Complete Guide to Typical Reactions of Alcohols | A-Level化学：醇的典型反应归纳

📚 Complete Guide to Typical Reactions of Alcohols | A-Level化学：醇的典型反应归纳

Alcohols are one of the most versatile functional groups in A-Level organic chemistry. Their reactions span substitution, elimination, oxidation, and esterification, making them a favourite topic in CIE examinations. This article systematically organises every key reaction you need to master, with conditions, mechanisms, and exam-focused observations.

醇是A-Level有机化学中最多变的官能团之一。其反应涵盖取代、消除、氧化和酯化，是CIE考试中的高频考点。本文系统归纳你需要掌握的所有关键反应，包括条件、机理和考试重点现象。

1. Structure and Bonding of Alcohols | 醇的结构与键合

The functional group of an alcohol is the hydroxyl group (–OH) attached to a saturated carbon atom. The general formula for aliphatic alcohols is CₙH₂ₙ₊₁OH. The electronegative oxygen atom creates a polar O–H bond, which underpins most of the chemical behaviour discussed below.

醇的官能团是连接在饱和碳原子上的羟基（–OH）。脂肪醇的通式为CₙH₂ₙ₊₁OH。电负性强的氧原子形成极性的O–H键，这决定了醇在以下讨论中的大部分化学行为。

Alcohols are classified by the number of carbon atoms attached to the carbon bearing the –OH group:

醇根据连接羟基碳原子上碳原子数目分类：

Primary (1°) alcohol – one alkyl group: RCH₂OH
伯醇（1°） – 连有一个烷基：RCH₂OH
Secondary (2°) alcohol – two alkyl groups: R₂CHOH
仲醇（2°） – 连有两个烷基：R₂CHOH
Tertiary (3°) alcohol – three alkyl groups: R₃COH
叔醇（3°） – 连有三个烷基：R₃COH

Classification formula: RCH₂OH (1°), R₂CHOH (2°), R₃COH (3°)

分类通式：RCH₂OH（1°）、R₂CHOH（2°）、R₃COH（3°）

This classification is not merely structural – it determines whether the alcohol can be oxidised, and to what product. Examiners frequently test this distinction, so memorise it early.

这种分类不仅仅是结构上的差异——它决定了醇能否被氧化以及氧化产物是什么。考官经常考查这一区分，请尽早牢记。

2. Combustion of Alcohols | 醇的燃烧反应

Alcohols burn readily in air or oxygen, releasing carbon dioxide and water. Complete combustion is highly exothermic, which is why ethanol is used as a fuel additive.

醇在空气或氧气中容易燃烧，释放二氧化碳和水。完全燃烧是高度放热的，因此乙醇常被用作燃料添加剂。

C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l) ΔH = –1371 kJ mol⁻¹

In limited oxygen, incomplete combustion occurs, producing carbon monoxide (CO) and carbon (soot). This is important when comparing the environmental impact of alcohol fuels versus petrol.

在氧气不足时发生不完全燃烧，生成一氧化碳（CO）和碳（烟灰）。这在比较醇类燃料与汽油的环境影响时尤为重要。

Exam tip: When writing combustion equations for any alcohol, always balance by counting carbon atoms first, then hydrogen, then oxygen last.

考试提示：写任何醇的燃烧方程式时，先数碳原子配平，再配氢，最后配氧。

3. Reaction with Sodium – Acidic Character | 与金属钠的反应——酸性特征

Alcohols react with sodium metal, releasing hydrogen gas. This demonstrates the weakly acidic nature of the O–H bond, although alcohols are far less acidic than water or carboxylic acids.

醇与金属钠反应放出氢气。这体现了O–H键的弱酸性特征，尽管醇的酸性远弱于水和羧酸。

2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂↑

Sodium ethoxide (C₂H₅ONa) is an ionic compound formed when the sodium replaces the hydrogen of the hydroxyl group. The reaction of ethanol with sodium is less vigorous than that of water with sodium, because the alkyl group donates electron density, making the O–H bond less polarised.

乙醇钠（C₂H₅ONa）是钠取代羟基中氢原子形成的离子化合物。乙醇与钠的反应不如水与钠剧烈，因为烷基推电子使O–H键极性减弱。

The trend in reactivity with sodium follows the order: methanol > primary > secondary > tertiary, due to increasing electron donation from additional alkyl groups. This reaction is a useful chemical test for the presence of an –OH group, as vigorous effervescence of hydrogen gas is observed.

