📚 Carboxylic Acids and Their Derivatives | 羧酸及其衍生物
Carboxylic acids are organic compounds containing the carboxyl functional group, –COOH. This group combines a carbonyl (C=O) and a hydroxyl (O–H) group, giving the compounds a distinctive range of chemical and physical properties. Carboxylic acids can be transformed into several reactive derivatives – esters, acyl chlorides, acid anhydrides and amides – each with its own characteristic reactions. This article covers their nomenclature, properties, preparation, and key reactions, in line with the AQA A-Level Chemistry specification.
羧酸是含有羧基（–COOH）的有机化合物。羧基由羰基（C=O）与羟基（O–H）组合而成，使这类化合物具有独特的物理和化学性质。羧酸可以转化为多种活泼的衍生物——酯、酰氯、酸酐和酰胺——每种衍生物都有其独特的反应。本文依据 AQA A-Level 化学考纲，系统介绍这些化合物的命名、性质、制备及关键反应。
1. Structure and Importance | 结构与重要性
In a carboxylic acid, the carboxyl carbon is sp² hybridised and is bonded to an –OH group and a C=O group. The electron-withdrawing oxygen atoms make the carboxyl group strongly polar, which explains the acidity and the ability to form hydrogen bonds.
在羧酸中，羧基碳为 sp² 杂化，与 –OH 和 C=O 基团相连。氧原子的吸电子效应使羧基极性很强，这解释了羧酸的酸性以及形成氢键的能力。
The simplest member is methanoic acid (HCOOH), followed by ethanoic acid (CH₃COOH; vinegar). Carboxylic acids are ubiquitous in nature – many fatty acids, amino acids, and metabolic intermediates contain the –COOH group.
最简单的羧酸是甲酸（HCOOH），其次是乙酸（CH₃COOH，即食醋的主要成分）。羧酸在自然界中普遍存在——许多脂肪酸、氨基酸及代谢中间体都含有 –COOH 基团。
Derivatives of carboxylic acids are formed by replacing the –OH group with another atom or group. The acyl group (RCO–) is retained in all derivatives, and therefore they all undergo nucleophilic acyl substitution reactions.
羧酸衍生物是通过将 –OH 替换为其他原子或基团而形成的。所有衍生物都保留酰基（RCO–），因此它们都能发生亲核酰基取代反应。
2. Nomenclature | 命名
In IUPAC nomenclature, the longest continuous chain containing the –COOH group is the parent chain. The final –e of the alkane name is replaced with –oic acid. For example, CH₃CH₂CH₂COOH is butanoic acid.
在 IUPAC 命名法中，包含 –COOH 的最长碳链作为母链，将烷烃名称末尾的 “e” 改为 “oic acid”。例如，CH₃CH₂CH₂COOH 称为丁酸（butanoic acid）。
For derivatives, the suffix changes according to the functional group. Esters use the alkyl group from the alcohol followed by –oate; acyl chlorides end in –oyl chloride; amides end in –amide. The table below summarises common examples.
对于衍生物，后缀根据官能团而改变。酯以醇部分的烷基开头，后接 “oate”；酰氯以 “oyl chloride” 结尾；酰胺以 “amide” 结尾。下表总结了常见例子。
| Compound / 化合物 | Name / 名称 |
|---|---|
| CH₃COOCH₂CH₃ | Ethyl ethanoate / 乙酸乙酯 |
| CH₃COCl | Ethanoyl chloride / 乙酰氯 |
| CH₃CONH₂ | Ethanamide / 乙酰胺 |
| (CH₃CO)₂O | Ethanoic anhydride / 乙酸酐 |
3. Physical Properties | 物理性质
Carboxylic acids have significantly higher boiling points than similar alkanes or alcohols because two carboxylic acid molecules form a hydrogen-bonded dimer. For example, ethanoic acid boils at 118 °C, while ethanol (similar formula) boils at 78 °C.
羧酸的沸点比相应烷烃或醇高得多，因为两个羧酸分子可通过氢键形成二聚体。例如，乙酸的沸点为 118 °C，而分子式相近的乙醇沸点为 78 °C。
Lower carboxylic acids (up to C₄) are soluble in water because they can form hydrogen bonds with water molecules. Solubility decreases as the alkyl chain lengthens.
低分子羧酸（C₄以下）可溶于水，因为能与水分子形成氢键。随着碳链增长，溶解度下降。
Esters, on the other hand, have pleasant, sweet smells and are volatile. They are insoluble in water. Acyl chlorides are colourless, pungent liquids that fume in moist air due to acidic gas release.
相反，酯类具有怡人甜味且易挥发，不溶于水。酰氯是无色、有刺激性气味的液体，在潮湿空气中产生烟雾（因释放酸性气体）。
4. Acidity of Carboxylic Acids | 羧酸的酸性
Carboxylic acids are weak acids. In water, they dissociate partially to form a carboxylate anion and an oxonium ion (H₃O⁺).
