📚 Carboxylic Acids | 羧酸考点精讲
Carboxylic acids are a fundamental class of organic compounds characterised by the presence of the carboxyl functional group (-COOH). They appear widely in nature, from the ethanoic acid in vinegar to the long-chain fatty acids in lipids, and play a central role in synthetic chemistry. Mastery of their nomenclature, physical properties, acidity, and key reactions is essential for success in IB and CCEA chemistry examinations. This article provides a comprehensive, bilingual review of the core concepts and frequently examined reactions of carboxylic acids.
羧酸是一类以羧基（-COOH）为特征官能团的基础有机化合物。它们广泛存在于自然界中，从食醋中的乙酸到脂类中的长链脂肪酸，并在合成化学中扮演核心角色。掌握羧酸的命名、物理性质、酸性及其关键反应，对于在 IB 和 CCEA 化学考试中取得好成绩至关重要。本文将以双语形式，系统梳理羧酸的核心概念与常考反应。
1. Structure and Nomenclature | 结构与命名
The carboxyl group consists of a carbonyl (C=O) and a hydroxyl (-OH) attached to the same carbon. In IUPAC nomenclature, the parent chain must include the carboxyl carbon, and the suffix “-oic acid” is used. The carbon of the carboxyl group is always numbered position 1. For example, methanoic acid (HCOOH) is the simplest member, while ethanoic acid (CH₃COOH) has two carbons. When substituents are present, their positions are indicated by numbers, with the carboxyl carbon as C-1, e.g., 2-chloropropanoic acid (CH₃CHClCOOH).
羧基由连接在同一个碳上的羰基（C=O）和羟基（-OH）组成。按照 IUPAC 命名法，主链必须包含羧基碳，并使用后缀“-oic acid”（中文为“酸”）。羧基的碳始终被编号为 1 位。例如，甲酸（HCOOH）是最简单的羧酸，而乙酸（CH₃COOH）含有两个碳。当存在取代基时，其位置用数字标示，以羧基碳为第1 位，如 2-氯丙酸（CH₃CHClCOOH）。
2. Physical Properties | 物理性质
Lower carboxylic acids are polar liquids with sharp, often unpleasant odours. They have significantly higher boiling points than alcohols of comparable molar mass because they can form strong intermolecular hydrogen bonds in a dimeric arrangement, effectively doubling the molecular weight of the vapour-state species. Solubility in water decreases as the length of the non-polar hydrocarbon chain increases; methanoic, ethanoic, and propanoic acids are fully miscible with water, whereas longer-chain acids are practically insoluble.
低级羧酸是具有刺激性气味的极性液体。它们的沸点显著高于相对分子质量相近的醇，因为它们可以通过二聚体形式形成强烈的分子间氢键，使得气相物种的有效分子量加倍。在水中的溶解度随着非极性烃链的增长而降低；甲酸、乙酸和丙酸可与水以任意比例混溶，而长链酸几乎不溶于水。
3. Acidity and pKₐ | 酸性与pKₐ
Carboxylic acids are weak Brønsted–Lowry acids, partially dissociating in water to give carboxylate anions and hydronium ions. The acid dissociation constant, Kₐ, typically lies in the range 10⁻⁴ to 10⁻⁵ mol dm⁻³, giving pKₐ values around 4–5. For instance, ethanoic acid has a pKₐ of 4.76. The strength arises from the resonance stabilisation of the carboxylate ion, where the negative charge is delocalised over two oxygen atoms, making the anion more stable than the undissociated acid.
羧酸是弱 Brønsted–Lowry 酸，在水中部分电离生成羧酸根离子和水合氢离子。酸电离常数 Kₐ 通常在 10⁻⁴ 至 10⁻⁵ mol dm⁻³ 范围内，pKₐ 约在 4–5。例如，乙酸的 pKₐ 为 4.76。其酸性强度源于羧酸根离子的共振稳定化作用：负电荷离域在两个氧原子之间，使得阴离子比未电离的酸更稳定。
4. Effect of Substituents on Acidity | 取代基对酸性的影响
Electron-withdrawing groups (EWGs), especially those at the α-carbon, increase acid strength by stabilising the resulting negative charge through inductive effect. Chloroethanoic acids illustrate this trend: the pKₐ of ethanoic acid is 4.76, while monochloroethanoic acid has 2.86, dichloroethanoic acid 1.29, and trichloroethanoic acid 0.65. The closer and more numerous the electronegative substituents, the stronger the acid. Conversely, electron-donating alkyl groups slightly reduce acidity, making methanoic acid (pKₐ 3.75) marginally stronger than ethanoic acid.
