📚 Carboxylic Acids and Their Derivatives | 羧酸及其衍生物
Carboxylic acids are a homologous series containing the carboxyl functional group, -COOH. Their derivatives, including acyl chlorides, acid anhydrides, esters and amides, share the acyl group RCO- but differ in the group attached to the carbonyl carbon. These compounds are central to organic synthesis, biological chemistry and industrial polymer production.
羧酸是含有羧基官能团 -COOH 的同系物。它们的衍生物包括酰氯、酸酐、酯和酰胺，都含有共同的酰基 RCO-，但羰基碳上连接的基团不同。这些化合物在有机合成、生物化学和工业聚合物生产中处于核心地位。
At A-Level, this topic is examined through structure and bonding, acidity trends, preparative routes, reaction mechanisms and the relative reactivity of derivatives. Mastering the interconversions in this family is essential for tackling multi-step synthesis questions and for understanding polymers, fats, proteins and pharmaceuticals.
在 A-Level 考试中，本主题通过结构成键、酸性变化规律、制备路线、反应机理以及衍生物的相对活性来进行考查。掌握这一化合物家族内部的相互转化，是解答多步合成题以及理解聚合物、油脂、蛋白质和药物的关键。
1. Structure and Nomenclature | 结构与命名
The carboxyl group consists of a carbonyl group (C=O) directly attached to a hydroxyl group (-OH). The carboxyl carbon is sp² hybridised, so the functional group is planar with bond angles close to 120°, and the C=O and C-O bonds show partial double-bond character due to electron delocalisation.
羧基由直接与羟基 (-OH) 相连的羰基 (C=O) 组成。该碳原子为 sp² 杂化，因此官能团为平面结构，键角接近 120°；由于电子离域，C=O 和 C-O 键都具有一定的双键性质。
In IUPAC nomenclature, the longest chain containing the -COOH group is selected and the suffix -oic acid is used. The carboxyl carbon is always position 1, so no locant is needed, for example methanoic acid HCOOH and ethanoic acid CH₃COOH. Substituents are numbered from the carboxyl carbon, as in 2-chloropropanoic acid CH₃CHClCOOH.
在 IUPAC 命名中，选择含 -COOH 的最长碳链，后缀使用 -oic acid（中文为酸）。羧基碳总是编号为 1，因此无需标明位置，例如甲酸 HCOOH 和乙酸 CH₃COOH。取代基从羧基碳开始编号，例如 2-氯丙酸 CH₃CHClCOOH。
Dicarboxylic acids such as ethanedioic acid HOOC-COOH and propanedioic acid HOOC-CH₂-COOH contain two carboxyl groups and are named with -dioic acid. Aromatic acids are named as benzoic acid derivatives, and the carboxyl group may also appear as a substituent, in which case the prefix carboxy- is used.
二元羧酸如乙二酸 HOOC-COOH 和丙二酸 HOOC-CH₂-COOH 含有两个羧基，命名时使用 -dioic acid（中文为二酸）。芳香酸按苯甲酸衍生物命名；当羧基作为取代基出现时，使用前缀 carboxy-（羧基）。
Salts are named by replacing -oic acid with -oate: sodium ethanoate CH₃COONa is the salt formed when ethanoic acid is neutralised by sodium hydroxide. Recognising the -oate ending is important because examiners frequently use salt names in reaction schemes.
盐的命名是把 -oic acid 换成 -oate：乙酸钠 CH₃COONa 就是乙酸被氢氧化钠中和生成的盐。识别 -oate（中文为“某酸盐”）词尾很重要，因为考题常在反应路线中使用盐的名称。
2. Physical Properties and Solubility | 物理性质与溶解性
Carboxylic acids have unusually high boiling points for their molecular mass because two acid molecules can form a hydrogen-bonded dimer. This dimer involves two O-H···O interactions between the carbonyl oxygen of one molecule and the hydroxyl hydrogen of another, giving an effective molar mass roughly double the monomer.
