📚 AS Chemistry: Alcohols – Key Points | AS 化学：醇 考点精讲
Alcohols are a fundamental homologous series in AS Chemistry, containing the hydroxyl functional group (–OH). Understanding their structure, classification, physical properties, and characteristic reactions is essential for achieving top marks. This concise revision guide covers the key exam points: nomenclature, hydrogen bonding and boiling points, production of ethanol, oxidation, esterification, halogenation, dehydration, and distinguishing tests. Each concept is presented in English and Chinese to support bilingual learners at TutorHao and aleveler.com.
醇是AS化学中一个基本的同系物，含有羟基(–OH)官能团。理解醇的结构、分类、物理性质与特征反应是取得高分的关键。本篇简练的复习指南涵盖了醇的命名、氢键与沸点、乙醇的制备、氧化反应、酯化、卤化、脱水以及鉴别测试等核心考点。每个知识点均以中英双语呈现，为TutorHao及aleveler.com的双语学习者提供支持。
1. Nomenclature and Classification | 命名与分类
Alcohols are named by replacing the final ‘e’ of the corresponding alkane with ‘-ol’. The parent chain is the longest continuous carbon chain bearing the –OH group, and the position of the hydroxyl group is given the lowest possible number. For diols, the suffix ‘-diol’ is used with two position numbers.
醇的命名方法是将相应烷烃词尾的“e”替换为“-ol”。选择含有–OH的最长连续碳链作为主链，羟基的位置用尽可能小的数字标出。对于二元醇，使用后缀“-diol”并标出两个羟基的位置。
Example: CH₃CH₂CH₂OH is propan-1-ol; CH₃CH(OH)CH₃ is propan-2-ol; CH₃CH₂CH(OH)CH₂OH is butane-1,2-diol.
例如：CH₃CH₂CH₂OH为1-丙醇；CH₃CH(OH)CH₃为2-丙醇；CH₃CH₂CH(OH)CH₂OH为1,2-丁二醇。
Classification is based on the carbon atom attached to –OH. Primary (1°) alcohols have the –OH on a carbon bonded to only one other carbon (or to no carbons in methanol). Secondary (2°) alcohols have the –OH carbon bonded to two other carbons. Tertiary (3°) alcohols have the –OH carbon bonded to three other carbons.
醇的分类基于连接–OH的碳原子。伯醇(1°)：–OH所在的碳只与一个其他碳相连（甲醇无碳）；仲醇(2°)：该碳与另外两个碳相连；叔醇(3°)：该碳与另外三个碳相连。
This classification directly determines the products of oxidation – a key exam focus.
这一分类直接决定了氧化反应的产物，是考试的重点。
2. Physical Properties: Boiling Point and Solubility | 物理性质：沸点与溶解度
Alcohols exhibit significantly higher boiling points than alkanes of comparable molar mass. This is due to intermolecular hydrogen bonding between the polar –OH groups. Energy is required to overcome these relatively strong hydrogen bonds in addition to van der Waals’ forces.
醇的沸点明显高于相对分子质量相近的烷烃。这是因为极性的–OH基团之间能够形成分子间氢键。除了克服范德华力外，还需要额外的能量来破坏这些较强的氢键。
As the alkyl chain length increases, the influence of the hydrophobic hydrocarbon part grows, and boiling points rise accordingly, but the difference from alkanes becomes proportionally smaller.
随着烷基链增长，疏水的碳氢部分影响增大，沸点也随之升高，但与相应烷烃的差距会相对变小。
Short-chain alcohols (methanol, ethanol, propanol) are miscible with water in all proportions because the –OH group can form hydrogen bonds with water molecules. As the hydrocarbon chain lengthens, solubility decreases rapidly – butan-1-ol is only partially soluble, and longer-chain alcohols are virtually insoluble.
短链醇（甲醇、乙醇、丙醇）与水以任意比例互溶，因为–OH基团能与水分子形成氢键。随着碳链增长，溶解度迅速降低——1-丁醇仅部分可溶，更长的醇几乎不溶于水。
3. Preparation of Ethanol: Fermentation vs Hydration | 乙醇的制备：发酵与水化
Ethanol can be produced by two main industrial routes. Fermentation uses renewable carbohydrate sources; direct hydration of ethene uses a petrochemical feedstock. Both reactions are commonly examined.