羧酸是弱酸。在水中部分电离，生成羧酸根阴离子和质子化的水离子（H₃O⁺）。
CH₃COOH + H₂O ⇌ CH₃COO⁻ + H₃O⁺
The acidity is due to the resonance stabilisation of the carboxylate ion, in which the negative charge is delocalised over two oxygen atoms. This makes the conjugate base more stable than that of an alcohol.
酸性来源于羧酸根离子的共振稳定性——负电荷离域在两个氧原子上，因此其共轭碱比醇相应的共轭碱更稳定。
Electron-withdrawing substituents such as Cl or NO₂ on the carbon adjacent to the –COOH group increase acidity by stabilising the negative charge further. Hence, chloroethanoic acid (pKa 2.87) is stronger than ethanoic acid (pKa 4.76).
若在 –COOH 的邻位碳上引入吸电子取代基如 Cl 或 NO₂，可进一步稳定负电荷，从而增大酸性。因此氯乙酸（pKa 2.87）强于乙酸（pKa 4.76）。
Carboxylic acids react with bases, carbonates, and reactive metals to form salts. For example, with sodium hydrogen carbonate they produce carbon dioxide, which is used as a simple test.
羧酸能与碱、碳酸盐和活泼金属反应生成盐。例如与碳酸氢钠反应产生二氧化碳，这可用作简单鉴别。
5. Preparation of Carboxylic Acids | 羧酸的制备
Three common laboratory routes to carboxylic acids are:
实验室制备羧酸的三种常见途径为：
- Oxidation of primary alcohols and aldehydes using acidified potassium dichromate(VI) or potassium manganate(VII). The aldehyde cannot be distilled off, so full oxidation occurs.
- 用酸化重铬酸钾(VI)或高锰酸钾氧化伯醇或醛。由于醛不能被蒸馏分离，氧化得以完全进行。
- Hydrolysis of nitriles: reflux with a dilute acid or alkali.
- 腈的水解：与稀酸或稀碱加热回流。
- Hydrolysis of esters or acyl chlorides with water or aqueous hydroxide.
- 酯或酰氯的水解：与水或氢氧化物水溶液反应。
The oxidation equation for ethanol to ethanoic acid is:
乙醇氧化为乙酸的方程式为：
CH₃CH₂OH + 2[O] → CH₃COOH + H₂O
6. Esterification with Alcohols | 酯化反应
Carboxylic acids react with alcohols in the presence of a strong acid catalyst (usually concentrated H₂SO₄) to form esters and water. This is an equilibrium reaction, so the yield is improved by using an excess of alcohol or by removing water.
羧酸在强酸催化剂（通常为浓 H₂SO₄）存在下与醇反应生成酯和水。该反应是可逆的，可通过使用过量醇或移除水来提高产率。
CH₃COOH + CH₃CH₂OH ⇌ CH₃COOCH₂CH₃ + H₂O
The mechanism is a nucleophilic addition-elimination pathway. The alcohol’s oxygen attacks the carbonyl carbon, followed by elimination of water. The –OH group of the acid is replaced by the –OR group from the alcohol.
反应机理为亲核加成–消除历程。醇中的氧进攻羰基碳，然后消除一分子水。酸中的 –OH 被醇的 –OR 基团取代。
The reverse reaction is ester hydrolysis. In acid conditions, this is an equilibrium; in alkaline conditions, the carboxylate salt is formed irreversibly (saponification).
其逆反应是酯的水解。酸性条件下达到平衡；碱性条件下不可逆地生成羧酸盐（皂化反应）。
7. Acyl Chlorides and Acid Anhydrides | 酰氯与酸酐
Acyl chlorides (RCOCl) and acid anhydrides (RCO−O−COR) are much more reactive than carboxylic acids. The C=O carbon is more electrophilic because of the highly electronegative Cl or the acyloxy group, which are good leaving groups.
酰氯（RCOCl）和酸酐（RCO−O−COR）比羧酸活泼得多。由于氯原子或酰氧基吸电子能力强，且为好的离去基团，它们的羰基碳亲电性更强。
Both react readily with water, alcohols and ammonia/amines to form carboxylic acids, esters and amides respectively. For example:
它们都能与水、醇和氨/胺迅速反应，分别生成羧酸、酯和酰胺。例如：
CH₃COCl + H₂O → CH₃COOH + HCl
CH₃COCl + CH₃CH₂OH → CH₃COOCH₂CH₃ + HCl
CH₃COCl + NH₃ → CH₃CONH₂ + HCl
The reaction with ammonia proceeds via a nucleophilic addition-elimination mechanism. Acyl chlorides fume in air because they react with moisture, releasing HCl. Acid anhydrides are slightly less reactive but are cheaper and release a less harmful by-product (a carboxylic acid instead of HCl).