羧酸的沸点相对其分子质量异常高，因为两个酸分子可以通过氢键形成二聚体。该二聚体包含一个分子的羰基氧与另一个分子的羟基氢之间的两个 O-H···O 相互作用，使有效分子质量约为单体两倍。
Short-chain carboxylic acids such as methanoic and ethanoic acid are fully miscible with water because the -COOH group forms hydrogen bonds with water molecules. Solubility falls sharply as the hydrocarbon chain lengthens: butanoic acid is only moderately soluble, while hexanoic acid and above are essentially insoluble and behave like oils.
甲酸、乙酸等短链羧酸可与水完全互溶，因为 -COOH 基团能与水分子形成氢键。随着烃链增长，溶解性急剧下降：丁酸仅中等溶解，己酸及更高级的羧酸基本不溶，表现为油状。
Low members (C1-C4) are colourless, pungent liquids that are corrosive to skin; higher members are waxy solids with faint odours. Melting points show an alternation pattern for long chains because even- and odd-numbered acids pack differently in the crystal lattice.
低碳成员（C1-C4）为无色、有刺激性气味的液体，对皮肤有腐蚀性；高碳成员为蜡状固体，气味微弱。长链酸的熔点呈现奇偶交替规律，因为偶数碳与奇数碳酸在晶格中的堆积方式不同。
Because they possess both a hydrogen-bond donor (-OH) and a hydrogen-bond acceptor (C=O), carboxylic acids dissolve well in polar solvents such as ethanol and acetone, and they are also soluble in ether, which is exploited in solvent extraction during practical work.
由于同时具有氢键给体 (-OH) 和氢键受体 (C=O)，羧酸易溶于乙醇、丙酮等极性溶剂，也可溶于乙醚，这一性质在实验课的溶剂萃取中被充分利用。
3. Acidity and Inductive Effects | 酸性与诱导效应
lies well to the left, with a Ka of about 1.8 × 10⁻⁵ mol dm⁻³ and a pKa of 4.76 at 298 K.
羧酸是弱酸，在水中只部分电离。乙酸的电离平衡明显偏左，在 298 K 时 Ka 约为 1.8 × 10⁻⁵ mol dm⁻³，pKa 为 4.76。
CH₃COOH + H₂O ⇌ CH₃COO⁻ + H₃O⁺
ving two equivalent resonance forms. In an alcohol, by contrast, the alkoxide ion has no such delocalisation, so alcohols (pKa ≈ 16) are far weaker acids than carboxylic acids.
酸性增强的原因是羧酸根离子因离域而稳定：负电荷平均分布在两个氧原子上，形成两个等价的共振结构。相比之下，醇钠中的烷氧基负离子没有这种离域，因此醇（pKa ≈ 16）的酸性远弱于羧酸。
Electron-withdrawing groups increase acidity by the inductive effect. Chlorine pulls electron density away from the carboxylate, dispersing the negative charge and stabilising the anion; the closer the substituent and the more substituents there are, the stronger the acid.
吸电子基团通过诱导效应增强酸性。氯原子把电子密度从羧酸根上拉走，使负电荷分散、阴离子更稳定；取代基越靠近羧基、数目越多，酸性越强。
| Acid 酸 | pKa | Reason 原因 |
| CHCl₂COOH (dichloroethanoic) | 1.29 | Two Cl atoms withdraw inductively 两个氯吸电子 |
| CH₂ClCOOH (chloroethanoic) | 2.87 | One Cl atom withdraws 一个氯吸电子 |
| HCOOH (methanoic) | 3.75 | H is less electron-donating than CH₃ 氢推电子弱于甲基 |
| CH₃COOH (ethanoic) | 4.76 | Reference acid 参照酸 |
Alkyl groups have the opposite, electron-donating effect, so long-chain acids are slightly weaker than methanoic acid. Carboxylic acids are nevertheless strong enough to liberate carbon dioxide from carbonates and hydrogencarbonates, which is a key test for the group.
