📚 Edexcel A-Level Chemistry Topic 17: Carboxylic Acids, Esters & Acyl Chlorides | 羧酸、酯和酰氯
ters, and acyl chlorides. These functional groups are built around the carbonyl unit (C=O) bonded to an electronegative atom, and they share many patterns of nucleophilic addition– for success in both Paper 2 and Paper 3, as well as for linking to biological molecules and industrial processes.
Edexcel A-Level 化学大纲的第 17 主题将三个密切相关的羰基化合物家族——羧酸、酯和酰氯——有机地整合在一起。这些官能团都以连接电负性原子的羰基（C＝O）为骨架，共享许多亲核加成–消除反应模式。深刻理解其结构、命名、分子间力、酸性和取代反应活性，不仅是应对 Paper 2 和 Paper 3 的关键，也有助于联系生物分子与工业过程。
1. Functional Group Overview | 官能团总览
A carboxylic acid contains the –COOH group, which combines a carbonyl (C=O) and a hydroxyl (–OH) on the same carbon. Esters have the –COOR group, where the –OH of the acid is replaced by an –OR alkoxy group. Acyl chlorides (ROCl) replace the – m, creating the most reactive derivative of the three. All three contain the acyl group (R–C=O) and are susceptible to attack by nucleophiles at the carbonyl carbon.
羧酸含有 –COOH 基团，它由同一碳上的羰基和羟基组成。酯的官能团是 –COOR，即酸中的 –OH 被 –OR 烷氧基取代。酰氯则将 –OH 换为氯原子，成为三者中活性最高的衍生物。它们都含有酰基（R–C＝O），羰基碳容易受到亲核试剂的进攻。
2. Nomenclature of Carboxylic Acids | 羧酸的命名
Carboxylic acids are named by replacing the final 'e' of the parent alkane with '-oic acid'. The carboxyl carbon is always carbon 1, so the position is not numbered. For example, methanoic acid (HCOOH), ethanoic acid (CH₃COOH), propanoic acid (CH₃CH₂COOH), and butanoic acid (CH₃CH₂CH₂COOH). Branched and substituted acids are named by numbering the chain starting from the carboxyl carbon; e.g. 2-methylpropanoic acid. Aromatic acids use benzoic acid (C₆H₅COOH) as the parent.
羧酸的命名是通过将母体烷烃末尾的“e”替换为“-oic acid”。羧基碳总是定为 1 号，故不需标位次。例如：甲酸（HCOOH）、乙酸（CH₃COOH）、丙酸（CH₃CH₂COOH）、丁酸（CH₃CH₂CH₂COOH）。支链和取代羧酸以羧基碳为起点编号，如 2-甲基丙酸。芳香酸以苯甲酸（C₆H₅COOH）为母体。
- HCOOH – methanoic acid / 甲酸
- CH₃COOH – ethanoic acid / 乙酸
- CH₃CH₂COOH – propanoic acid / 丙酸
- C₆H₅COOH – benzoic acid / 苯甲酸
3. Physical Properties: Hydrogen Bonding and Boiling Points | 物理性质：氢键与沸点
Carboxylic acids have significantly higher boiling points than alcohols of comparable molar mass because they form two intermolecular hydrogen bonds per molecule, leading to stable dimers in the liquid and vapour phases. This dimeric structure increases the effective molar mass and requires more energy to separate. As the alkyl chain length increases, London dispersion forces grow, but the acid group always dominates:
羧酸的沸点远高于相近摩尔质量的醇，因为每个分子能形成两个分子间氢键，从而在液相和气相中形成稳定的二聚体。二聚体结构提高了有效摩尔质量，需要更多能量才能分离。尽管随烷基链增长，色散力增大，但羧基始终主导物性：
| Compound | Molar mass (g mol⁻¹) | Boiling point (°C) |
|---|---|---|
| Ethanoic acid | 60 | 118 |
| Propan-1-ol | 60 | 97 |
| Propanoic acid | 74 | 141 |
| Butan-1-ol | 74 | 118 |
Esters cannot form hydrogen bonds between their own molecules, so their boiling points are much lower than the parent carboxylic acids of similar size. Acyl chlorides also lack hydrogen bonding and have boiling points lower still, as the Cl atom is less polarisable than an O–H group would suggest.