乙醇可通过两种主要工业路径制备。发酵法使用可再生的碳水化合物原料，而乙烯直接水化法则以石化产品为原料。这两个反应都是常见考点。
| Feature | Fermentation | Hydration of Ethene |
|---|---|---|
| Raw Material | Sugars, starch (renewable) | Ethene from crude oil (non-renewable) |
| 原料 | 糖类、淀粉（可再生） | 来自石油的乙烯（不可再生） |
| Conditions | Yeast, 35–40 °C, anaerobic, aqueous | H₃PO₄ catalyst, 300 °C, 60–70 atm |
| 条件 | 酵母，35–40 °C，厌氧，水溶液 | 磷酸催化剂，300 °C，60–70个大气压 |
| Equation | C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ | C₂H₄ + H₂O ⇌ C₂H₅OH |
| 反应方程式 | C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ | C₂H₄ + H₂O ⇌ C₂H₅OH |
| Product Purity | ≈15% ethanol; fractional distillation required | Pure ethanol obtained directly |
| 产品纯度 | ~15%乙醇，需分馏浓缩 | 直接获得纯乙醇 |
Fermentation is sustainable but slow, while hydration is fast and continuous but relies on non-renewable resources. Exam questions often ask to compare atom economy and environmental impact.
发酵法可持续但速率慢，水化法快速连续但依赖不可再生资源。试题常要求比较原子经济性和环境影响。
4. Combustion of Alcohols | 醇的燃烧
Alcohols burn readily in an excess of oxygen, producing carbon dioxide and water. The general equation for complete combustion is: CₙH₂ₙ₊₁OH + (3n/2)O₂ → nCO₂ + (n+1)H₂O.
醇在充足的氧气中容易燃烧，生成二氧化碳和水。完全燃烧的通式为：CₙH₂ₙ₊₁OH + (3n/2)O₂ → nCO₂ + (n+1)H₂O。
For ethanol: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O. The flame is clean and pale blue. Because alcohols are already partially oxidised, they release less energy per mole than the corresponding alkane.
以乙醇为例：C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O，火焰洁净，呈淡蓝色。由于醇已经是部分氧化的分子，每摩尔燃烧放出的热量低于相应烷烃。
In limited oxygen, incomplete combustion can produce carbon monoxide (CO) or soot (C). This is a safety concern and may feature in evaluation questions.
在氧气不足时，不完全燃烧会生成一氧化碳(CO)或碳黑(C)。这是一个安全隐患，也可能出现在评价类试题中。
5. Oxidation of Alcohols | 醇的氧化反应
The oxidising agent is acidified potassium dichromate(VI), K₂Cr₂O₇/ H₂SO₄(aq), which is reduced from orange Cr₂O₇²⁻ to green Cr³⁺ ions. The ease and extent of oxidation depend on the class of alcohol.
氧化剂为酸化重铬酸钾溶液(K₂Cr₂O₇ / 稀H₂SO₄)，其本身由橙色的Cr₂O₇²⁻被还原为绿色的Cr³⁺。氧化的难易和程度取决于醇的分类。
Primary alcohols are oxidised first to aldehydes and then to carboxylic acids. To isolate the aldehyde, the alcohol is heated with acidified dichromate and the aldehyde is distilled off as soon as it forms (immediate distillation). To obtain the carboxylic acid, the mixture is heated under reflux with excess oxidising agent.
伯醇首先被氧化为醛，继而氧化为羧酸。分离醛的方法是将醇与酸化重铬酸钾一起加热，并立即将生成的醛蒸馏出来（即时蒸馏）。若要制取羧酸，则需在过量氧化剂存在下回流加热。
CH₃CH₂OH + [O] → CH₃CHO + H₂O
CH₃CHO + [O] → CH₃COOH
Secondary alcohols are oxidised to ketones. Ketones resist further oxidation, so the reaction need only be heated under reflux. For example, propan-2-ol gives propanone:
仲醇被氧化为酮。酮不易继续氧化，因此只需回流加热即可。例如，2-丙醇生成丙酮：
CH₃CH(OH)CH₃ + [O] → CH₃COCH₃ + H₂O
Tertiary alcohols are not oxidised by acidified dichromate. The carbon bearing –OH has no hydrogen atom to be removed, so no reaction occurs and the orange colour persists. This forms the basis of a simple distinguishing test.
