📚 Alcohols, Esters and Carboxylic Acids | 醇、酯和羧酸
Alcohols, esters and carboxylic acids are three closely linked oxygen-containing organic families. Their interconversions, especially esterification and oxidation, form a central part of Cambridge A-Level Chemistry. A single carbon atom can move up and down this oxidation ladder, and examiners test that journey again and again in both AS and A2 papers.
醇、酯和羧酸是三类密切相关的含氧有机物家族。它们的相互转化——尤其是酯化和氧化——是剑桥 A-Level 化学的核心内容。同一个碳原子可以在这条氧化阶梯上上下移动，而考官在 AS 和 A2 试卷中反复考查这一转化路线。
This revision guide covers functional groups and classification, IUPAC naming, physical properties, oxidation and dehydration of alcohols, the acidity and preparation of carboxylic acids, esterification, ester hydrolysis, and the full interconversion map with typical exam traps.
本篇复习指南涵盖官能团与分类、IUPAC 命名、物理性质、醇的氧化与脱水、羧酸的酸性及制备、酯化反应、酯的水解，以及完整的相互转化关系图和常见考试陷阱。
1. Functional Groups and Classification | 官能团与分类
Alcohols contain the hydroxyl group –OH attached to a saturated carbon atom. Carboxylic acids contain the carboxyl group –COOH, while esters contain the –COO– linkage. These three groups all contain oxygen, but the way the oxygen is bonded changes the chemistry completely.
醇含有连接在饱和碳原子上的羟基 –OH；羧酸含有羧基 –COOH；酯则含有 –COO– 键。这三种官能团都含氧，但氧原子的连接方式不同，化学性质也就完全不同。
The general formulae are R–OH for an alcohol, R–COOH for a carboxylic acid and R–COO–R' for an ester, where R and R' are alkyl groups. An alcohol with n carbon atoms has the molecular formula CₙH₂ₙ₊₁OH, and a saturated monocarboxylic acid has the formula CₙH₂ₙ₊₁COOH.
醇的通式为 R–OH，羧酸为 R–COOH，酯为 R–COO–R'，其中 R 与 R' 为烷基。含 n 个碳原子的饱和一元醇分子式为 CₙH₂ₙ₊₁OH，饱和一元羧酸为 CₙH₂ₙ₊₁COOH。
Alcohols are classified as primary, secondary or tertiary according to the number of carbon atoms bonded to the carbon bearing the –OH group. A primary alcohol has one alkyl group on that carbon, a secondary alcohol has two, and a tertiary alcohol has three.
根据与羟基所连碳原子相连的碳原子数目，醇可分为伯醇、仲醇和叔醇。伯醇的羟基碳上连有一个烷基，仲醇连有两个，叔醇连有三个。
| Type | Structure | Example |
|---|---|---|
| Primary (1°) | RCH₂OH | Ethanol CH₃CH₂OH |
| Secondary (2°) | R₂CHOH | Propan-2-ol CH₃CH(OH)CH₃ |
| Tertiary (3°) | R₃COH | 2-Methylpropan-2-ol (CH₃)₃COH |
Two classification traps are worth memorising. First, phenol has an –OH group bonded directly to a benzene ring, so it is not classified as an alcohol and does not undergo the usual alcohol reactions. Second, an ether (R–O–R') contains oxygen but no –OH, so it is neither an alcohol nor an acid.
有两个分类陷阱必须记住。第一，苯酚的 –OH 直接连在苯环上，因此不属于醇，也不发生醇的常规反应。第二，醚（R–O–R'）含氧但没有 –OH，既不是醇也不是酸。
2. Naming Alcohols | 醇的命名
For IUPAC naming, choose the longest carbon chain that contains the carbon atom bearing the –OH group. Number the chain from the end that gives the –OH group the lowest possible number, and use the suffix -ol with that number.
IUPAC 命名时，先选择包含羟基所连碳原子的最长碳链。编号应使羟基获得尽可能小的位次，并以 -ol 为后缀标出该位次。
Some worked examples. CH₃CH₂CH₂OH is propan-1-ol; CH₃CH(OH)CH₃ is propan-2-ol; CH₃CH₂CH(OH)CH₃ is butan-2-ol; and (CH₃)₃COH is 2-methylpropan-2-ol, where the -ol number is given last.
