📚 A-Level WJEC Chemistry: Alcohols Key Points | A-Level WJEC 化学：醇 考点精讲
Alcohols are a cornerstone of organic chemistry in the WJEC A-Level specification. This guide covers their structure, classification, physical properties, key preparation methods, and the full range of chemical reactions you must master. Pay close attention to oxidation pathways, elimination mechanisms, and the distinguishing tests, as these appear frequently in structured and extended-response questions.
醇是 WJEC A-Level 有机化学的重要基础。本指南涵盖醇的结构、分类、物理性质、关键制备方法以及所有必须掌握的化学反应。请特别关注氧化途径、消除机理和鉴别测试，因为这些内容经常出现在结构题和拓展解答题中。
1. Homologous Series and General Formula | 同系物与通式
Alcohols belong to a homologous series with the general formula CₙH₂ₙ₊₁OH (or CₙH₂ₙ₊₂O). They contain the hydroxyl functional group –OH attached to a saturated carbon atom. The simplest alcohol is methanol, CH₃OH.
醇属于同系物，通式为 CₙH₂ₙ₊₁OH（或 CₙH₂ₙ₊₂O）。它们含有连接在饱和碳原子上的羟基官能团 –OH。最简单的醇是甲醇 CH₃OH。
2. Classification: Primary, Secondary, Tertiary | 分类：伯醇、仲醇、叔醇
Alcohols are classified based on the number of alkyl groups attached to the carbon atom bearing the –OH group. A primary (1°) alcohol has one alkyl group, a secondary (2°) alcohol has two, and a tertiary (3°) alcohol has three alkyl groups attached to the α‑carbon. This classification dictates oxidation behaviour.
醇的分类取决于连接 –OH 基团的碳原子上所连的烷基数目。伯醇 (1°) 有一个烷基，仲醇 (2°) 有两个，叔醇 (3°) 有三个烷基连在 α‑碳上。这种分类决定了氧化行为。
Example of 1°: ethanol CH₃CH₂OH
1° 示例：乙醇 CH₃CH₂OH
Example of 2°: propan-2-ol CH₃CH(OH)CH₃
2° 示例：丙-2-醇 CH₃CH(OH)CH₃
Example of 3°: 2-methylpropan-2-ol (CH₃)₃COH
3° 示例：2-甲基丙-2-醇 (CH₃)₃COH
3. Physical Properties and Hydrogen Bonding | 物理性质与氢键
The –OH group allows alcohols to form hydrogen bonds. This results in relatively high melting and boiling points compared to alkanes of similar molar mass. Short-chain alcohols (up to propanol) are miscible with water due to hydrogen bonding; solubility decreases as the hydrocarbon chain lengthens.
–OH 基团使醇能形成氢键。与摩尔质量相近的烷烃相比，醇的熔点和沸点相对较高。由于氢键作用，短链醇（至丙醇）可与水混溶；随着烃链增长，溶解度下降。
The boiling point trend: methanol (65°C) < ethanol (78°C) < propan-1-ol (97°C) increases with molar mass and van der Waals forces, but the dominant intermolecular force remains hydrogen bonding.
沸点趋势：甲醇 (65°C) < 乙醇 (78°C) < 丙-1-醇 (97°C) 随摩尔质量与范德华力增加而升高，但主导的分子间力仍是氢键。
4. Preparation of Ethanol: Fermentation vs Hydration | 乙醇的制备：发酵与水合法
Ethanol can be produced by fermentation of glucose using yeast at 30– naerobic conditions: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. This yields an aqueous solution of about 15% ncentrated by fractional distillation. Fermentation is renewable but slow and batch-based.
乙醇可通过葡萄糖在酵母作用下于 30–40°C 厌氧条件下发酵制得：C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂。得到约 15% 的乙醇水溶液，可通过分馏浓缩。发酵是可再生方法，但速度较慢且为分批生产。
Alternatively, industrial hydration of ethene with steam (H₃PO₄ catalyst, 300°C, 60 atm) produces ethanol directly: C₂H₄ + H₂O ⇌ C₂H₅OH. This is a continuous, fast process but uses non-renewable petroleum feedstock.
另一种方法是用乙烯与蒸汽在磷酸催化、300°C、60 atm 下水合：C₂H₄ + H₂O ⇌ C₂H₅OH。这是一种连续快速过程，但使用不可再生的石油原料。
5. Combustion of Alcohols | 醇的燃烧
O₂ + (n+1)H₂O. The enthalpy of combustion becomes more exothermic per mole as chain length increases, due to more C–C and C–H bonds being broken and formed.