叔醇不能被酸化重铬酸钾氧化。连接–OH的碳上没有可被脱去的氢原子，因此不发生反应，溶液保持橙色。这构成了一个简单的鉴别测试的基础。
6. Reaction with Sodium | 与金属钠的反应
Alcohols react with reactive metals such as sodium, producing an alkoxide (sodium alkoxide) and hydrogen gas. The reaction is similar to that of water with sodium, but less vigorous.
醇与钠等活泼金属反应，生成醇钠（烷氧基钠）和氢气。该反应与水与钠的反应相似，但较为温和。
2C₂H₅OH + 2Na → 2C₂H₅O⁻Na⁺ + H₂
Effervescence of hydrogen gas is observed, and the sodium dissolves. Sodium ethoxide (C₂H₅ONa) is a white solid once solvent is evaporated. Longer-chain alcohols react more slowly due to steric hindrance and lower solubility in the reaction medium.
观察到氢气气泡冒出，钠逐渐溶解。蒸去溶剂后得到白色固体乙醇钠(C₂H₅ONa)。碳链较长的醇由于位阻和在反应介质中的溶解度降低，反应速率更慢。
This reaction is a test for the –OH group, but it cannot distinguish between alcohol and carboxylic acid (which also reacts), so it is used alongside other tests.
该反应可用于检验–OH基团，但无法区分醇和羧酸（羧酸也会反应），因此需与其他测试联合使用。
7. Esterification | 酯化反应
Alcohols react with carboxylic acids to form esters and water in a reversible condensation reaction, catalysed by concentrated sulfuric acid. The ester linkage is –COO–.
醇与羧酸在浓硫酸催化下发生可逆的缩合反应，生成酯和水。酯键为–COO–。
RCOOH + R’OH ⇌ RCOOR’ + H₂O
For example, ethanol and ethanoic acid give ethyl ethanoate, a sweet-smelling ester used as a solvent and in flavourings. Heating under reflux with a few drops of concentrated H₂SO₄ drives the equilibrium to the right; the ester is often distilled off to improve yield.
例如，乙醇与乙酸反应生成乙酸乙酯，这是一种具有果香、常用作溶剂和调味剂的酯。加入少量浓硫酸并回流加热，可使平衡向右移动；通常通过蒸馏移走酯来提高产率。
Acid anhydrides can also react with alcohols to form esters at room temperature, producing a more vigorous and less reversible reaction, but this is usually covered in A2 rather than AS.
酸酐也可与醇在室温下反应生成酯，反应更剧烈且可逆性低，但这通常是A2的内容，AS阶段以羧酸法为主。
8. Halogenation of Alcohols | 醇的卤化反应
The –OH group can be substituted by a halogen atom to produce a haloalkane. Different reagents offer varying levels of reactivity and product purity.
醇的–OH基团可被卤素原子取代，生成卤代烷。不同的试剂提供不同的活性和产物纯度。
With phosphorus(V) chloride, PCl₅, the reaction is vigorous at room temperature, giving the chloroalkane, phosphoryl chloride and HCl gas (observed as white fumes). For example:
使用五氯化磷(PCl₅)时，反应在室温下即剧烈进行，生成氯代烷、三氯氧磷和氯化氢气体（可观察到白雾）。例如：
C₂H₅OH + PCl₅ → C₂H₅Cl + POCl₃ + HCl
Thionyl chloride, SOCl₂, is often preferred in synthesis because the by-products (SO₂ and HCl) are gases, leaving a pure chloroalkane. This reaction is usually carried out under reflux.
在有机合成中常使用亚硫酰氯(SOCl₂)，因为副产物(SO₂和HCl)为气体，容易除去，可得到较纯的氯代烷。此反应通常在回流下进行。
Alcohols also react with concentrated hydrohalic acids (e.g. HBr) in a nucleophilic substitution, but the mixture often requires heating and is less clean than phosphorus- or thionyl-based methods.
醇也可与浓氢卤酸（如HBr）发生亲核取代反应，但混合物通常需要加热，反应纯度不如用磷或亚硫酰类试剂。
Published by TutorHao | Chemistry Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导