与氨的反应按亲核加成–消除机理进行。酰氯在空气中发烟，因为与水汽反应放出 HCl。酸酐活性略低，但更便宜且副产物（羧酸）危害较小。
8. Esters: Properties and Hydrolysis | 酯的性质与水解
Esters are neutral, volatile liquids with distinctive fragrances. They are used in flavourings, perfumes and as solvents. In nature, many fats and oils are esters of glycerol (triesters).
酯是中性、易挥发液体，具有特征香气，广泛用于香料、食品添加剂和溶剂。天然脂肪和油脂是甘油的三酯。
Esters can be hydrolysed in two ways:
酯可通过两种方式水解：
- Acid hydrolysis: reflux with a dilute acid such as H₂SO₄. This is the reverse of esterification, giving the carboxylic acid and alcohol as an equilibrium mixture.
- 酸性水解：与稀硫酸等稀酸回流。这是酯化的逆反应，得到羧酸和醇的平衡混合物。
- Alkaline hydrolysis: reflux with aqueous NaOH or KOH. This is irreversible and produces a carboxylate salt and an alcohol. This is used in the manufacture of soap (saponification of fats).
- 碱性水解：与 NaOH 或 KOH 水溶液回流。反应不可逆，生成羧酸盐和醇。工业上用于制皂（脂肪皂化）。
Because the alkaline hydrolysis is irreversible, it is often used to confirm the presence of an ester group: the salt is converted back to the carboxylic acid by adding strong acid.
由于碱性水解不可逆，常用于确认酯基的存在：先制得羧酸盐，再用强酸酸化为羧酸。
9. Amides | 酰胺
Amides (RCONH₂ or RCONHR′) are derived from carboxylic acids and ammonia or amines. Primary amides are solids (except methanamide) and have high melting points due to extensive hydrogen bonding.
酰胺（RCONH₂ 或 RCONHR′）由羧酸与氨或胺反应制得。伯酰胺（除甲酰胺外）为固体，因广泛氢键而具有较高熔点。
A convenient laboratory preparation uses acyl chlorides or acid anhydrides with ammonia or amines:
实验室常用酰氯或酸酐与氨或胺反应制备酰胺：
CH₃COCl + 2NH₃ → CH₃CONH₂ + NH₄Cl
The 2:1 stoichiometry ensures the acid HCl is neutralised. Amides are neutral compounds. They can be hydrolysed by boiling with strong acid or base to produce a carboxylic acid (or carboxylate salt) and ammonium/amine salt.
反应计量比为2:1，以便中和产生的 HCl。酰胺呈中性，可通过与强酸或强碱煮沸水解，得到羧酸（或羧酸盐）和铵盐/胺盐。
10. Interconversion and Reactivity | 相互转化与反应活性
The reactivity of carboxylic acid derivatives towards nucleophilic substitution varies significantly with the leaving group ability. The order is:
羧酸衍生物发生亲核取代反应的活性随离去基团的能力不同而显著变化，顺序为：
Acyl chloride > Acid anhydride > Carboxylic acid > Ester > Amide
Acyl chlorides react violently with water, while amides require prolonged heating to hydrolyse. This trend reflects the leaving group – Cl⁻, RCOO⁻, –OH, –OR, –NH₂ – and the electronegativity of the atom attached to the carbonyl carbon.
酰氯与水激烈反应，而酰胺则需要长时间加热才能水解。这一趋势反映了离去基团（Cl⁻、RCOO⁻、–OH、–OR、–NH₂）的离去能力以及羰基碳上所连原子的电负性。
All these reactions are nucleophilic addition-elimination processes: a nucleophile attacks the C=O carbon, forms a tetrahedral intermediate, and then eliminates the leaving group to restore the carbonyl. Understanding this common mechanism helps predict products across all derivatives.
所有这些反应均为亲核加成–消除过程：亲核试剂进攻羰基碳，形成四面体中间体，然后消除离去基团，重新形成羰基。掌握这一共同机制有助于预测各类衍生物的反应产物。
In summary, carboxylic acids and their derivatives exhibit a rich and systematic chemistry. From their acidic behaviour to the formation of esters and amides, these compounds underpin many industrial and biological processes. A clear grasp of naming, physical properties, mechanisms and relative reactivity is essential for A-Level success.
总之，羧酸及其衍生物具有丰富而系统的化学性质。从酸性行为到酯和酰胺的形成，这些化合物支撑着许多工业与生物过程。清晰掌握命名、物理性质、反应机理和相对活性是 A-Level 考试成功的关键。
Published by TutorHao | Chemistry Revision Series | aleveler.com
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