醇在过量氧气中容易燃烧，生成二氧化碳和水。完全燃烧通式为：CₙH₂ₙ₊₁OH + (3n/2)O₂ → nCO₂ + (n+1)H₂O。随着链长增加，每摩尔的燃烧焓变得更负，因为断裂和形成了更多的 C–C 和 C–H 键。
Ethanol is used as a biofuel; methanol burns with a clean, almost invisible flame. Incomplete combustion can produce toxic CO and carbon soot.
乙醇用作生物燃料；甲醇燃烧时火焰清洁且几乎不可见。不完全燃烧会产生有毒的 CO 和碳烟。
6. Oxidation of Alcohols | 醇的氧化
Oxidation is a key distinguishing reaction. Acidified potassium dichromate(VI) (K₂Cr₂O₇/H₂SO₄) is the typical oxidising agent, turning from orange to green as Cr₂O₇²⁻ is reduced to Cr³⁺. Primary alcohols are oxidised first to aldehydes, then to carboxylic acids. Secondary alcohols are oxidised to ketones. Tertiary alcohols resist oxidation under these conditions.
氧化是关键的鉴别反应。酸化重铬酸钾(VI)（K₂Cr₂O₇/H₂SO₄）是常用的氧化剂，橙色变为绿色，因为 Cr₂O₇²⁻ 被还原为 Cr³⁺。伯醇先氧化成醛，再氧化成羧酸。仲醇氧化成酮。叔醇在这些条件下不被氧化。
To stop at the aldehyde, distillation is used to remove the aldehyde as it forms (lower boiling point than the alcohol and acid). Full oxidation to a carboxylic acid requires heating under reflux with excess oxidising agent.
如需停留在醛的阶段，可使用蒸馏在醛生成时立即移出（其沸点低于醇和酸）。完全氧化为羧酸需与过量氧化剂一起回流加热。
Equations (using [O] for oxygen from oxidising agent):
1°: RCH₂OH + [O] → RCHO + H₂O; RCHO + [O] → RCOOH
方程式（用 [O] 表示氧化剂提供的氧）：
1°: RCH₂OH + [O] → RCHO + H₂O; RCHO + [O] → RCOOH
2°: R₂CHOH + [O] → R₂C=O + H₂O
7. Elimination (Dehydration) to Alkenes | 消除反应（脱水）生成烯烃
ntrated phosphoric acid (H₃PO₄) or concentrated sulfuric acid (H₂SO₄) at around 170°C. The – n elimination reaction (E1 mechanism for tertiary, E2 for primary under these conditions).
醇与浓磷酸 (H₃PO₄) 或浓硫酸 (H₂SO₄) 在约 170°C 下加热，发生酸催化脱水。–OH 基团和相邻碳上的一个氢原子被移除，形成 C=C 双键。这是一个消除反应（叔醇遵循 E1 机理，伯醇在此条件下为 E2）。
ical alcohols, a mixture of alkenes may be produced, and the major product follows Zaitsev's rule (the more substituted alkene predominates).
示例：乙醇 → 乙烯：C₂H₅OH → CH₂=CH₂ + H₂O。对于不对称的醇，可能生成烯烃混合物，主产物遵循扎伊采夫规则（取代较多的烯烃为主）。
8. Reaction with Sodium | 与钠的反应
he product is a sodium alkoxide and hydrogen: 2ROH + 2Na → 2RO⁻Na⁺ + H₂. This demonstrates the weakly acidic nature of the –OH proton.
醇与钠等活泼金属反应，置换出氢气。该反应类似于水与钠的反应，但较不剧烈。产物为醇钠和氢气：2ROH + 2Na → 2RO⁻Na⁺ + H₂。这表明 –OH 质子具有弱酸性。
Observations: effervescence, sodium dissolves, colourless solution forms. This reaction can be used to test for the presence of an –OH group, although carboxylic acids and water also give positive results.
现象：产生气泡，钠溶解，形成无色溶液。该反应可用于检验 –OH 基团的存在，但羧酸和水也会产生阳性结果。
9. Esterification with Carboxylic Acids | 与羧酸的酯化反应
Alcohols react with carboxylic acids in the presence of a concentrated sulfuric acid catalyst to form esters and water. This is a reversible condensation reaction, often described as nucleophilic acyl substitution. The equilibrium can be driven to the right by using an excess of one reactant or removing water.