烷基具有相反的推电子效应，因此长链羧酸的酸性比甲酸略弱。不过羧酸的酸性足以与碳酸盐和碳酸氢盐反应放出二氧化碳，这正是该官能团的关键检验方法。
4. Preparation of Carboxylic Acids | 羧酸的制备
Oxidation of a primary alcohol with acidified potassium dichromate(VI) under reflux gives the carboxylic acid; if the product is distilled out as it forms, the aldehyde can be isolated instead. Secondary alcohols oxidise to ketones and tertiary alcohols resist oxidation under these conditions.
伯醇用酸化重铬酸钾(VI)回流氧化可得到羧酸；若边生成边蒸出产物，则可分离出醛。仲醇氧化成酮，叔醇在该条件下难以被氧化。
CH₃CH₂OH + 2[O] → CH₃COOH + H₂O
Aldehydes are readily oxidised by Tollens' reagent or Fehling's solution to the corresponding carboxylic acid, and this is also the basis of the test that distinguishes an aldehyde from a ketone. Ketones are not oxidised by these mild reagents.
醛可被托伦试剂或斐林试剂轻易氧化为相应的羧酸，这也是区分醛与酮的检验基础。酮不能被这些温和试剂氧化。
Hydrolysis of a nitrile is a valuable one-carbon-lengthening route. Refluxing a nitrile with dilute hydrochloric acid and water produces the carboxylic acid and ammonium chloride; the intermediate is an amide, so partial hydrolysis can be used to make the amide deliberately.
腈的水解是一条重要的增长一个碳原子的路线。将腈与稀盐酸和水一起回流，生成羧酸和氯化铵；中间体是酰胺，因此控制部分水解可有目的地制备酰胺。
Esters and amides can be hydrolysed back to the parent acid under acidic or alkaline conditions, and alkylbenzenes with a benzylic hydrogen can be oxidised to aromatic acids, for example methylbenzene to benzoic acid with hot alkaline KMnO₄.
酯和酰胺可在酸性或碱性条件下水解回到母体酸；侧链有苄基氢的烷基苯可被氧化为芳香酸，例如甲基苯用热的碱性 KMnO₄ 氧化生成苯甲酸。
5. Reactions of the Carboxyl Group | 羧基的反应
With reactive metals, bases and carbonates, carboxylic acids act as acids and form salts. Sodium carbonate gives vigorous effervescence of carbon dioxide, and this behaviour distinguishes a carboxylic acid from a phenol, which is not acidic enough to react with carbonate.
与活泼金属、碱和碳酸盐反应时，羧酸表现为酸并生成盐。碳酸钠会产生剧烈的二氧化碳气泡，这一现象可把羧酸与酚区分开，因为酚的酸性不足以与碳酸盐反应。
2CH₃COOH + Na₂CO₃ → 2CH₃COONa + CO₂ + H₂O
Esterification with an alcohol requires a strong acid catalyst such as concentrated sulfuric acid and heating under reflux. The reaction is reversible and slow, with the equilibrium favouring the ester only when water is removed or an excess of one reagent is used.
与醇的酯化反应需要浓硫酸等强酸催化并加热回流。该反应可逆且较慢，只有除去水或使用过量的一种试剂时，平衡才有利于酯的生成。
Reduction with lithium tetrahydridoaluminate(III), LiAlH₄, in dry ether converts the carboxyl group into a primary alcohol; the hydride reagent is a powerful nucleophile that attacks the carbonyl carbon twice. Sodium tetrahydridoborate, NaBH₄, is not strong enough to reduce a carboxylic acid.