与钠反应的活性顺序为：甲醇 > 伯醇 > 仲醇 > 叔醇，原因是烷基数目增加使推电子效应增强。该反应是检验–OH基团存在的有效化学方法，观察到剧烈产生氢气泡。

4. Substitution with Hydrogen Halides | 与氢卤酸的取代反应

Primary alcohols react with hydrogen halides to form haloalkanes. However, the reaction with HCl alone is slow; a catalyst such as anhydrous zinc chloride (ZnCl₂) is required.

伯醇与氢卤酸反应生成卤代烷。但单纯与HCl反应较慢，需加入无水氯化锌（ZnCl₂）作催化剂。

C₂H₅OH + HCl → C₂H₅Cl + H₂O

Alternatively, the use of PCl₅, PCl₃, or SOCl₂ provides much faster and more complete substitution. These reagents are preferred for preparing pure haloalkanes in the laboratory.

或者，使用PCl₅、PCl₃或SOCl₂可以实现更快、更完全的取代反应。这些试剂更适合在实验室中制备纯净的卤代烷。

With phosphorus pentachloride (PCl₅), misty white fumes of HCl gas are observed – a classic test for the presence of an –OH group:

与五氯化磷（PCl₅）反应时，可观察到HCl气体的白色酸雾——这是检验–OH基团的经典方法：

C₂H₅OH + PCl₅ → C₂H₅Cl + POCl₃ + HCl↑

With thionyl chloride (SOCl₂), the by-products SO₂ and HCl are both gaseous, so pure chloroalkane can be isolated easily by simple distillation:

与氯化亚砜（SOCl₂）反应时，副产物SO₂和HCl均为气体，通过简单蒸馏即可获得纯净的氯代烷：

C₂H₅OH + SOCl₂ → C₂H₅Cl + SO₂↑ + HCl↑

Exam tip: The order of reactivity of hydrogen halides is HI > HBr > HCl. For HBr, sodium bromide mixed with dilute sulfuric acid is commonly used in situ.

考试提示：氢卤酸的反应活性顺序为HI > HBr > HCl。制备HBr时常用溴化钠与稀硫酸现场反应。

5. Dehydration to Alkenes | 脱水生成烯烃

Alcohols undergo elimination (dehydration) when heated in the presence of an acid catalyst, producing an alkene and water. The most common conditions are concentrated sulfuric acid (H₂SO₄) at approximately 170°C, or passing alcohol vapour over heated aluminium oxide (Al₂O₃) catalyst.

醇在酸催化下加热发生消除反应（脱水），生成烯烃和水。最常用的条件是浓硫酸（H₂SO₄）在约170°C下加热，或将醇蒸气通过加热的氧化铝（Al₂O₃）催化剂。

C₂H₅OH → C₂H₄ + H₂O (conc. H₂SO₄, 170°C)

C₂H₅OH → C₂H₄ + H₂O （浓H₂SO₄，170°C）

The reaction proceeds via protonation of the –OH group, followed by loss of water and formation of a carbocation intermediate, then loss of a β-hydrogen to form the C=C double bond. For unsymmetrical alcohols, dehydration often gives a mixture of alkene isomers, with the more substituted alkene (Zaitsev’s rule) as the major product.

反应机理为：–OH基团先被质子化，随后脱去水生成碳正离子中间体，再失去β-氢形成C=C双键。对于不对称醇，脱水常得到烯烃异构体混合物，取代更多的烯烃（扎伊采夫规则）为主要产物。

It is crucial to note the temperature dependence: at 140°C, ethanol undergoes intermolecular dehydration to form ethoxyethane (diethyl ether), whereas at 170°C, intramolecular dehydration yields ethene:

务必注意温度的影响：在140°C时，乙醇发生分子间脱水生成乙氧基乙烷（乙醚）；而在170°C时，分子内脱水生成乙烯：

2C₂H₅OH → C₂H₅OC₂H₅ + H₂O (140°C, ether formation)

2C₂H₅OH → C₂H₅OC₂H₅ + H₂O （140°C，生成醚）

6. Oxidation of Primary Alcohols | 伯醇的氧化反应

Primary alcohols are oxidised first to aldehydes and then to carboxylic acids. The typical oxidising agent is acidified potassium dichromate(VI), K₂Cr₂O₇/H₂SO₄, which changes from orange to green as Cr(VI) is reduced to Cr(III).

