📚 AS Chemistry: Carboxylic Acids – Key Points | AS化学：羧酸考点精讲
Carboxylic acids are a fundamental homologous series in organic chemistry, characterised by the –COOH functional group. At AS level, you need to master their structure, nomenclature, physical properties, acidity, preparation and key reactions including esterification, reduction and conversion to derivatives. A clear understanding of these topics will enable you to explain trends in boiling points, solubility and acid strength, and to write balanced equations for typical transformations.
羧酸是有机化学中重要的同系列化合物，其特征官能团为–COOH。在AS阶段，你需要掌握其结构、命名、物理性质、酸性、制备以及典型反应，包括酯化、还原和转化为衍生物。深入理解这些内容能帮助你解释沸点、溶解度和酸性强度的变化规律，并正确书写各类转化反应的方程式。
Published by TutorHao | Chemistry Revision Series | aleveler.com
1. Structure of the Carboxyl Group | 羧基的结构
The carboxyl group combines a carbonyl (C=O) and a hydroxyl (–OH) on the same sp² hybridised carbon. The carbon–oxygen bonds are not identical due to resonance; the C=O bond has partial double-bond character in both C–O linkages, which stabilises the carboxylate anion after deprotonation.
羧基由连接在同一碳原子上的羰基(C=O)和羟基(–OH)组成，该碳为sp²杂化。由于存在共振，两根碳氧键并不完全相同，C=O键和C–OH键都带有部分双键性质，这使得失去质子后形成的羧酸根阴离子更加稳定。
The carbonyl oxygen withdraws electron density from the O–H bond, making the hydrogen considerably more acidic than that in alcohols (pKa ~16) or water.
羰基氧原子从O–H键中拉走电子云密度，使得羧酸中的氢原子远比醇(pKa约16)和水中的氢更具酸性。
2. Nomenclature | 命名法
IUPAC names are formed by replacing the -e of the parent alkane with -oic acid. The carboxyl carbon is always assigned the number 1 position in the chain. Examples: methanoic acid (HCOOH), ethanoic acid (CH₃COOH), propanoic acid (CH₃CH₂COOH).
IUPAC命名规则是将母体烷烃词尾的 -e 替换为 -oic acid，羧基碳总是编号为1。示例：methanoic acid (甲酸，HCOOH)，ethanoic acid (乙酸，CH₃COOH)，propanoic acid (丙酸，CH₃CH₂COOH)。
For dicarboxylic acids, the suffix becomes -dioic acid, e.g. ethanedioic acid (HOOC–COOH). Many simple acids also have widely used common names: formic acid (methanoic), acetic acid (ethanoic), benzoic acid (C₆H₅COOH).
对于二元羧酸，词尾变为 -dioic acid，如 ethanedioic acid (乙二酸，HOOC–COOH)。许多简单羧酸仍有广泛使用的俗名：甲酸(formic acid)、乙酸(acetic acid)、苯甲酸(benzoic acid, C₆H₅COOH)。
3. Physical Properties | 物理性质
Carboxylic acids exhibit high boiling points relative to hydrocarbons, haloalkanes and even alcohols of similar molecular mass. This is because they can form two hydrogen bonds between molecules, creating stable dimers in the liquid and vapour states.
羧酸的沸点相对于分子质量相近的烃、卤代烃甚至醇都更高。这是因为羧酸分子间能够形成两个氢键，在液态和气态中都能通过氢键形成稳定的二聚体。
| Compound | Mᵣ | b.p. / °C |
|---|---|---|
| Propane | 44 | −42 |
| Ethanal | 44 | 21 |
| Ethanol | 46 | 78 |
| Methanoic acid | 46 | 101 |
The first four members are completely miscible with water due to extensive hydrogen bonding with water molecules. As the hydrocarbon chain lengthens, the hydrophobic alkyl part dominates and solubility decreases rapidly.
前四个成员能与水以任意比例混溶，因为它们可与水分子形成广泛的氢键。随着碳链增长，疏水性烷基部分的影响变大，溶解度迅速下降。
4. Acidity and pKa | 酸性与pKa
Carboxylic acids are weak acids that partially dissociate in aqueous solution: RCOOH + H₂O ⇌ RCOO⁻ + H₃O⁺. Their pKa values generally lie in the range 4–5, reflecting equilibrium constants of the order of 10⁻⁵.
羧酸是弱酸，在水中部分电离：RCOOH + H₂O ⇌ RCOO⁻ + H₃O⁺。它们的pKa值通常在4–5之间，对应的平衡常数约为10⁻⁵。
The carboxylate ion (RCOO⁻) is resonance-stabilised, as the negative charge is delocalised equally over both oxygen atoms. This enhanced stability makes the acid stronger than alcohols (pKa ~16) and phenols (pKa ~10).
