📚 Carboxylic Acids and Their Derivatives | 羧酸及其衍生物
Carboxylic acids and their derivatives form a central family of organic compounds, characterised by the carboxyl group (-COOH) and its modified forms such as acid chlorides, anhydrides, esters, and amides. These substances underpin biological processes, polymer manufacture, pharmaceutical synthesis, and laboratory transformations. A clear understanding of their structure, bonding, acidity, interconversion reactions, and relative reactivity is essential for success in A Level Chemistry. This article systematically covers nomenclature, physical trends, preparation methods, key reactions, and spectroscopic identification, linking each derivative’s behaviour to the leaving group ability and resonance effects that govern the acyl substitution mechanism.
羧酸及其衍生物构成有机化学的核心家族，它们的特征官能团是羧基（–COOH）及其修饰形式，如酰氯、酸酐、酯和酰胺。这些物质支撑着生物过程、聚合物制造、药物合成和实验室转化。透彻理解它们的结构、键合、酸性、相互转化反应以及相对反应活性，是学好 A Level 化学的基础。本文系统涵盖命名法、物理趋势、制备方法、关键反应和光谱鉴定，将每种衍生物的行为与离去基团能力和共振效应联系起来，阐明亲核酰基取代的支配规律。
1. Structure and Nomenclature of Carboxylic Acids | 羧酸的结构与命名
The carboxyl functional group consists of a carbonyl (C=O) and a hydroxyl (-OH) attached to the same carbon. This carbon is sp² hybridised, giving a planar arrangement with bond angles close to 120°. IUPAC names replace the terminal ‘-e’ of the parent alkane with ‘-oic acid’, so methanoic acid (HCOOH) is the simplest, followed by ethanoic acid (CH₃COOH) and propanoic acid (CH₃CH₂COOH). When substituents are present, the carboxyl carbon is always C-1; for example, 2-methylpropanoic acid. Dicarboxylic acids use the suffix ‘-dioic acid’, as in ethanedioic acid (HOOC-COOH). Aromatic carboxylic acids such as benzoic acid (C₆H₅COOH) are named with the ring as the parent.
羧基官能团由连接在同一个碳上的羰基（C=O）和羟基（–OH）组成。这个碳是 sp² 杂化，呈平面构型，键角接近 120°。IUPAC 命名将母体烷烃末尾的“-e”替换为“-oic acid”，因此甲酸（HCOOH）是最简单的羧酸，接着是乙酸（CH₃COOH）和丙酸（CH₃CH₂COOH）。当有取代基时，羧基碳始终为 C-1 位，例如 2-甲基丙酸。二羧酸使用后缀“-dioic acid”，如乙二酸（HOOC–COOH）。芳香族羧酸如苯甲酸（C₆H₅COOH）以苯环为母体命名。
2. Physical Properties and Acidity | 物理性质与酸性
Carboxylic acids exhibit exceptionally high boiling points for their molecular mass because two molecules can form a dimer through a pair of intermolecular hydrogen bonds. Lower members (up to C4) are fully miscible with water, but solubility falls sharply beyond five carbon atoms as the hydrophobic alkyl chain dominates. The -COOH group is acidic: a typical pKₐ value is around 4.8 for ethanoic acid, much stronger than alcohols (pKₐ ~16) and phenols (pKₐ ~10). The strength stems from resonance stabilisation of the carboxylate anion, RCOO⁻, which delocalises the negative charge equally over two oxygen atoms, thus lowering the energy of the conjugate base. Electron-withdrawing substituents further enhance acidity; chloroethanoic acid (ClCH₂COOH) has a pKₐ of 2.9 due to the –I effect of chlorine.
羧酸的沸点相对于其分子质量异常高，因为两个分子可通过一对分子间氢键形成二聚体。低级成员（最多 C4）与水完全混溶，但超过五个碳原子后溶解度急剧下降，疏水烷基链占据主导。–COOH 基团显酸性：乙酸的典型 pKₐ 值约为 4.8，远强于醇（pKₐ ~16）和酚（pKₐ ~10）。酸性来源于羧酸根负离子 RCOO⁻ 的共振稳定，负电荷均等地离域在两个氧原子上，从而降低了共轭碱的能量。吸电子取代基进一步增强酸性；氯乙酸（ClCH₂COOH）因氯的 –I 效应，pKₐ 为 2.9。
3. Preparation of Carboxylic Acids | 羧酸的制备
Primary alcohols and aldehydes are oxidised to carboxylic acids using strong oxidising agents such as acidified potassium dichromate(VI) under reflux. For example, ethanol gives ethanoic acid. Alkylbenzenes with at least one benzylic hydrogen are oxidised by hot alkaline potassium manganate(VII) followed by acidification, producing benzoic acid. Nitriles, RCN, hydrolyse when heated with dilute hydrochloric acid or sodium hydroxide, giving the corresponding carboxylic acid (or its salt) plus ammonium ions or ammonia. In the laboratory, esters themselves can be hydrolysed to regenerate the parent acid, providing a useful purification step.