吸电子基团（EWG），特别是位于 α-碳上的，通过诱导效应稳定产生的负电荷，从而增强酸性。氯代乙酸系列清楚显示了这一趋势：乙酸的 pKₐ 为 4.76，而一氯乙酸为 2.86，二氯乙酸为 1.29，三氯乙酸为 0.65。吸电子取代基越靠近羧基、数量越多，酸性越强。相反，给电子烷基会略微降低酸性，因此甲酸（pKₐ 3.75）比乙酸稍强。
5. Preparation Methods | 制备方法
In the laboratory, carboxylic acids are commonly synthesised by the oxidation of primary alcohols or aldehydes using acidified potassium dichromate(VI) under reflux. Another reliable route is the hydrolysis of nitriles, which yields carboxylic acids after heating with dilute HCl or NaOH followed by acidification. Aromatic carboxylic acids, such as benzoic acid, can be obtained by oxidising alkylbenzenes with hot alkaline KMnO₄, where the entire alkyl side chain is converted to a -COOH group regardless of its length.
在实验室中，羧酸通常通过使用酸化重铬酸钾（VI）在回流下氧化伯醇或醛来制备。另一条可靠的途径是腈的水解：将腈与稀盐酸或氢氧化钠溶液加热，随后酸化即可得到羧酸。芳香族羧酸，如苯甲酸，可通过用热的碱性高锰酸钾溶液氧化烷基苯制得，此时无论烷基侧链多长，都会被全部氧化为 -COOH 基团。
6. Reactions as Acids: Salt Formation | 作为酸的反应：成盐
Carboxylic acids react with reactive metals (e.g., Na, Mg) to liberate hydrogen gas and form carboxylate salts. They are neutralised by bases such as NaOH and by carbonates/hydrogencarbonates, producing CO₂ gas that can be used as a diagnostic test for the carboxyl group. With sodium carbonate, effervescence is observed:
2CH₃COOH + Na₂CO₃ → 2CH₃COONa + CO₂ + H₂O
The resulting sodium salts are generally water-soluble and can be converted back to the acid by adding a stronger mineral acid.
羧酸能与活泼金属（如 Na、Mg）反应放出氢气，并生成羧酸盐。它们可被碱（如 NaOH）以及碳酸盐/碳酸氢盐中和，产生 CO₂ 气体，这一现象可用于检验羧基。与碳酸钠反应时，可观察到明显的气泡产生：
2CH₃COOH + Na₂CO₃ → 2CH₃COONa + CO₂ + H₂O
生成的钠盐通常溶于水，加入更强的无机酸可将其转回为羧酸。
7. Esterification | 酯化反应
When a carboxylic acid is heated with an alcohol in the presence of a strong acid catalyst (commonly concentrated H₂SO₄), an equilibrium is established producing an ester and water. This Fischer esterification is a nucleophilic acyl substitution proceeding through a tetrahedral intermediate. The reaction is slow and reversible; yield can be improved by using an excess of one reactant or by removing water. Esters have characteristic sweet, fruity odours and are used in flavourings and solvents.
当羧酸与醇在强酸催化剂（通常为浓 H₂SO₄）存在下加热时，会建立一个生成酯和水的平衡。这种费歇尔酯化反应是经过四面体中间体的亲核酰基取代反应。反应缓慢且可逆；可通过使用过量的一种反应物或移除水来提高产率。酯具有特征的甜美果香，广泛用作香料和溶剂。
8. Reduction to Alcohols | 还原成醇
Carboxylic acids are resistant to many reducing agents but can be reduced to primary alcohols by powerful hydride donors such as lithium aluminium hydride (LiAlH₄) in dry ether. The reaction proceeds via an aldehyde intermediate that is further reduced in situ. Sodium borohydride (NaBH₄) is insufficient for this transformation. A typical example is the reduction of ethanoic acid to ethanol. This reaction is important in synthetic organic chemistry for the interconversion of functional groups.