举几个实例：CH₃CH₂CH₂OH 为丙-1-醇；CH₃CH(OH)CH₃ 为丙-2-醇；CH₃CH₂CH(OH)CH₃ 为丁-2-醇；(CH₃)₃COH 为 2-甲基丙-2-醇，其中 -ol 的位次写在最后。
When a molecule contains more than one –OH group, keep the final -e of the parent alkane and add -diol, -triol and so on. Ethane-1,2-diol (HOCH₂CH₂OH) is the main component of antifreeze, and propane-1,2,3-triol (HOCH₂CH(OH)CH₂OH) is glycerol.
当分子含有一个以上 –OH 时，保留母体烷烃的词尾 -e 并加上 -diol、-triol 等。乙-1,2-二醇（HOCH₂CH₂OH）是防冻液的主要成分，丙-1,2,3-三醇（HOCH₂CH(OH)CH₂OH）即甘油。
Substituents are listed alphabetically, and ring alcohols take the suffix -ol with no number, for example cyclohexanol. Note that the older names (ethylene glycol, glycerol) still appear in textbooks, but examiners expect the systematic IUPAC name.
取代基按字母顺序列出；环状醇直接加后缀 -ol 且不需位次，例如环己醇。注意，旧称（乙二醇、甘油）在教材中仍会出现，但考试要求写出系统 IUPAC 名称。
3. Physical Properties of Alcohols | 醇的物理性质
Alcohol molecules are polar because the O–H bond is strongly polarised, and the –OH group allows hydrogen bonding between molecules. As a result, alcohols have much higher boiling points than the alkanes or ethers of similar relative molecular mass.
由于 O–H 键高度极化，醇分子具有极性，且 –OH 之间可以形成氢键。因此，醇的沸点远高于相对分子质量相近的烷烃或醚。
−42 °C. The difference is not caused by relative molecular mass but by the energy needed to break hydrogen bonds during boiling.
例如，乙醇（Mr = 46）沸点为 78 °C，而丙烷（Mr = 44）沸点为 −42 °C。这一差异并非源于相对分子质量，而是源于沸腾时需要破坏氢键所消耗的能量。
Solubility in water depends on chain length. Methanol, ethanol and propan-1-ol are completely miscible with water because the –OH group forms hydrogen bonds with water molecules. As the hydrocarbon chain grows, the non-polar part dominates and solubility falls sharply; hexan-1-ol is only slightly soluble.
在水中的溶解度取决于碳链长度。甲醇、乙醇和丙-1-醇与水完全互溶，因为 –OH 能与水分子形成氢键。随着烃链增长，非极性部分占主导，溶解度迅速下降；己-1-醇仅微溶于水。
Alcohols also have higher melting points and viscosities than comparable alkanes, and the boiling point rises as the number of –OH groups increases, because more hydrogen bonds must be broken per molecule.
醇的熔点和黏度也高于相应的烷烃；随着 –OH 数目增加，每个分子需要破坏的氢键更多，沸点进一步升高。
4. Combustion and Oxidation of Alcohols | 醇的燃烧与氧化
Alcohols burn readily in excess oxygen to give carbon dioxide and water, releasing a large amount of energy. Ethanol is used as a fuel and as a petrol additive.
醇在过量氧气中容易燃烧，生成二氧化碳和水，并释放大量能量。乙醇被用作燃料和汽油添加剂。
C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O
The more important exam reaction is oxidation by acidified potassium dichromate(VI), K₂Cr₂O₇/H₂SO₄, which acts as the oxidising agent and is written as [O]. The characteristic colour change is orange to green.
考试中更重要的反应是用酸性重铬酸钾 K₂Cr₂O₇/H₂SO₄ 氧化，它作为氧化剂可写作 [O]。特征现象是橙色变为绿色。
A primary alcohol is first oxidised to an aldehyde, and the aldehyde can be oxidised further to a carboxylic acid. If the aldehyde is distilled off as it forms, it escapes further oxidation; if the mixture is heated under reflux with excess oxidising agent, the carboxylic acid is obtained.