酯分子间无法形成氢键，因此沸点远低于同碳数的母体羧酸。酰氯同样缺乏氢键，且因氯原子的极化能力弱于 O–H，其沸点更低。
4. Acidity of Carboxylic Acids | 羧酸的酸性
Carboxylic acids are weak acids, partially dissociating in water to produce the carboxylate anion and H₃O⁺. The pKₐ of ethanoic acid is 4.76, making it a stronger acid than ethanol (pKₐ ~16) and phenol (pKₐ ~10). The acid strength arises from the resonance stabilisation of the carboxylate anion, which delocalises the negative charge over two oxygen atoms:
羧酸是弱酸，在水中部分解离生成羧酸根阴离子和 H₃O⁺。乙酸的 pKₐ 为 4.76，酸性强于乙醇（pKₐ≈16）和苯酚（pKₐ≈10）。酸性源于羧酸根离子的共振稳定化，负电荷离域在两个氧原子上：
RCOOH + H₂O ⇌ RCOO⁻ + H₃O⁺
The carboxylate ion can be represented as a hybrid with two equivalent C–O bonds. Electron-withdrawing substituents (e.g. –Cl, –NO₂) near the carboxyl group increase acidity by stabilising the negative charge further through inductive effects. Conversely, electron-donating alkyl groups slightly reduce acidity.
羧酸根离子可用两个等价的 C–O 键共振式表示。靠近羧基的吸电子取代基（如 –Cl、–NO₂）通过诱导效应进一步稳定负电荷，从而增强酸性；相反，给电子的烷基会略微减弱酸性。
5. Reactions of Carboxylic Acids with Metals, Bases and Carbonates | 羧酸与金属、碱和碳酸盐的反应
As acids, they react with reactive metals (Mg, Zn) to form salts and hydrogen gas; with metal hydroxides to form salts and water; and with carbonates/hydrogencarbonates to form salts, water, and carbon dioxide. The effervescence of CO₂ with NaHCO₃ is a useful test distinguishing carboxylic acids from other acidic phenols, which do not react with HCO₃⁻.
作为酸，它们与活泼金属（镁、锌）反应生成盐和氢气；与金属氢氧化物反应生成盐和水；与碳酸盐/碳酸氢盐反应生成盐、水和二氧化碳。与碳酸氢钠放出 CO₂ 气体的现像是区分羧酸与酚类酸性物质的实用检测，因为酚不与 HCO₃⁻ 反应。
2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂
CH₃COOH + NaOH → CH₃COONa + H₂O
2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂
6. Esterification: Equilibrium and Catalysis | 酯化反应：平衡与催化
Esterification is the reaction of a carboxylic acid with an alcohol in the presence of a concentrated sulfuric acid catalyst, forming an ester and water. The reaction is reversible and reaches equilibrium. Concentrated H₂SO₄ acts as both an acid catalyst (protonating the carbonyl oxygen to enhance electrophilicity) and a dehydrating agent (removing water to shift equilibrium towards the product). The general equation is:
酯化反应是羧酸与醇在浓硫酸催化下生成酯和水的可逆平衡过程。浓 H₂SO₄ 既是酸催化剂（质子化羰基氧以提高亲电性），又是脱水剂（移去水使平衡正向移动）。反应通式为：
RCOOH + R'OH ⇌ RCOOR' + H₂O
For example, ethanoic acid reacts with ethanol to give ethyl ethanoate, a sweet-smelling ester used as a solvent and in flavourings. Heating under reflux with excess alcohol or acid can improve yield, as can the removal of the ester or water by distillation.