在干燥乙醚中用四氢合铝酸锂 LiAlH₄ 还原，可把羧基转变为伯醇；该氢化物试剂是强亲核体，会两次进攻羰基碳。四氢合硼酸钠 NaBH₄ 的还原能力不足以还原羧酸。
Reaction with phosphorus(V) chloride, PCl₅, or thionyl chloride, SOCl₂, replaces the -OH with -Cl to give an acyl chloride, and the reaction with PCl₅ is detected by misty fumes of hydrogen chloride. Decarboxylation of a sodium carboxylate with soda lime yields an alkane.
与五氯化磷 PCl₅ 或亚硫酰氯 SOCl₂ 反应时，-OH 被 -Cl 取代生成酰氯；与 PCl₅ 的反应可通过氯化氢的白雾来检测。羧酸钠与碱石灰共热脱羧可得到烷烃。
6. Acyl Chlorides | 酰氯
Acyl chlorides such as ethanoyl chloride CH₃COCl are made from the parent acid using PCl₅, PCl₃ or SOCl₂, the last being preferred because the by-products sulfur dioxide and hydrogen chloride are gases that escape, leaving a pure product.
乙酰氯 CH₃COCl 等酰氯由母体酸与 PCl₅、PCl₃ 或 SOCl₂ 反应制得，其中 SOCl₂ 更受青睐，因为副产物二氧化硫和氯化氢都是气体，逸出后留下纯净产物。
They are extremely reactive towards water, alcohols and ammonia because chloride is an excellent leaving group and the carbonyl carbon is strongly electrophilic. Hydrolysis is vigorous and produces the carboxylic acid plus hydrogen chloride mist.
酰氯对水、醇和氨极其活泼，因为氯离子是很好的离去基团，且羰基碳是强亲电中心。水解反应剧烈，生成羧酸并放出氯化氢白雾。
CH₃COCl + H₂O → CH₃COOH + HCl
With alcohols, acyl chlorides give esters rapidly at room temperature without a catalyst; with ammonia they give primary amides; with primary amines they give N-substituted amides. The hydrogen chloride produced is usually absorbed by a base or removed by a drying tube.
与醇反应时，酰氯在室温下无需催化剂即可快速生成酯；与氨反应生成伯酰胺；与伯胺反应生成 N-取代酰胺。生成的氯化氢通常用碱吸收或用干燥管排除。
Because of their speed and completeness, acyl chlorides are widely used in the laboratory to convert alcohols to esters and to prepare amides that would otherwise be difficult to make. They must be stored dry and handled in a fume cupboard.
由于反应快速且完全，酰氯在实验室中广泛用于把醇转化为酯，以及制备难以用其他方法得到的酰胺。它们必须干燥保存，并在通风橱中操作。
7. Acid Anhydrides | 酸酐
Acid anhydrides are formed by removing a molecule of water from two carboxylic acid molecules; ethanoic anhydride, (CH₃CO)₂O, is the most common example. Industrially they are prepared from acyl chlorides and carboxylate salts.
酸酐可由两分子羧酸脱去一分子水形成；乙酸酐 (CH₃CO)₂O 是最常见的例子。工业上则由酰氯与羧酸盐反应制备。
2CH₃COOH ⇌ (CH₃CO)₂O + H₂O
Their reactions mirror those of acyl chlorides but proceed more slowly and less violently, and the leaving group is a carboxylate ion rather than chloride. This makes them safer and more controllable for large-scale work.
它们的反应与酰氯相似，但更慢、更温和，离去基团是羧酸根离子而不是氯离子。这使它们在大规模操作中更安全、更易控制。
With water they hydrolyse back to the acid; with alcohols they give esters and the parent acid; with ammonia and amines they give amides. Esterification of an alcohol with an anhydride needs no strong acid catalyst.
与水反应水解回酸；与醇反应生成酯和母体酸；与氨和胺反应生成酰胺。醇与酸酐的酯化不需要强酸催化。
The classic examination example is the synthesis of aspirin, where salicylic acid (2-hydroxybenzoic acid) is treated with ethanoic anhydride to form 2-ethanoyloxybenzoic acid. The reaction belongs to the ester-forming family, even though it is a phenol that is being acylated.