伯醇和醛通过强氧化剂（如酸化重铬酸钾(VI)）在回流条件下氧化成羧酸。例如，乙醇可生成乙酸。含有至少一个苄位氢的烷基苯可被热的碱性高锰酸钾氧化，随后酸化得到苯甲酸。腈类 RCN 在与稀盐酸或氢氧化钠共热时水解，生成相应的羧酸（或其盐）以及铵离子或氨。在实验室中，酯本身可水解再生成母体酸，是一种有用的纯化手段。
4. General Reactions of Carboxylic Acids | 羧酸的主要反应
Carboxylic acids are weak acids but still react with reactive metals such as sodium to release hydrogen gas and form carboxylate salts. With bases like sodium hydroxide, a neutralisation occurs, producing the salt and water. Treatment with carbonates or hydrogencarbonates yields a carboxylate salt, carbon dioxide, and water — a test that distinguishes carboxylic acids from phenols, which do not liberate CO₂. Esterification, the acid-catalysed condensation with an alcohol, is a reversible equilibrium that requires heating under reflux with concentrated sulfuric acid and is pushed to completion using excess reactant or removal of water. Strong reducing agents such as LiAlH₄ in dry ether reduce -COOH all the way to the primary alcohol. Phosphorus pentachloride (PCl₅) or thionyl chloride (SOCl₂) converts the -OH group into -Cl, generating an acyl chloride — a key gateway to derivatives.
羧酸是弱酸，但仍能与活泼金属如钠反应，放出氢气并生成羧酸盐。与氢氧化钠等碱发生中和反应，生成盐和水。与碳酸盐或碳酸氢盐反应产生羧酸盐、二氧化碳和水——这一试验可将羧酸与酚区分开，因为酚不释放 CO₂。酯化反应是酸催化的与醇的缩合，是一个可逆平衡，需在浓硫酸催化下回流加热，并通过过量反应物或除水使平衡右移。强还原剂如氢化铝锂在干醚中将 –COOH 彻底还原为伯醇。五氯化磷（PCl₅）或亚硫酰氯（SOCl₂）则将 –OH 基团转化为 –Cl，生成酰氯——这是通往衍生物的关键门户。
RCOOH + R’OH ⇌ RCOOR’ + H₂O RCOOH + SOCl₂ → RCOCl + SO₂ + HCl
5. Acyl Chlorides | 酰氯
Acyl chlorides, general formula RCOCl, are named by replacing the ‘-oic acid’ ending with ‘-oyl chloride’, e.g. ethanoyl chloride CH₃COCl. They are prepared from carboxylic acids using PCl₅ or SOCl₂, reactions that proceed readily at room temperature without the need for a catalyst. Acyl chlorides are the most reactive of the common carboxylic acid derivatives. Their reactions follow an addition-elimination mechanism: a nucleophile adds to the electrophilic carbonyl carbon, eliminating chloride ion, which is an excellent leaving group because Cl⁻ is a very weak base. With water, they hydrolyse vigorously, releasing HCl fumes; with alcohols, they produce esters smoothly without acid catalysis; with ammonia and amines, they yield amides and N-substituted amides. Because of their high reactivity, acyl chlorides are kept anhydrous and used immediately.
酰氯的通式为 RCOCl，命名时将“-oic acid”结尾改为“-oyl chloride”，如乙酰氯 CH₃COCl。它们可由羧酸与 PCl₅ 或 SOCl₂ 制备，这些反应在室温下即可顺利进行，无需催化剂。酰氯是常见羧酸衍生物中反应活性最高的。其反应遵循加成-消除机理：亲核试剂进攻具有亲电性的羰基碳，同时氯离子离去——Cl⁻ 是一种极弱的碱，因而是极佳的离去基团。与水反应时剧烈水解，释放 HCl 烟雾；与醇反应可平稳生成酯，无需酸催化；与氨和胺反应分别得到酰胺和 N-取代酰胺。由于反应活性高，酰氯需保持无水并现制现用。
6. Acid Anhydrides | 酸酐
Acid anhydrides have the structure (RCO)₂O and are named by replacing ‘acid’ with ‘anhydride’, such as ethanoic anhydride (CH₃CO)₂O. They are prepared by the dehydration of carboxylic acids with a powerful dehydrating agent like phosphorus pentoxide (P₂O₅) or by reacting an acyl chloride with a carboxylate salt. Anhydrides are less vigorous than acyl chlorides but still react with water, alcohols, ammonia, and amines to produce carboxylic acids, esters, and amides. Their moderate reactivity makes them especially useful in industrial syntheses; for instance, aspirin is made by acetylating salicylic acid with ethanoic anhydride, avoiding the harsh conditions and corrosive by-products associated with acyl chlorides.
