H] → RCH₂OH + H₂O
5. Esters: Formation and Uses | 酯：生成与用途
Esters have the general formula RCOOR'. They are named with the alcohol-derived alkyl group first and the carboxylic acid-derived alkanoate part second. For example, ethyl ethanoate is formed from ethanol and ethanoic acid.
酯的通式为 RCOOR'。命名时先写来自醇的烷基，再写来自羧酸的酸根部分。例如乙酸乙酯由乙醇和乙酸反应生成。
Esters are volatile liquids with distinctive sweet, fruity smells. They are used as solvents, plasticisers and artificial flavourings. Their low boiling points compared with carboxylic acids arise from the lack of hydrogen bonding between ester molecules.
酯是具有独特甜味和水果香气的挥发性液体。它们用作溶剂、增塑剂和人工香料。与羧酸相比，酯的沸点较低，因为酯分子之间不能形成氢键。
Esters can be hydrolysed under acidic or alkaline conditions. Alkaline hydrolysis using aqueous sodium hydroxide goes to completion because the carboxylate salt formed is not able to reverse the reaction. This process is used in soap making from natural fats and oils.
酯可在酸性或碱性条件下水解。使用氢氧化钠溶液的碱性水解反应进行完全，因为生成的羧酸盐不能使反应逆转。该过程用于从天然油脂制造肥皂。
CH₃COOC₂H₅ + NaOH → CH₃COONa + C₂H₅OH
6. Acyl Chlorides | 酰氯
Acyl chlorides have the general formula RCOCl and are named by replacing -oic acid with -oyl chloride. For example, CH₃COCl is ethanoyl chloride. The chlorine atom makes the carbonyl carbon highly electrophilic, so acyl chlorides are the most reactive carboxylic acid derivatives.
酰氯的通式为 RCOCl，命名时将“酸”改为“酰氯”。例如 CH₃COCl 是乙酰氯。氯原子使羰基碳高度亲电，因此酰氯是最活泼的羧酸衍生物。
Acyl chlorides react vigorously with water at room temperature to produce the parent carboxylic acid and hydrochloric acid. The reaction is exothermic and produces steamy white fumes of HCl gas in moist air.
酰氯在室温下与水剧烈反应，生成相应的羧酸和盐酸。该反应放热，并在潮湿空气中产生 HCl 气体的白色烟雾。
CH₃COCl + H₂O → CH₃COOH + HCl
With alcohols, acyl chlorides form esters rapidly at room temperature. With ammonia and primary amines, they form amides and N-substituted amides. These reactions are useful because they occur without strong heating required for esterification with carboxylic acids.
酰氯与醇在室温下迅速生成酯。与氨和伯胺反应生成酰胺和 N-取代酰胺。这些反应非常有用，因为它们不需要像羧酸酯化反应那样强加热。
CH₃COCl + C₂H₅OH → CH₃COOC₂H₅ + HCl
CH₃COCl + 2NH₃ → CH₃CONH₂ + NH₄Cl
7. Acid Anhydrides | 酸酐
Acid anhydrides have the general formula RCOOOCR and are named by replacing -oic acid with -oic anhydride. For example, (CH₃CO)₂O is ethanoic anhydride. They are formed by the condensation of two carboxylic acid molecules with the elimination of water.
酸酐的通式为 RCOOOCR，命名时将“酸”改为“酸酐”。例如 (CH₃CO)₂O 是乙酸酐。它们由两分子羧酸缩合脱去一分子水形成。
Acid anhydrides are less reactive than acyl chlorides but still react readily with water, alcohols and ammonia. A key advantage in the laboratory is that acid anhydride reactions are less violent and do not produce corrosive HCl gas, making them safer for preparing esters and amides.
