📚 A-Level WJEC Chemistry: Carboxylic Acids Exam Essentials | A-Level WJEC 化学：羧酸 考点精讲
Carboxylic acids are a fundamental homologous series in organic chemistry, featuring the carboxyl functional group –COOH. Their reactivity as weak acids, ability to form esters and derivatives, and their physical properties dominated by hydrogen bonding make them a core topic for WJEC A-Level Chemistry. This revision guide distils the essential knowledge, from nomenclature and acidity to key reactions and preparation methods, equipping you for both AS and A2 examination success.
羧酸是有机化学中一个基本的同系物，其特征官能团是羧基 –COOH。作为弱酸，它们能发生酯化反应并形成多种衍生物，其物理性质受氢键主导，这些内容都是 WJEC A-Level 化学的核心考点。这份考点精讲提炼了从命名、酸性到关键反应与制备方法的必要知识，帮助你在 AS 和 A2 考试中稳操胜券。
1. Structure and Nomenclature | 结构与命名
Carboxylic acids contain the carboxyl group, –COOH, which consists of a carbonyl (C=O) and a hydroxyl (–) attached to the same carbon. The general formula for aliphatic monocarboxylic acids is CnH2n+1COOH. In systematic IUPAC naming, the suffix '-oic acid' is used, and the carboxyl carbon is always designated as carbon 1. Examples include methanoic acid (HCOOH), ethanoic acid (CH3COOH) and propanoic acid (CH3CH2COOH). When substituents are present, the parent chain is numbered starting from the –COOH carbon, e.g. 2-methylpropanoic acid.
羧酸含有羧基 –COOH，它由连接在同一个碳原子上的羰基 (C=O) 和羟基 (–OH) 组成。脂肪族一元羧酸的通式为 CnH2n+1COOH。在 IUPAC 系统命名中，使用后缀“-酸”，并且羧基碳的编号总是定为 1 位。例如，甲酸 (HCOOH)、乙酸 (CH3COOH) 和丙酸 (CH3CH2COOH)。当存在取代基时，主链从 –COOH 的碳开始编号，如 2-甲基丙酸。
2. Physical Properties: Boiling Points and Solubility | 物理性质：沸点与溶解度
Carboxylic acids exhibit significantly higher boiling points than alcohols of comparable relative molecular mass. This is because they form strong intermolecular hydrogen bonds and exist as stable dimers in the liquid and vapour phase, effectively doubling their molecular mass. For instance, ethanoic acid (b.p. 118 °C) boils much higher than propan-1-ol (b.p. 97 °C). Regarding solubility, the first four straight-chain members (methanoic to butanoic acid) are completely miscible with water. The highly polar –COOH group can engage in hydrogen bonding with water molecules. As the hydrophobic alkyl chain lengthens, solubility decreases sharply.
羧酸的沸点显著高于相对分子质量相近的醇。这是因为它们能形成强烈的分子间氢键，并且在液态和气相中以稳定的二聚体形式存在，这实际上使分子量翻倍。例如，乙酸（沸点 118 °C）的沸点远高于 1-丙醇（沸点 97 °C）。就溶解度而言，前四个直链成员（甲酸到丁酸）能与水完全互溶。强极性的 –COOH 基团可以与水分子形成氢键。随着疏水烷基链的增长，溶解度急剧下降。
3. Acidity of Carboxylic Acids | 羧酸的酸性
Carboxylic acids are weak acids, partially dissociating in water to form carboxylate ions and H3O+. The acid dissociation constant Ka is typically in the range of 10-4 to 10-5 mol dm-3, corresponding to pKa values around 4–5. The acidity arises from the stability of the carboxylate ion, RCOO-, where the negative charge is delocalised over the two oxygen atoms through resonance. This delocalisation makes the conjugate base far more stable than that of an alcohol (alkoxide ion), explaining why carboxylic acids are much stronger acids than alcohols.
羧酸是弱酸，在水中部分电离，生成羧酸根离子和 H3O+。酸解离常数 Ka 通常在 10-4 到 10-5 mol dm-3 范围内，相应的 pKa 约为 4–5。酸性源于羧酸根离子 RCOO- 的稳定性，其负电荷通过共振离域到两个氧原子上。这种离域使得共轭碱远比醇的共轭碱（烷氧负离子）稳定，从而解释了为什么羧酸的酸性远强于醇。
4. Inductive Effects on Acid Strength | 诱导效应对酸性强度的影响
Electron-withdrawing groups attached to the carbon chain near the carboxyl group increase acid strength by stabilising the negative charge on the carboxylate ion through a negative inductive effect (–I). Chlorine atoms are a prime example: chloroethanoic acid (pKa 2.86) is stronger than ethanoic acid (pKa 4.76), and trichloroethanoic acid is even stronger. Conversely, electron-donating alkyl groups (+I effect) slightly decrease acidity, which is why methanoic acid is a stronger acid than ethanoic acid.