羧酸根离子 (RCOO⁻) 因共振得到稳定，负电荷平均分布在两个氧原子上。这种额外的稳定性使羧酸的酸性强于醇(pKa约16)和酚(pKa约10)。
| Acid | Approximate pKa |
|---|---|
| Ethanol | 15.9 |
| Phenol | 10.0 |
| Ethanoic acid | 4.76 |
| Methanoic acid | 3.75 |
5. Factors Affecting Acid Strength | 影响酸性强度的因素
Electron-withdrawing groups (e.g. –Cl, –NO₂, –CN) near the –COOH group increase acidity by stabilising the resulting carboxylate anion through the negative inductive effect. The closer the group and the more electronegative the atom, the greater the effect.
靠近–COOH的吸电子基团(如–Cl、–NO₂、–CN)通过负诱导效应稳定生成的羧酸根阴离子，从而增强酸性。基团离羧基越近、原子电负性越强，效应越显著。
Conversely, electron-donating alkyl groups (e.g. –CH₃) slightly decrease acidity because they intensify the negative charge on the carboxylate ion. Thus, methanoic acid (pKa 3.75) is stronger than ethanoic acid (pKa 4.76).
相反，给电子的烷基(如–CH₃)会使羧酸根上的负电荷更集中，从而略微降低酸性。因此，甲酸(pKa 3.75)的酸性强于乙酸(pKa 4.76)。
Substituting chlorine atoms dramatically increases acidity: chloroethanoic acid (pKa 2.86), dichloroethanoic acid (pKa 1.29), trichloroethanoic acid (pKa 0.65).
氯原子的取代可使酸性大幅增强：一氯乙酸(pKa 2.86)、二氯乙酸(pKa 1.29)、三氯乙酸(pKa 0.65)。
6. Preparation of Carboxylic Acids | 羧酸的制备方法
Primary alcohols can be fully oxidised to carboxylic acids by heating under reflux with acidified potassium dichromate(VI). The aldehyde intermediate is first formed and then further oxidised, so it cannot be isolated unless distilled off immediately.
伯醇可通过与酸化重铬酸钾(VI)溶液加热回流被完全氧化为羧酸。中间产物醛会继续被氧化，除非立即蒸馏移出，否则无法单独获得。
Similarly, aldehydes are oxidised to carboxylic acids using the same oxidising agent. This reaction is often used to distinguish aldehydes from ketones.
类似地，醛也可用同样的氧化剂氧化成羧酸。该反应常用于区分醛和酮。
Nitriles (R–C≡N) can be hydrolysed by heating with dilute hydrochloric acid or sodium hydroxide solution, followed by acidification if a base was used. The reaction proceeds through an amide intermediate and ultimately yields a carboxylic acid.
腈(R–C≡N)可通过与稀盐酸或氢氧化钠溶液加热水解，若使用碱则最后需酸化，从而得到羧酸。反应经过酰胺中间体最终生成羧酸。
Esters undergo acid- or base-catalysed hydrolysis to regenerate the parent carboxylic acid and alcohol.
酯通过酸催化或碱催化水解可以再生为原来的羧酸和醇。
7. Reactions as Acids | 作为酸的典型反应
Carboxylic acids react with reactive metals such as sodium or magnesium to give carboxylate salts and hydrogen gas. For example: 2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂.
羧酸与活泼金属如钠或镁反应生成羧酸盐和氢气。例如：2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂。
They neutralise aqueous sodium hydroxide or other bases to form a salt and water: CH₃CH₂COOH + NaOH → CH₃CH₂COONa + H₂O.
它们能中和氢氧化钠等碱，生成盐和水：CH₃CH₂COOH + NaOH → CH₃CH₂COONa + H₂O。
With sodium carbonate or sodium hydrogencarbonate, vigorous effervescence of carbon dioxide occurs: RCOOH + NaHCO₃ → RCOONa + CO₂ + H₂O. This reaction serves as a simple chemical test for the –COOH group.
与碳酸钠或碳酸氢钠反应会产生二氧化碳气体，观察到剧烈冒泡：RCOOH + NaHCO₃ → RCOONa + CO₂ + H₂O。该反应可作为–COOH基团的简易化学检验。
8. Esterification | 酯化反应
When a carboxylic acid is heated with an alcohol in the presence of a concentrated sulfuric acid catalyst, an ester and water are formed. This reversible condensation reaction is often called Fischer esterification.
羧酸与醇在浓硫酸催化下加热，生成酯和水。该可逆缩合反应常称为Fischer酯化反应。
RCOOH + R’OH ⇌ RCOOR’ + H₂O
The reaction mixture is heated under reflux to overcome the slow rate at room temperature. The ester can be separated by distillation, often washed with sodium carbonate solution to remove unreacted acid.