伯醇先被氧化为醛，醛还可继续被氧化为羧酸。若生成的醛及时蒸出，便可避免进一步氧化；若与过量氧化剂在回流条件下加热，则得到羧酸。
CH₃CH₂OH + [O] → CH₃CHO + H₂O
CH₃CHO + [O] → CH₃COOH
A secondary alcohol is oxidised to a ketone and cannot be oxidised further under these conditions because the carbonyl carbon carries no hydrogen atom.
仲醇被氧化为酮，且在该条件下不能继续氧化，因为羰基碳上没有氢原子。
CH₃CH(OH)CH₃ + [O] → CH₃COCH₃ + H₂O
A tertiary alcohol is resistant to oxidation by acidified dichromate(VI) because the carbon bearing the –OH has no hydrogen atom attached. This gives a simple test: primary and secondary alcohols turn the orange solution green on warming, whereas a tertiary alcohol does not.
叔醇不能被酸性重铬酸盐氧化，因为连有 –OH 的碳上没有氢原子。这提供了一个简易鉴别方法：伯醇和仲醇温热后使橙色溶液变绿，而叔醇不会。
5. Dehydration and Substitution of Alcohols | 醇的脱水与取代反应
Heating an alcohol with concentrated sulfuric acid, or passing its vapour over heated aluminium oxide, removes a molecule of water and produces an alkene. This is an elimination reaction, and the C–O bond and a C–H bond on the adjacent carbon are both broken.
将醇与浓硫酸共热，或使其蒸气通过加热的氧化铝，可脱去一分子水生成烯烃。这是消去反应，C–O 键与相邻碳上的 C–H 键同时断裂。
CH₃CH₂OH → CH₂=CH₂ + H₂O
The typical conditions are concentrated H₂SO₄ at about 170 °C, or Al₂O₃ at 300–400 °C. If a secondary or tertiary alcohol can dehydrate in two directions, the more substituted alkene is usually the major product.
典型条件是约 170 °C 的浓 H₂SO₄，或 300–400 °C 的 Al₂O₃。若仲醇或叔醇可向两个方向脱水，取代基较多的烯烃通常是主要产物。
Alcohols also undergo substitution to give haloalkanes. Warming with a mixture of sodium bromide and concentrated sulfuric acid generates hydrogen bromide in situ, or phosphorus pentachloride and thionyl chloride can be used to introduce chlorine.
醇还可发生取代反应生成卤代烃。与溴化钠和浓硫酸的混合物共热可原位生成溴化氢；也可用五氯化磷或亚硫酰氯引入氯原子。
CH₃CH₂OH + HBr → CH₃CH₂Br + H₂O
Both dehydration and substitution begin with protonation of the –OH group, converting it into a good leaving group (H₂O). Recognising this shared first step helps you to explain mechanisms rather than just recall products.
脱水与取代的第一步都是 –OH 被质子化，从而转化为良好的离去基团（H₂O）。认识到这一共同起始步骤，有助于解释机理而不只是背诵产物。
6. Carboxylic Acids: Structure, Naming and Physical Properties | 羧酸：结构、命名与物理性质
A carboxylic acid contains the carboxyl group –COOH, in which a carbonyl group and a hydroxyl group are attached to the same carbon. The carboxyl carbon is sp² hybridised and the group is planar, and this conjugation is what makes the –OH hydrogen unusually acidic.
羧酸含有羧基 –COOH，即羰基和羟基连在同一个碳原子上。羧基碳为 sp² 杂化，该基团呈平面结构，正是这种共轭作用使 –OH 上的氢具有异常强的酸性。
For naming, replace the -e of the parent alkane with -oic acid. Methanoic acid (HCOOH), ethanoic acid (CH₃COOH, the main component of vinegar) and propanoic acid (CH₃CH₂COOH) are the first three members. Dicarboxylic acids use -dioic acid, for example ethanedioic acid (oxalic acid).
命名时，将母体烷烃的词尾 -e 换成 -oic acid。甲酸（HCOOH）、乙酸（CH₃COOH，食醋的主要成分）和丙酸（CH₃CH₂COOH）是最初三个成员。二元羧酸使用 -dioic acid，例如乙二酸（草酸）。
ogen bonds and the molecules pair up into dimers. Ethanoic acid (Mr = 60) boils at 118 °C.