羧酸对许多还原剂较为稳定，但可被强氢负离子给体，如在干燥乙醚中的氢化铝锂（LiAlH₄）还原为伯醇。反应经过一个醛中间体，并被进一步原位还原。硼氢化钠（NaBH₄）无法完成此转变。一个典型例子是将乙酸还原为乙醇。该反应在合成有机化学中对于官能团间的相互转化具有重要意义。
9. Reaction with Amines / Amide Formation | 与胺反应/酰胺形成
Direct reaction between a carboxylic acid and an amine initially produces an ammonium carboxylate salt. Heating this salt above 100 °C drives off water and yields an amide. The overall transformation is a condensation reaction. In biological systems, amide bonds (peptide bonds) link amino acids together in proteins, formed via ribosomal synthesis rather than direct thermal condensation. Amides are much less basic than amines due to delocalisation of the nitrogen lone pair into the carbonyl group.
羧酸与胺直接反应首先生成羧酸铵盐。将该盐加热至 100 °C 以上，可脱去水分子并生成酰胺。总的变化是一个缩合反应。在生物体系中，酰胺键（肽键）将氨基酸连接成蛋白质，这一过程由核糖体催化完成，而非直接热缩合。由于氮上的孤对电子离域至羰基，酰胺的碱性远弱于胺。
10. Decarboxylation | 脱羧反应
Decarboxylation is the loss of carbon dioxide from a carboxyl group. Simple alkanoic acids are generally resistant, but certain structural features facilitate the reaction. β-keto acids undergo decarboxylation readily upon warming, via a cyclic transition state. Sodium salts of carboxylic acids, when heated with soda lime (NaOH/CaO), lose CO₂ and form a hydrocarbon with one less carbon atom — a useful method for shortening a carbon chain in laboratory synthesis.
脱羧反应是指羧基失去二氧化碳。简单的烷酸通常不易脱羧，但某些结构特征可促进该反应。β-酮酸在加热时易于通过环状过渡态脱羧。羧酸的钠盐与碱石灰（NaOH/CaO）共热时，会失去 CO₂ 并生成少一个碳原子的烃，这是实验室合成中缩短碳链的实用方法。
11. Test for Carboxyl Group | 羧基的检测
The presence of a carboxyl group is indicated by the evolution of carbon dioxide when the sample is reacted with sodium carbonate or sodium hydrogencarbonate solution. This test distinguishes carboxylic acids from weaker phenols, which do not react with carbonates. Additionally, carboxylic acids turn blue litmus red and have a characteristic peak in the infrared spectrum: a broad O–H stretching absorption around 2500–3300 cm⁻¹ and a strong C=O stretch near 1700–1725 cm⁻¹.
羧基的存在可通过样品与碳酸钠或碳酸氢钠溶液反应释放出二氧化碳来指示。该检验法能够区分羧酸与酸性较弱的酚类，后者不与碳酸盐反应。此外，羧酸可使蓝色石蕊试纸变红，并在红外光谱中具有特征吸收峰：在 2500–3300 cm⁻¹ 处的宽 O–H 伸缩振动峰和 1700–1725 cm⁻¹ 附近的强 C=O 伸缩振动峰。
12. Summary of Key Reactions | 关键反应总结
The carboxyl group undergoes a diverse set of reactions, most of which proceed via the acyl carbon. Acidity leads to salt and CO₂ formation. Nucleophilic acyl substitution gives esters (with alcohols) and amides (via ammonium salts). Reduction yields primary alcohols, and decarboxylation shortens the carbon skeleton. A summary table of reagents and products is a powerful revision tool for tackling synthesis and mechanism questions on the exam.
羧基可以发生多种反应，其中大多数通过酰基碳进行。酸性反应导致成盐和 CO₂ 的生成。亲核酰基取代可生成酯（与醇）和酰胺（通过铵盐）。还原得到伯醇，而脱羧则缩短碳骨架。列出试剂和产物的总结表格是攻克考试中合成与机理题目的有力复习工具。
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