酸酐的反应活性低于酰氯，但仍能容易地与水、醇和氨反应。实验室中的一个主要优点是酸酐反应不那么剧烈，且不产生腐蚀性 HCl 气体，因此制备酯和酰胺时更安全。
(CH₃CO)₂O + H₂O → 2CH₃COOH
(CH₃CO)₂O + C₂H₅OH → CH₃COOC₂H₅ + CH₃COOH
8. Amides | 酰胺
Amides contain the functional group -CONH₂. Simple amides are named by replacing -oic acid with -amide, such as ethanamide, CH₃CONH₂. The carbonyl carbon is bonded to the nitrogen atom, making the C-N bond partially double in character due to resonance.
酰胺含有官能团 -CONH₂。简单酰胺命名时将“酸”改为“酰胺”，例如乙酰胺 CH₃CONH₂。羰基碳与氮原子相连，由于共振作用，C-N 键具有部分双键性质。
Amides are much less reactive than acyl chlorides, anhydrides and esters. They are neutral compounds and do not react readily with water at room temperature. Hydrolysis requires prolonged heating with either a strong acid or a strong base.
酰胺的反应活性远低于酰氯、酸酐和酯。它们是中性化合物，室温下不易与水反应。水解需要与强酸或强碱长时间加热。
s high melting points and, in the case of polymers such as proteins, strong intermolecular forces stabilising the structure.
伯酰胺和仲酰胺可在 N-H 与 C=O 基团之间形成氢键。这使得简单酰胺具有高熔点，对于蛋白质等聚合物而言，强分子间作用力可稳定结构。
9. Hydrolysis of Carboxylic Acid Derivatives | 羧酸衍生物的水解
All carboxylic acid derivatives undergo hydrolysis, but the conditions required depend on their reactivity. Acyl chlorides and acid anhydrides hydrolyse rapidly with cold water. Esters need acid or alkali and heating, while amides require prolonged reflux with concentrated acid or alkali.
所有羧酸衍生物都能水解，但所需条件取决于其反应活性。酰氯和酸酐在冷水中迅速水解。酯需要在酸性或碱性条件下加热，而酰胺则需要与浓酸或浓碱长时间回流。
Acidic hydrolysis of an amide produces the parent carboxylic acid and an ammonium salt. Alkaline hydrolysis produces the carboxylate salt and ammonia gas, which can be detected by its ability to turn damp red litmus paper blue.
酰胺的酸性水解生成相应的羧酸和铵盐。碱性水解生成羧酸盐和氨气，氨气可使湿润的红色石蕊试纸变蓝，以此进行检验。
CH₃CONH₂ + H₂O + HCl → CH₃COOH + NH₄Cl
CH₃CONH₂ + NaOH → CH₃COONa + NH₃
10. Condensation Polymers: Polyesters and Polyamides | 缩合聚合物：聚酯和聚酰胺
Condensation polymerisation occurs when bifunctional monomers react together with the elimination of a small molecule such as water. Two important classes are polyesters and polyamides, both formed from carboxylic acid derivatives.
缩聚反应是双官能团单体之间反应并脱去水等小分子的过程。两类重要的缩聚物是聚酯和聚酰胺，它们均由羧酸衍生物形成。
Polyesters are made from diols and dicarboxylic acids or diacyl chlorides. A common example is polyethylene terephthalate, PET, formed from ethane-1,2-diol and benzene-1,4-dicarboxylic acid.
聚酯由二醇与二元羧酸或二酰氯制成。常见的例子是聚对苯二甲酸乙二醇酯，即 PET，由乙二醇和对苯二甲酸形成。
n HOCH₂CH₂OH + n HOOC-C₆H₄-COOH → [OCH₂CH₂OOC-C₆H₄-CO]ₙ + 2n H₂O
Polyamides are made from diamines and dicarboxylic acids or diacyl chlorides. Nylon 6,6 is produced from hexane-1,6-diamine and hexanedioic acid. The amide link -CONH- is the repeating structural feature.