最经典的考题例子是阿司匹林的合成：水杨酸（2-羟基苯甲酸）与乙酸酐反应生成 2-乙酰氧基苯甲酸。尽管被酰化的是酚羟基，该反应仍属于成酯反应一类。
8. Esters: Formation and Hydrolysis | 酯：生成与水解
Esters have the general formula RCOOR', with the alkyl part named first and the acyl part second: CH₃COOC₂H₅ is ethyl ethanoate. They are volatile, neutral, often sweet-smelling liquids that are responsible for the aromas of many fruits and flowers.
酯的通式为 RCOOR'，命名时先写烷基、后写酰基：CH₃COOC₂H₅ 为乙酸乙酯。它们是易挥发、中性的液体，常有怡人香味，是许多水果和花朵香气的来源。
Esterification with an alcohol and a concentrated sulfuric acid catalyst is reversible; the reaction is slow at room temperature and is normally carried out under reflux for some time. The reaction can be followed by the disappearance of the acid or by the ester smell.
与醇在浓硫酸催化下的酯化反应是可逆的；室温下反应缓慢，通常需回流一段时间。可通过酸的消耗或酯的气味来跟踪反应进程。
Acid hydrolysis of an ester is the reverse of esterification and reaches an equilibrium, so it is not suitable for complete conversion. Alkaline hydrolysis, by contrast, is irreversible because the carboxylate salt is formed, and this is the reaction used in saponification of fats and oils to make soap.
酯的酸性水解是酯化的逆反应，会达到平衡，因此不适合完全转化。碱性水解则不可逆，因为生成了羧酸盐，这正是油脂皂化制皂所用的反应。
CH₃COOC₂H₅ + NaOH → CH₃COONa + C₂H₅OH
Esters are also important industrially as solvents, plasticisers and flavourings, and the transesterification of vegetable oils with methanol produces biodiesel plus glycerol. Fats and oils are triesters of glycerol with long-chain carboxylic acids.
酯在工业上还用做溶剂、增塑剂和香料；植物油与甲醇的酯交换反应生成生物柴油和甘油。脂肪和油脂就是甘油与长链羧酸形成的三酯。
9. Amides | 酰胺
Amides contain the group -CONH₂ and are best prepared by reacting an acyl chloride or an anhydride with ammonia or an amine. The carbonyl carbon is attacked by the lone pair on nitrogen, expelling chloride and losing a proton to give the amide.
酰胺含有 -CONH₂ 基团，最好由酰氯或酸酐与氨或胺反应制得。氮原子上的孤对电子进攻羰基碳，排出氯离子并失去一个质子，从而生成酰胺。
Amides have high boiling points because the -NH₂ group forms strong hydrogen bonds, and they are usually solids unless the hydrocarbon chain is long. Unlike amines, they are essentially neutral because the nitrogen lone pair is delocalised into the carbonyl group.
酰胺沸点很高，因为 -NH₂ 基团形成强氢键；除烃链较长者外，它们通常是固体。与胺不同，酰胺基本呈中性，因为氮的孤对电子离域进入羰基。
Heating a primary amide with phosphorus(V) oxide, P₄O₁₀, dehydrates it to a nitrile; hydrolysis with dilute acid or alkali regenerates the carboxylic acid. These two reactions link the nitrile, amide and acid routes in synthesis questions.
伯酰胺与五氧化二磷 P₄O₁₀ 共热脱水生成腈；用稀酸或稀碱水解则重新得到羧酸。这两个反应在合成题中把腈、酰胺和酸的路线串联起来。
In biology, amide linkages are peptide bonds joining amino acids into proteins. Industrially, the same linkage appears in nylon-6,6, a polyamide formed from a dicarboxylic acid and a diamine, and in Kevlar-type aramids.