羧酸的沸点甚至高于相对分子质量相近的醇，因为每个分子可形成两个氢键，分子间缔合成二聚体。乙酸（Mr = 60）沸点为 118 °C。
The first four members are completely miscible with water, but solubility decreases as the alkyl chain lengthens. All the carboxylic acids up to C₄ have a sharp, sour smell; those from C₅ upwards smell unpleasant, which is why butanoic acid is associated with rancid butter.
前四个成员与水完全互溶，但随着烷基链增长溶解度下降。C₄ 以内的羧酸都有刺鼻酸味，C₅ 以上气味难闻，丁酸便与腐臭黄油的气味相关。
7. Acidity of Carboxylic Acids | 羧酸的酸性
Carboxylic acids are weak acids. In water they dissociate only partially, and the position of equilibrium lies well to the left. They are, however, much stronger acids than alcohols and phenols.
羧酸是弱酸。在水中它们只部分电离，平衡位置明显偏左。但它们比醇和苯酚的酸性强得多。
CH₃COOH ⇌ CH₃COO⁻ + H⁺
The carboxylate ion is stabilised by delocalisation of the negative charge over both oxygen atoms, which is why the acid readily loses a proton. In an alcohol, the negative charge formed on ionisation cannot be delocalised, so the equilibrium lies far to the left.
羧酸根离子的负电荷在两个氧原子上离域而得到稳定，因此羧酸容易失去质子。而醇电离后产生的负电荷无法离域，所以平衡强烈偏向左侧。
Acid strength is measured by Ka or pKa. The smaller the pKa, the stronger the acid. A 0.1 mol dm⁻³ solution of ethanoic acid has a pH of about 2.9, compared with pH 1 for 0.1 mol dm⁻³ hydrochloric acid.
酸性强弱用 Ka 或 pKa 衡量。pKa 越小，酸性越强。0.1 mol dm⁻³ 乙酸溶液的 pH 约为 2.9，而 0.1 mol dm⁻³ 盐酸的 pH 为 1。
| Acid | pKa | Reason |
|---|---|---|
| Ethanoic acid CH₃COOH | 4.76 | reference weak acid |
| Chloroethanoic acid CH₂ClCOOH | 2.87 | Cl withdraws electrons inductively |
| Dichloroethanoic acid CHCl₂COOH | 1.29 | two Cl atoms, stronger effect |
| Trichloroethanoic acid CCl₃COOH | 0.66 | strongest of this series |
Electron-withdrawing groups such as Cl, Br, NO₂ and CN increase acid strength by pulling electron density away from the carboxylate ion, which spreads and stabilises the negative charge. Electron-donating alkyl groups have the opposite effect, which is why methanoic acid (pKa 3.75) is stronger than ethanoic acid.
Cl、Br、NO₂、CN 等吸电子基团把电子密度从羧酸根上拉走，使负电荷更加分散和稳定，从而增强酸性。给电子的烷基则相反，因此甲酸（pKa 3.75）比乙酸酸性强。
Standard test-tube reactions confirm the acidity. Carboxylic acids liberate carbon dioxide from sodium carbonate or sodium hydrogencarbonate, turn blue litmus red, and react with sodium to give hydrogen.
标准试管实验可验证其酸性。羧酸能使碳酸钠或碳酸氢钠放出二氧化碳，使蓝色石蕊变红，并与钠反应放出氢气。
2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂
8. Preparation and Reactions of Carboxylic Acids | 羧酸的制备与反应
In the laboratory, carboxylic acids are most often made by heating a primary alcohol with excess acidified dichromate(VI) under reflux, or by oxidising an aldehyde with Fehling's solution or Tollens' reagent.
实验室中，羧酸最常通过将伯醇与过量酸性重铬酸盐回流加热制得，或用斐林试剂、托伦试剂氧化醛制得。
Industrially, ethanoic acid is produced by the oxidation of ethanol or by the carbonylation of methanol with carbon monoxide using a rhodium catalyst. Nitriles can also be hydrolysed to carboxylic acids by heating with dilute hydrochloric acid.
工业上，乙酸通过乙醇氧化制得，或在铑催化下用一氧化碳对甲醇进行羰基化制得。腈也可与稀盐酸共热水解为羧酸。
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