反应混合物需加热回流以克服室温下较慢的速率。酯可通过蒸馏分离，并常用碳酸钠溶液洗涤除去未反应的酸。
Common examples include the preparation of ethyl ethanoate (CH₃COOCH₂CH₃) from ethanoic acid and ethanol.
常见实例包括用乙酸和乙醇制备乙酸乙酯(CH₃COOCH₂CH₃)。
9. Reduction of Carboxylic Acids | 羧酸的还原
Carboxylic acids are resistant to reduction by mild reagents. The powerful reducing agent lithium aluminium hydride (LiAlH₄) in dry ether reduces –COOH all the way to the primary alcohol.
羧酸不易被温和还原剂还原。强还原剂氢化铝锂(LiAlH₄)在干燥乙醚中可将–COOH直接还原为伯醇。
RCOOH + 4[H] → RCH₂OH + H₂O
Sodium borohydride (NaBH₄) is not sufficiently strong and does not reduce carboxylic acids, though it reduces aldehydes and ketones. This selectivity is important in synthetic planning.
硼氢化钠(NaBH₄)还原能力不够，无法还原羧酸，但可还原醛和酮。这种选择性在合成路线设计中有重要意义。
10. Conversion to Acyl Chlorides | 转化为酰氯
Carboxylic acids react with phosphorus(V) chloride (PCl₅) at room temperature to produce an acyl chloride, phosphoryl chloride (POCl₃) and steamy fumes of hydrogen chloride.
羧酸与五氯化磷(PCl₅)在室温下反应生成酰氯、三氯氧磷(POCl₃)和氯化氢白雾。
RCOOH + PCl₅ → RCOCl + POCl₃ + HCl
Thionyl chloride (SOCl₂) is often preferred because it gives gaseous by-products (SO₂ and HCl) that are easily removed, leaving the acyl chloride pure: RCOOH + SOCl₂ → RCOCl + SO₂ + HCl.
氯化亚砜(SOCl₂)更常被选用，因为其副产物(SO₂和HCl)为气体易除去，可直接得到较纯的酰氯：RCOOH + SOCl₂ → RCOCl + SO₂ + HCl。
11. Formation of Amides | 酰胺的生成
Direct reaction of a carboxylic acid with ammonia initially produces an ammonium carboxylate salt. Heating this salt leads to dehydration and formation of a primary amide.
羧酸与氨直接反应首先生成羧酸铵盐，加热该盐可脱水生成伯酰胺。
RCOO⁻NH₄⁺ → RCONH₂ + H₂O
A more efficient laboratory route converts the carboxylic acid to an acyl chloride first, then reacts it with ammonia or an amine. The acyl chloride method works at room temperature and gives high yields: RCOCl + 2NH₃ → RCONH₂ + NH₄Cl.
更高效的实验室方法是将羧酸先转化为酰氯，再与氨或胺反应。酰氯法在室温下即可进行且产率较高：RCOCl + 2NH₃ → RCONH₂ + NH₄Cl。
12. Summary of Key Reactions | 反应考点总结
The following summary outlines the interconversions that every AS candidate must know. Mastering this reaction map allows you to answer synthesis and identification questions confidently.
以下总结列出了每位AS考生必须掌握的相互转化关系。掌握这个反应网络能让你自如地应对合成路线和鉴别类题目。
- As an acid: RCOOH → carboxylate salt (with metal, base or carbonate)
- 酸性反应: RCOOH → 羧酸盐 (与金属、碱或碳酸盐)
- Esterification: RCOOH + R’OH ⇌ RCOOR’ + H₂O (conc. H₂SO₄, heat)
- 酯化: RCOOH + R’OH ⇌ RCOOR’ + H₂O (浓H₂SO₄, 加热)
- Reduction: RCOOH + 4[H] → RCH₂OH (LiAlH₄ in dry ether)
- 还原: RCOOH + 4[H] → RCH₂OH (LiAlH₄/干燥乙醚)
- Acyl chloride: RCOOH + PCl₅ → RCOCl; or + SOCl₂ → RCOCl
- 酰氯: RCOOH + PCl₅ → RCOCl; 或 + SOCl₂ → RCOCl
- Amide: via acyl chloride + NH₃/amine
- 酰胺: 经由酰氯 + 氨/胺
Remember to write balanced equations with correct structures, and be ready to explain the relative acidity or boiling points using hydrogen bonding and inductive effects.
务必书写配平且结构正确的方程式，并随时准备运用氢键和诱导效应解释相对酸性或沸点的高低。
Published by TutorHao | Chemistry Revision Series | aleveler.com
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