例如乙酸与乙醇反应生成具有果香味的乙酸乙酯，常用作溶剂和调味剂。加热回流并使用过量的醇或酸可提高产率，通过蒸馏移走酯或水亦可促进产率。
7. Naming Esters and Their Occurrence | 酯的命名与存在
Esters are named as alkyl alkanoates: the alkyl group from the alcohol appears first, followed by the carboxylate stem derived from the acid with the ending '-oate'. For example, CH₃COOCH₂CH₃ is ethyl ethanoate; HCOOCH₃ is methyl methanoate; CH₃CH₂COOCH₃ is methyl propanoate. Many esters are naturally occurring in fruits and flowers, responsible for pleasant odours and used in perfumes and food flavourings.
酯的命名采取“某酸某酯”的形式：醇部烷基在前，羧酸部分取主链并以“-oate”结尾。例如 CH₃COOCH₂CH₃ 是乙酸乙酯；HCOOCH₃ 是甲酸甲酯；CH₃CH₂COOCH₃ 是丙酸甲酯。许多酯天然存在于水果和花卉中，带来宜人的香味，广泛用于香水和食品调味。
| Ester name | Formula | Typical odour |
|---|---|---|
| Ethyl ethanoate | CH₃COOC₂H₅ | Pear drops |
| Methyl butanoate | C₃H₇COOCH₃ | Apple |
| Ethyl butanoate | C₃H₇COOC₂H₅ | Pineapple |
8. Hydrolysis of Esters: Acidic and Alkaline | 酯的水解：酸性与碱性条件
Esters can be hydrolysed back to the parent acid and alcohol. Acid-catalysed hydrolysis uses dilute HCl or H₂SO₄ and heat under reflux; it is the reverse of esterification and reaches an equilibrium. Alkaline hydrolysis uses aqueous NaOH and is irreversible, forming the carboxylate salt and alcohol. This alkaline process is often called saponification, particularly when used to make soaps from long-chain fatty esters:
酯可水解回到母体酸和醇。酸催化水解使用稀盐酸或稀硫酸，加热回流，是酯化的逆反应，达到平衡。碱性水解使用 NaOH 水溶液，反应不可逆，生成羧酸盐和醇。碱性水解常被称为皂化反应，尤其是以长链脂肪酸酯制取肥皂时：
Acidic: RCOOR' + H₂O ⇌ RCOOH + R'OH
Alkaline: RCOOR' + NaOH → RCOO⁻Na⁺ + R'OH
In examination questions, alkaline hydrolysis is often used to produce the sodium salt of a carboxylic acid; subsequent acidification yields the free acid. The irreversibility is driven by the formation of the resonance-stabilised carboxylate, which removes the acid from the equilibrium.
考试中常会要求利用碱性水解制备羧酸钠盐，再经酸化得到游离酸。反应不可逆是因为生成了共振稳定的羧酸盐，从平衡中移走了酸。
9. Introduction to Acyl Chlorides: Structure and Naming | 酰氯简介：结构和命名
Acyl chlorides contain the –COCl group. They are named by replacing the '-oic acid' suffix of the parent acid with '-oyl chloride'. For example, ethanoic acid becomes ethanoyl chloride (CH₃COCl), propanoic acid becomes propanoyl chloride (CH₃CH₂COCl), and benzoic acid becomes benzoyl chloride (C₆H₅COCl). The carbonyl carbon is sp² hybridised and highly electrophilic due to the electron-withdrawing effects of both oxygen and chlorine.
酰氯含有 –COCl 基团。命名时把母体羧酸的“-oic acid”换成“-oyl chloride”。如乙酸变为乙酰氯（CH₃COCl），丙酸变为丙酰氯（CH₃CH₂COCl），苯甲酸变为苯甲酰氯（C₆H₅COCl）。羰基碳为 sp² 杂化，因氧和氯的吸电子效应而高度缺电子，表现出极强的亲电性。
10. Preparation of Acyl Chlorides | 酰氯的制备
Acyl chlorides are prepared by reacting carboxylic acids with phosphorus pentachloride (PCl₅), phosphorus trichloride (PCl₃), or thionyl chloride (SOCl₂). The latter is often favoured because it produces gaseous by-products (SO₂ and HCl) that escape, driving the reaction to completion and simplifying purification. Example:
酰氯可通过羧酸与五氯化磷、三氯化磷或亚硫酰氯反应制备。亚硫酰氯因副产物为气体（SO₂ 和 HCl）逸出，更容易推动反应进行完全，产物提纯也更简单。例如：
CH₃COOH + SOCl₂ → CH₃COCl + SO₂ + HCl
With PCl₅, the reaction gives acyl chloride, HCl, and POCl₃ (phosphoryl chloride), which must be separated by distillation. Acyl chlorides are moisture-sensitive and must be handled under anhydrous conditions.