聚酰胺由二胺与二元羧酸或二酰氯制成。尼龙 6,6 由己二胺和己二酸生产。酰胺键 -CONH- 是重复结构特征。
n H₂N(CH₂)₆NH₂ + n HOOC(CH₂)₄COOH → [HN(CH₂)₆NHOC(CH₂)₄CO]ₙ + 2n H₂O
11. Identifying Functional Groups and Reaction Pathways | 官能团鉴别与反应路径
Carboxylic acids can be identified by adding solid sodium carbonate or sodium hydrogencarbonate solution. Effervescence of carbon dioxide confirms the presence of the -COOH group. This test is less positive for most substituted phenols, which are too weakly acidic to release CO₂ from carbonate.
羧酸可通过加入固体碳酸钠或碳酸氢钠溶液进行鉴别。产生二氧化碳气泡即证明存在 -COOH 基团。大多数取代酚酸性太弱，不能从碳酸盐中释放出 CO₂，因此该检验可用于区分。
Esters can be hydrolysed and the products tested to identify the parent alcohol and acid. For example, alkaline hydrolysis followed by acidification gives the carboxylic acid, which can be tested with carbonate. Distillation of the alcohol and testing with acidified dichromate can confirm a primary or secondary alcohol.
酯可水解后检测产物以鉴定原来的醇和酸。例如碱性水解后酸化得到羧酸，可用碳酸盐检验。蒸馏出醇并用酸化重铬酸盐检验，可确认是伯醇或仲醇。
Amides can be distinguished from esters and carboxylic acids by their resistance to room-temperature hydrolysis and their production of ammonia on strong alkaline heating. Acyl chlorides are identified by rapid hydrolysis and steamy HCl fumes, while acid anhydrides hydrolyse more gently to the parent acid.
酰胺可通过其抗室温水解性以及强碱加热时释放氨气来与酯和羧酸区分。酰氯可通过快速水解和 HCl 白烟鉴别，而酸酐则较温和地水解为相应的酸。
12. Summary of Relative Reactivity | 衍生物相对反应活性总结
The relative reactivity of carboxylic acid derivatives follows the order: acyl chloride > acid anhydride > ester > amide. This trend is explained by two factors: the electron-withdrawing power of the leaving group and the stability of the leaving group as an anion.
羧酸衍生物的相对反应活性顺序为：酰氯 > 酸酐 > 酯 > 酰胺。这一趋势可用两个因素解释：离去基团的吸电子能力以及离去基团作为阴离子的稳定性。
Acyl chlorides are most reactive because the chloride ion is a weak base and a good leaving group, and the electronegative chlorine atom increases the partial positive charge on the carbonyl carbon. Amides are least reactive because the nitrogen lone pair donates electron density into the carbonyl group, reducing its electrophilicity.
酰氯最活泼，因为氯离子是弱碱且是良好的离去基团，同时电负性的氯原子增加了羰基碳上的部分正电荷。酰胺最不活泼，因为氮上的孤对电子向羰基提供电子密度，降低了羰基的亲电性。
| Derivative | Reaction with water | Reaction with alcohol | Reaction with ammonia |
|---|---|---|---|
| Acyl chloride | Violent, room temperature | Fast, room temperature | Fast, room temperature |
| Acid anhydride | Gentle, warm | Warm | Warm |
| Ester | Acid or alkali, heat | No reaction | No reaction |
| Amide | Concentrated acid or alkali, prolonged reflux | No reaction | No reaction |
Understanding this reactivity order allows chemists to select the appropriate derivative for a synthesis. For example, an ester can be made directly from a carboxylic acid and alcohol, but using an acyl chloride or acid anhydride gives a faster, higher-yielding reaction under milder conditions.
理解这一反应活性顺序有助于化学家为合成选择合适的衍生物。例如酯可直接由羧酸和醇制备，但使用酰氯或酸酐可在更温和条件下实现更快、更高产率的反应。
Published by TutorHao | Chemistry Revision Series | aleveler.com
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