酸酐的结构为 (RCO)₂O，命名时将“酸”替换为“酸酐”，如乙酸酐 (CH₃CO)₂O。它们可以通过强脱水剂如五氧化二磷（P₂O₅）使羧酸脱水制得，或通过酰氯与羧酸盐反应制备。酸酐的活泼性不如酰氯，但仍能与水、醇、氨和胺反应，生成羧酸、酯和酰胺。其适中的反应活性使它们在工业合成中特别有用；例如，阿司匹林就是通过水杨酸与乙酸酐乙酰化制得的，避免了酰氯所伴随的苛刻条件和腐蚀性副产物。
7. Esters | 酯
Esters have the general formula RCOOR’ and are well-known for their pleasant, fruity odours. They are volatile liquids that serve as solvents and flavourings. The three principal ways to prepare an ester are: (i) Fischer esterification (carboxylic acid + alcohol, H⁺, Δ, reversible); (ii) acyl chloride + alcohol at room temperature; (iii) acid anhydride + alcohol with warming. Esters undergo acid-catalysed hydrolysis to regenerate the parent acid and alcohol — a reversible process — and base-catalysed hydrolysis (saponification), which goes to completion because the carboxylate salt formed does not react with alcohol. With concentrated ammonia, esters produce amides and the parent alcohol. Reduction with LiAlH₄ cleaves the ester, yielding two alcohol molecules.
酯的通式为 RCOOR’，因其宜人的果香而广为人知。它们是挥发性液体，可用作溶剂和香料。制备酯的三种主要方法为：(i) 费歇尔酯化（羧酸 + 醇，H⁺ 催化，加热，可逆）；(ii) 酰氯与醇在室温下反应；(iii) 酸酐与醇温热反应。酯水解在酸催化下生成母体酸和醇——这是一个可逆过程；而在碱催化水解（皂化）中，反应可进行到底，因为生成的羧酸盐不与醇反应。与浓氨反应时，酯生成酰胺和母体醇。氢化铝锂还原可使酯断裂，得到两分子醇。
RCOOR’ + H₂O ⇌ RCOOH + R’OH RCOOR’ + OH⁻ → RCOO⁻ + R’OH
8. Amides | 酰胺
Amides contain the -CONH₂ group; they are named by dropping ‘-oic acid’ and adding ‘-amide’, giving ethanamide, CH₃CONH₂. They can be prepared from acyl chlorides or acid anhydrides with ammonia, from ammonium carboxylates by dehydration on heating, or from esters with ammonia. Amides are neutral solids (except methanamide, a liquid) with high melting points due to strong hydrogen bonding. Hydrolysis requires prolonged heating with either acidic or alkaline solutions: acid hydrolysis yields the carboxylic acid and ammonium ions; alkaline hydrolysis gives the carboxylate salt and ammonia. Dehydration with phosphorus pentoxide (P₄O₁₀) produces nitriles, RCN. Reduction with LiAlH₄ converts the carbonyl group to -CH₂-, affording the corresponding primary amine.
酰胺含有 –CONH₂ 基团；命名时去掉“-oic acid”加上“-amide”，得乙酰胺 CH₃CONH₂。它们可通过酰氯或酸酐与氨反应制得，也可由羧酸铵盐加热脱水制备，或由酯与氨反应得到。除甲酰胺为液体外，酰胺多为中性固体，因强氢键作用而具有高熔点。酰胺水解需在酸性或碱性溶液中长时间加热：酸水解生成羧酸和铵离子；碱水解则生成羧酸盐和氨。与五氧化二磷（P₄O₁₀）共热脱水生成腈 RCN。氢化铝锂还原将羰基转化为 –CH₂–，得到相应的伯胺。
9. Relative Reactivity of Carboxylic Acid Derivatives | 羧酸衍生物的相互反应活性
The reactivity order of carboxylic acid derivatives towards nucleophilic acyl substitution is: acyl chloride > acid anhydride > ester > amide. This sequence is directly related to the basicity—and hence the leaving-group ability—of the group attached to the carbonyl: Cl⁻ is an extremely weak base and departs readily; the carboxylate ion is a slightly stronger base but still a good leaving group; alkoxide ions are much stronger bases and poorer leaving groups; amide ion NH₂⁻ is a very strong base, making amides the least reactive. This trend is reflected in the hydrolysis conditions: acyl chlorides react violently with cold water, anhydrides require warming, esters need acid or base and heat, and amides demand prolonged heating with concentrated acid or alkali.