醇在浓硫酸催化下与羧酸反应，生成酯和水。这是一个可逆的缩合反应，常称为亲核酰基取代。可通过使用过量反应物或移除水来使平衡向右移动。
General equation: RCOOH + R'OH ⇌ RCOOR' + H₂O. Esters have characteristic sweet, fruity smells and are used in flavourings and solvents. Naming: alkyl alkanoate, first part from alcohol, second from acid.
通式：RCOOH + R'OH ⇌ RCOOR' + H₂O。酯具有特征性的果香味，用于香料和溶剂。命名：烷基链酸酯，第一部分来自醇，第二部分来自酸。
10. Reaction with Halogenating Agents | 与卤化试剂的反应
The –OH group can be replaced by a halogen atom. Several reagents are used:
₅ → RCl + POCl₃ + HCl. Room temperature, vigorous, steamy fumes of HCl. Used as a test for –OH groups.
PCl₅（五氯化磷）：ROH + PCl₅ → RCl + POCl₃ + HCl。室温下剧烈反应，产生 HCl 白雾。用作 –OH 基团的检验。
hloride): 3ROH + PCl₃ → 3RCl + H₃PO₃. Warmer conditions, good for preparing chloroalkanes.
PCl₃（三氯化磷）：3ROH + PCl₃ → 3RCl + H₃PO₃。温热条件下进行，适于制备氯代烷。
SOCl₂ (thionyl chloride): ROH + SOCl₂ → RCl + SO₂ + HCl. Often preferred because the by‑products are gases, leaving pure chloroalkane.
SOCl₂（二氯亚砜）：ROH + SOCl₂ → RCl + SO₂ + HCl。常为首选，因为副产物为气体，可得纯净的氯代烷。
The reaction with red phosphorus and bromine (generating PBr₃ in situ) or with PI₃ similarly produces bromoalkanes and iodoalkanes.
与红磷和溴（原位生成 PBr₃）或与 PI₃ 反应，类似地生成溴代烷和碘代烷。
11. Oxidation Test with Acidified Dichromate – Distinguishing Alcohols | 酸化重铬酸盐氧化测试 – 鉴别醇
This simple test tube reaction distinguishes between 1°, 2°, and 3° alcohols. Add a few drops of the alcohol to a warm solution of acidified potassium dichromate(VI). Immediate colour change from orange to green indicates oxidation—primary and secondary alcohols give a positive result within a few minutes upon warming. Tertiary alcohols show no colour change.
这种简单的试管反应可区分伯、仲、叔醇。向温热的酸化重铬酸钾(VI) 溶液中滴加少量醇。颜色从橙色立即变为绿色表明发生了氧化——伯醇和仲醇在温热后几分钟内给出阳性结果。叔醇无颜色变化。
Use this alongside other tests (e.g., sodium metal) to build a profile for an unknown compound. Remember that aldehydes also give a positive dichromate test, so consider the full context.
将此测试与其它试验（如金属钠）结合使用，可为未知化合物建立特征谱。注意醛类也会使重铬酸盐呈阳性，因此需考虑整体情况。
12. Key Reaction Mechanisms and Summary Table | 关键反应机理与总结表格
WJEC expects you to recall and draw mechanisms for nucleophilic substitution (with hydrogen halides, where the alcohol is first protonated, then water leaves, giving a carbocation that is attacked by halide ion) and for acid-catalysed elimination to alkenes. Understand the role of the lone pair on oxygen in protonation and the formation of a good leaving group.
WJEC 要求你能够回忆并画出亲核取代机理（与卤化氢反应时，醇先被质子化，水离去生成碳正离子，再由卤离子进攻）以及酸催化消除生成烯烃的机理。理解氧上孤对电子在质子化中的作用，以及良好离去基团的形成。
| Reaction | Conditions | Product | Notes |
|---|---|---|---|
| Oxidation of 1° alcohol | K₂Cr₂O₇/H⁺, distil | Aldehyde | Orange → green |
| Oxidation of 1° alcohol | K₂Cr₂O₇/H⁺, reflux | Carboxylic acid | Orange → green |
| Oxidation of 2° alcohol | K₂Cr₂O₇/H⁺, warm | Ketone | No further oxidation |
| Dehydration | Conc. H₃PO₄ or H₂SO₄, 170°C | Alkene | Elimination |
| Esterification | Carboxylic acid, conc. H₂SO₄ | Ester + water | Equilibrium reaction |
| Reaction with Na | Room temperature | Alkoxide + H₂ | Effervescence |
Memorising reagents, conditions, and observations is essential for both practical assessments and written papers.
熟记试剂、条件和观察结果对于实验评定和书面考试都至关重要。
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