若使用 PCl₅，产物为酰氯、HCl 和 POCl₃（磷酰氯），需蒸馏分离。酰氯极易水解，须在无水条件下操作。
11. Reactions of Acyl Chlorides: Addition–Elimination Mechanism | 酰氯的反应：加成–消除机理
Acyl chlorides undergo nucleophilic addition–elimination with water, alcohols, ammonia, and amines. The mechanism proceeds in two steps: the nucleophile attacks the carbonyl carbon (addition) forming a tetrahedral intermediate, then the chloride ion is eliminated, and H⁺ is transferred. These reactions are vigorous at room temperature and produce HCl, so they are often carried out in the absence of moisture.
酰氯与水、醇、氨和胺发生亲核加成–消除反应。机理分两步：亲核试剂进攻羰基碳（加成）形成四面体中间体，随后消除氯离子，并发生质子转移。这些反应在室温下即可剧烈进行并放出 HCl，因此通常须在无水条件下操作。
| Nucleophile | Product | Typical equation |
|---|---|---|
| Water | Carboxylic acid | CH₃COCl + H₂O → CH₃COOH + HCl |
| Alcohol | Ester | CH₃COCl + C₂H₅OH → CH₃COOC₂H₅ + HCl |
| Ammonia (NH₃) | Amide | CH₃COCl + 2NH₃ → CH₃CONH₂ + NH₄Cl |
| Primary amine | N-substituted amide | CH₃COCl + CH₃NH₂ → CH₃CONHCH₃ + HCl |
Note that ammonia and amines require two equivalents: one to serve as the nucleophile and the second to neutralise the HCl formed. The reaction of acyl chlorides with alcohols is a better method for making esters than Fischer esterification because it is irreversible and gives a high yield without heating.
注意氨和胺需用两当量：一当量作亲核试剂，另一当量中和生成的 HCl。酰氯和醇反应是比费歇尔酯化更优的酯合成法，因为它不可逆且无需加热即可获得高产率。
12. Comparing Reactivity: Acyl Chlorides, Esters, and Carboxylic Acids | 反应性比较：酰氯、酯和羧酸
The reactivity order towards nucleophilic addition–elimination is: acyl chlorides > carboxylic acids > esters. This difference stems from the leaving group ability and the electrophilicity of the carbonyl carbon. Chloride is an excellent leaving group, making the tetrahedral intermediate collapse easily. The –OH group in acids becomes a poor leaving group (as –OH⁻ is a strong base), unless protonated. The –OR group in esters is a poorer leaving group still, requiring acid or base catalysis for hydrolysis. Thus, while acyl chlorides react rapidly with nucleophiles even at low temperatures, esters are relatively inert unless heated with catalysts.
亲核加成–消除反应活性顺序为：酰氯 ＞ 羧酸 ＞ 酯。这一差异源于离去基团的优劣以及羰基碳的亲电性。氯离子是极好的离去基团，使四面体中间体容易坍塌。羧酸中的 –OH 是差的离去基团（–OH⁻ 是强碱），除非被质子化。酯中的 –OR 离去性更差，需要酸或碱催化才水解。因此，酰氯即使在低温下也能与亲核试剂迅速反应，而酯相对惰性，除非加热并使用催化剂。
Reactivity: RCOCl > RCOOH > RCOOR'
This trend is also reflected in the difficulty of preparing the derivatives: acyl chlorides are made from acids using reactive chlorinating agents, while esters require equilibration and removal of water.
这一趋势也反映在制备难度上：酰氯需用强氯化试剂从酸制得，而酯则需要平衡操作并除水。
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