在生物体内，酰胺键就是连接氨基酸形成蛋白质的肽键。在工业上，同样的键出现在尼龙-6,6（由二元酸与二元胺形成的聚酰胺）以及凯夫拉类芳纶中。
10. Relative Reactivity and Mechanism | 相对活性与机理
The derivatives can be ranked in order of reactivity towards nucleophiles: acyl chloride > acid anhydride > ester > amide. All of them react by the same nucleophilic addition-elimination mechanism through a tetrahedral intermediate.
各衍生物对亲核试剂的活性顺序为：酰氯 > 酸酐 > 酯 > 酰胺。它们都通过同一亲核加成-消除机理反应，经由四面体中间体。
The rate depends on two factors: how electrophilic the carbonyl carbon is, and how good the leaving group is. Chloride is a weak base and an excellent leaving group, so acyl chlorides react fastest; the -NH₂ group in an amide is a strong base and a poor leaving group, and its lone pair also donates into the carbonyl, reducing the positive charge on carbon.
反应速率取决于两个因素：羰基碳的亲电性，以及离去基团的好坏。氯离子是弱碱、优良离去基团，因此酰氯反应最快；酰胺中的 -NH₂ 是强碱、差离去基团，且其孤对电子向羰基供电，降低了碳上的正电荷。
| Derivative 衍生物 | Leaving group 离去基团 | Reactivity 活性 |
| RCOCl 酰氯 | Cl⁻ | Highest 最高 |
| (RCO)₂O 酸酐 | RCOO⁻ | High 高 |
| RCOOR' 酯 | R'O⁻ | Moderate 中等 |
| RCONH₂ 酰胺 | NH₂⁻ | Lowest 最低 |
This ranking explains why a derivative can be converted into a less reactive one, but not easily the reverse: an acyl chloride can make an ester or an amide on demand, whereas an amide cannot be turned into an acyl chloride by a simple substitution.
这一顺序解释了为什么衍生物可以转化为活性更低的一种，而反向转化不易进行：酰氯可按需制得酯或酰胺，而酰胺不能通过简单的取代反应变成酰氯。
11. Identification Tests and Analysis | 鉴别与检测
The standard wet test for a carboxylic acid is addition of sodium carbonate solution, which produces steady effervescence of carbon dioxide; passing the gas through limewater turns it milky. Phenols give no reaction, which allows the two to be distinguished.
羧酸的标准湿法检验是加入碳酸钠溶液，产生持续的二氧化碳气泡；把气体通入石灰水会变浑浊。酚不反应，由此可将二者区分。
Esters are identified by their characteristic sweet smell and by hydrolysis, which gives an alcohol and a carboxylic acid or its salt. Tollens' reagent distinguishes an aldehyde from a carboxylic acid, since an acid gives no silver mirror.
酯可通过其特征性香味以及水解生成醇与羧酸（或羧酸盐）来识别。托伦试剂可区分醛与羧酸，因为羧酸不生成银镜。
In instrumental analysis, the infrared spectrum of a carboxylic acid shows a very broad O-H absorption between about 2500 and 3300 cm⁻¹ and a strong C=O stretch near 1700 cm⁻¹. An ester shows the C=O stretch at a slightly higher wavenumber but has no broad O-H band.
在仪器分析中，羧酸的红外光谱在约 2500–3300 cm⁻¹ 处有很宽的 O-H 吸收峰，并在约 1700 cm⁻¹ 处有强 C=O 伸缩振动。酯的 C=O 伸缩振动波数略高，但没有宽的 O-H 吸收带。
Quantitative determination of a carboxylic acid is carried out by titration against standard sodium hydroxide using phenolphthalein, and the result can be used with the mass of sample to deduce the relative molecular mass of the acid.
羧酸的定量测定用标准氢氧化钠溶液滴定、以酚酞为指示剂，所得结果结合样品质量可推算该酸的相对分子质量。