羧酸衍生物对亲核酰基取代的反应活性顺序为：酰氯 > 酸酐 > 酯 > 酰胺。这一序列直接与连接在羰基上的基团的碱性——即离去能力——相关：Cl⁻ 是极弱的碱，极易离去；羧酸根离子碱性稍强但仍为好离去基；烷氧基离子碱性更强，是较差的离去基；酰胺离子 NH₂⁻ 是很强的碱，致使酰胺活性最低。该趋势在水解条件上得到体现：酰氯与冷水剧烈反应，酸酐需要温热，酯需要酸或碱并加热，酰胺则需用浓酸或浓碱长时间加热。
| Derivative | Leaving group | Basic strength of leaving group | Typical hydrolysis conditions |
|---|---|---|---|
| Acyl chloride | Cl⁻ | Very weak | Cold water, vigorous |
| Acid anhydride | RCOO⁻ | Weak | Warm water |
| Ester | RO⁻ | Stronger | Dilute acid or alkali, heat |
| Amide | NH₂⁻ | Very strong | Conc. H⁺ or OH⁻, prolonged heat |
10. Polyesters and Polyamides | 聚酯和聚酰胺
When difunctional monomers react, condensation polymerisation produces polyesters and polyamides with the elimination of a small molecule such as water or HCl. Polyethylene terephthalate (PET, Terylene) is a polyester formed from benzene-1,4-dicarboxylic acid (terephthalic acid) and ethane-1,2-diol. Nylon-6,6 is a polyamide prepared from hexane-1,6-dioic acid and hexane-1,6-diamine; the two monomers are first combined as a nylon salt, which is then heated to drive off water and form the amide linkages. Both polymers contain repeating ester or amide units that are susceptible to hydrolysis, making them biodegradable under the right conditions. Drawing the repeat unit, identifying the monomers, and describing alkaline hydrolysis are typical exam skills.
当双官能团单体反应时，缩合聚合生成聚酯和聚酰胺，同时脱去小分子如水或氯化氢。聚对苯二甲酸乙二醇酯（PET，涤纶）是由对苯二甲酸和乙二醇形成的聚酯。尼龙-6,6 是由己二酸和己二胺制备的聚酰胺；两种单体首先结合成尼龙盐，然后加热脱水形成酰胺链节。两种聚合物都含有能够水解的重复酯基或酰胺单元，因而在合适条件下可生物降解。绘制重复单元、识别单体以及描述碱性水解，是典型的考试技能。
11. Spectroscopic Identification | 光谱鉴定
Infrared spectroscopy provides clear signatures for carboxylic acids and their derivatives. The O–H stretch in carboxylic acids appears as a very broad, often smeared band between 2500–3300 cm⁻¹, overlapping with C–H stretches. Carbonyl stretches vary systematically: acid C=O absorbs near 1700–1725 cm⁻¹; ester C=O near 1735–1750 cm⁻¹; acyl chloride C=O is pushed to about 1800 cm⁻¹ due to the electronegative chlorine; amide C=O absorbs at lower wavenumber, around 1650–1690 cm⁻¹. In ¹H NMR, the acidic proton of a carboxylic acid is highly deshielded, giving a signal typically between δ 10–13 ppm, often broadened by exchange phenomena. Esters display a distinctive quartet (or triplet) for the O-CH₂ group around 4.1–4.4 ppm. Mass spectrometry of esters often shows cleavage at the acyl-oxygen bond, producing acylium ion fragments that help deduce the structure.
红外光谱为羧酸及其衍生物提供了清晰的指纹特征。羧酸的 O–H 伸缩振动在 2500–3300 cm⁻¹ 范围内呈现一个非常宽、常连成一片的吸收带，与 C–H 伸缩重叠。羰基伸缩呈现出系统变化：酸的 C=O 在约 1700–1725 cm⁻¹ 吸收；酯的 C=O 接近 1735–1750 cm⁻¹；酰氯的 C=O 因电负性大的氯原子而推向约 1800 cm⁻¹；酰胺的 C=O 在较低波数 1650–1690 cm⁻¹ 附近。在 ¹H NMR 谱中，羧酸的酸性质子高度去屏蔽，信号通常出现在 δ 10–13 ppm 之间，且常因交换现象而变宽。酯在约 4.1–4.4 ppm 处显示 O–CH₂ 的特征四重峰（或三重峰）。酯的质谱常显示酰氧键断裂，产生酰基正离子碎片，有助于推定结构。
Key IR ranges: acid O–H 2500–3300 (broad); acid C=O ~1710; ester C=O ~1740; acyl chloride C=O ~1800; amide C=O ~1660 cm⁻¹
¹H NMR: Carboxylic acid proton δ 10–13 ppm; ester -OCH₂- δ 4.1–4.4 ppm
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