连接在靠近羧基的碳链上的吸电子基团可通过负诱导效应 (–I) 稳定羧酸根离子上的负电荷，从而增强酸性。氯原子是一个典型例子：氯乙酸 (pKa 2.86) 的酸性强于乙酸 (pKa 4.76)，而三氯乙酸的酸性更强。相反，给电子烷基 (+I 效应) 会略微减弱酸性，这就是甲酸比乙酸更强的缘故。
5. Reactions with Metals | 与金属的反应
Carboxylic acids react with reactive metals such as magnesium, zinc and iron, producing a salt (metal carboxylate) and hydrogen gas. The reaction is a typical acid–metal redox process, albeit slower than with strong mineral acids due to the weak acidity. For example, ethanoic acid reacts with magnesium to form magnesium ethanoate and hydrogen: 2CH3COOH + Mg → (CH3COO)2Mg + H2. The ionic equation reflects the role of H+: 2CH3COOH + Mg → 2CH3COO- + Mg2+ + H2. Effervescence of hydrogen gas is observable.
羧酸能与活泼金属（如镁、锌和铁）反应，生成盐（金属羧酸盐）和氢气。该反应是典型的酸-金属氧化还原过程，但由于酸性较弱，反应速率比强无机酸慢。例如，乙酸与镁反应生成乙酸镁和氢气：2CH3COOH + Mg → (CH3COO)2Mg + H2。离子方程式体现了 H+ 的作用：2CH3COOH + Mg → 2CH3COO- + Mg2+ + H2。可观察到氢气的气泡产生。
6. Reactions with Bases and Carbonates | 与碱和碳酸盐的反应
With bases such as sodium hydroxide, a neutralisation reaction occurs, forming a water-soluble carboxylate salt and water: CH3COOH + NaOH → CH3COONa + H2O. With carbonates or hydrogencarbonates, carboxylic acids liberate carbon dioxide gas. The test using sodium hydrogencarbonate solution is a classic way to distinguish carboxylic acids from phenols and alcohols, which do not react: CH3COOH + NaHCO3 → CH3COONa + H2O + CO2. The brisk effervescence of CO2, which turns limewater milky, confirms the presence of a carboxyl group.
与氢氧化钠等碱发生中和反应，生成水溶性羧酸盐和水：CH3COOH + NaOH → CH3COONa + H2O。与碳酸盐或碳酸氢盐反应时，羧酸会释放出二氧化碳气体。使用碳酸氢钠溶液的测试是区分羧酸与酚、醇的经典方法，因为后两者不发生反应：CH3COOH + NaHCO3 → CH3COONa + H2O + CO2。二氧化碳迅速冒出气泡，并能将石灰水变浑浊，从而确认羧基的存在。
7. Esterification | 酯化反应
Carboxylic acids react with alcohols in the presence of a concentrated sulfuric acid catalyst and heat to form esters. This is a reversible condensation reaction, where water is eliminated. The ester functional group –COO– links the acid and alcohol portions. For instance, ethanoic acid and ethanol yield ethyl ethanoate: CH3COOH + C2H5OH ⇌ CH3COOC2H5 + H2O. Concentrated H2SO4 acts as both a catalyst and a dehydrating agent, shifting equilibrium to the right. Naming esters follows the pattern: alkyl alkanoate.
羧酸在浓硫酸催化剂存在并加热的条件下与醇反应生成酯。这是一个可逆的缩合反应，同时脱去水。酯基 –COO– 连接酸和醇的部分。例如，乙酸和乙醇反应生成乙酸乙酯：CH3COOH + C2H5OH ⇌ CH3COOC2H5 + H2O。浓硫酸既作催化剂又作脱水剂，使平衡右移。酯的命名遵循“某酸某酯”的模式。
8. Hydrolysis of Esters and Derivatives | 酯的水解与衍生物的形成
Esters can be hydrolysed back to the parent carboxylic acid and alcohol. Acid-catalysed hydrolysis uses dilute HCl or H2SO4 and is the reverse of esterification. Base-catalysed hydrolysis (using NaOH) is often preferred because it goes to completion – the carboxylate salt is formed, and acidification afterwards releases the free acid. This is used in the preparation of soaps (saponification). Carboxylic acids can also be converted to acyl chlorides (using PCl5 or SOCl2) and acid anhydrides (by dehydration), which are key reactive derivatives studied at A2.
酯可以水解回到原来的羧酸和醇。酸催化水解使用稀盐酸或稀硫酸，本质是酯化反应的逆反应。碱催化水解（用 NaOH）往往更常用，因为它能进行到底——生成羧酸盐，之后酸化即得游离羧酸。这一方法用于制备肥皂（皂化反应）。羧酸还可转化为酰氯（用 PCl5 或 SOCl2）和酸酐（通过脱水），这些是 A2 阶段学习的重要活泼衍生物。
9. Reduction of Carboxylic Acids | 羧酸的还原
Carboxylic acids are reduced to primary alcohols by the powerful reducing agent lithium tetrahydridoaluminate (lithium aluminium hydride, LiAlH4) in dry ether. This reaction requires anhydrous conditions and is an important synthetic route because direct reduction of acids is not possible with milder agents like NaBH4. The general equation is: RCOOH + 4[H] → RCH2OH + H2O. For example, propanoic acid is reduced to propan-1-ol.
羧酸可被强还原剂四氢铝锂（氢化铝锂，LiAlH4）在干醚中还原为伯醇。该反应需要无水条件，是一条重要的合成路线，因为使用 NaBH4 等温和试剂无法直接还原羧酸。反应通式为：RCOOH + 4[H] → RCH2OH + H2O。例如，丙酸被还原为 1-丙醇。
10. Preparation of Carboxylic Acids | 羧酸的制备方法
WJEC examinations frequently test the synthesis of carboxylic acids. The main laboratory methods include: (a) Full oxidation of primary alcohols using acidified potassium dichromate(VI) under reflux; (b) Oxidation of aldehydes; (c) Hydrolysis of nitriles (R–C≡N) with dilute acid or alkali, which yields a carboxylic acid with one extra carbon atom; (d) Hydrolysis of esters, as discussed. The oxidation of a primary alcohol proceeds via the aldehyde, so the reaction mixture must be heated strongly under reflux with excess oxidant to ensure complete conversion to the acid.
WJEC 考试经常考查羧酸的合成。主要的实验室制备方法包括：(a) 在回流下用酸化重铬酸钾(VI) 将伯醇充分氧化；(b) 氧化醛；(c) 腈 (R–C≡N) 在稀酸或稀碱中水解，得到多一个碳原子的羧酸；(d) 如上所述的水解酯。伯醇的氧化会经过醛的阶段，因此反应混合物必须与过量氧化剂在强回流下加热，以确保完全转化为酸。
11. Testing for the Carboxyl Group | 羧基的检验
To confirm the presence of a –COOH group in an unknown organic compound, the sodium hydrogencarbonate test is the most definitive. Adding solid NaHCO3 or its saturated solution to the sample produces vigorous effervescence of CO2. Phenols and alcohols do not react. A positive result is also indicated by the formation of a steamy acidic mist with phosphorus pentachloride (PCl5), though this test is not unique to acids. Carboxylic acids turn blue litmus red and have characteristic sharp, often vinegary odours for the lower members.
要确认未知有机物中是否存在 –COOH 基团，碳酸氢钠测试最为确证。向样品中加入固体 NaHCO3 或其饱和溶液会产生剧烈的 CO2 气泡。酚和醇不发生反应。使用五氯化磷 (PCl5) 会产生酸雾，这也是一个阳性迹象，但并非羧酸独有。羧酸使蓝色石蕊试纸变红，并且低级成员具有特征性的刺鼻气味，常似醋味。
12. Key Data and Comparison Table | 核心数据与比较表
A concise summary of key constants and comparative properties helps solidify understanding. The table below presents pKa values, boiling points and solubility for selected carboxylic acids alongside typical exam reasoning.
一份简洁的核心常数与性质比较表有助于巩固理解。下表列出了选定羧酸的 pKa 值、沸点和溶解度，并附有典型的考试推理。
| Acid | pKa | Boiling Point (°C) | Solubility in Water |
|---|---|---|---|
| Methanoic acid | 3.75 | 101 | Miscible |
| Ethanoic acid | 4.76 | 118 | Miscible |
| Propanoic acid | 4.87 | 141 | Miscible |
| Butanoic acid | 4.82 | 164 | 6.0 g/100 mL |
| Chloroethanoic acid | 2.86 | 189 | Very soluble |
The trend in pKa clearly illustrates the electron-withdrawing effect of chlorine, while the boiling points reflect dimeric hydrogen bonding. As the chain length grows, the influence of the hydrophobic tail reduces water solubility.
pKa 的变化趋势清晰地展示了氯的吸电子效应，而沸点则反映了二聚体氢键的作用。随着碳链增长，疏水尾部的影响导致水溶性下降。
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