伯醇首先被氧化为醛，再进一步氧化为羧酸。典型氧化剂是酸化的重铬酸钾(VI)，即K₂Cr₂O₇/H₂SO₄，反应中Cr(VI)被还原为Cr(III)，溶液由橙色变为绿色。

To stop at the aldehyde stage, the aldehyde must be distilled off immediately since it has a lower boiling point than the alcohol. For example, ethanol (b.p. 78°C) is oxidised to ethanal (b.p. 21°C), which distils over before it can be further oxidised.

若要停留在醛阶段，必须立即蒸馏出醛，因为醛的沸点低于醇。例如，乙醇（沸点78°C）被氧化为乙醛（沸点21°C），乙醛在进一步氧化前即被蒸馏出来。

CH₃CH₂OH + [O] → CH₃CHO + H₂O (distillation)

CH₃CH₂OH + [O] → CH₃CHO + H₂O （蒸馏条件）

If reflux conditions are used, the aldehyde is further oxidised to the carboxylic acid:

若采用回流条件，醛会进一步被氧化为羧酸：

CH₃CHO + [O] → CH₃COOH (reflux)

CH₃CHO + [O] → CH₃COOH （回流条件）

Combining these, the overall oxidation of a primary alcohol under reflux is:

综合来看，伯醇在回流条件下的总氧化反应为：

CH₃CH₂OH + 2[O] → CH₃COOH + H₂O

7. Oxidation of Secondary and Tertiary Alcohols | 仲醇与叔醇的氧化

Secondary alcohols are oxidised to ketones under the same conditions. Ketones are resistant to further oxidation because the carbonyl carbon has no hydrogen atom attached and the C–C bonds are not easily cleaved under mild conditions.

仲醇在相同条件下被氧化为酮。酮不易被进一步氧化，因为羰基碳上没有氢原子，且在温和条件下C–C键不易断裂。

Propan-2-ol + [O] → Propanone + H₂O

丙-2-醇 + [O] → 丙酮 + H₂O

CH₃CH(OH)CH₃ + [O] → CH₃COCH₃ + H₂O

Tertiary alcohols do not undergo oxidation under these conditions because the carbon bearing the –OH group has no hydrogen atom available for elimination. The solution remains orange, which is a useful way to distinguish tertiary alcohols from primary and secondary alcohols.

叔醇在这些条件下不发生氧化反应，因为连接–OH基团的碳上没有可消除的氢原子。溶液保持橙色，这是区分叔醇与伯醇、仲醇的有效方法。

Summary of oxidation products:

氧化产物汇总：

Primary alcohol → Aldehyde → Carboxylic acid
伯醇 → 醛 → 羧酸
Secondary alcohol → Ketone
仲醇 → 酮
Tertiary alcohol → No reaction (no H on the alcohol carbon)
叔醇 → 不反应（醇碳上无氢）

Colour change: orange (Cr₂O₇²⁻) → green (Cr³⁺). This is a test-tube observation you must be able to describe clearly in the exam.

颜色变化：橙色（Cr₂O₇²⁻）→ 绿色（Cr³⁺）。这是考试中你必须能清晰描述的试管观察现象。

8. Esterification | 酯化反应

Alcohols react with carboxylic acids in the presence of a strong acid catalyst (concentrated H₂SO₄) to form esters and water. This is a reversible equilibrium reaction, so reflux is used to drive the reaction towards the ester product.

醇与羧酸在强酸催化剂（浓H₂SO₄）存在下反应生成酯和水。这是一个可逆平衡反应，因此使用回流促使反应向酯的方向进行。

CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O

Ethyl ethanoate (CH₃COOC₂H₅) is a volatile, sweet-smelling liquid. In the mechanism, the acid catalyst protonates the carboxylic acid, and the alcohol attacks the carbonyl carbon; water is eliminated. Esters are named as alkyl alkanoates: the alkyl group comes from the alcohol, and the alkanoate part comes from the carboxylic acid.

乙酸乙酯（CH₃COOC₂H₅）是一种易挥发、具有甜味的液体。在机理中，酸催化剂先质子化羧酸，醇亲核进攻羰基碳，脱去一分子水。酯的命名为”烷基某酸酯”：烷基来自醇，某酸酯部分来自羧酸。

For example, methanol + butanoic acid gives methyl butanoate. Esters are widely used as flavouring agents, fragrances, and solvents, and they are the basis of biodiesel.

例如，甲醇 + 丁酸生成丁酸甲酯。酯广泛用作调味剂、香料和溶剂，也是生物柴油的基础。

9. Iodoform Reaction – Distinguishing Methyl Alcohols | 碘仿反应——区分甲基醇

The iodoform test identifies compounds containing the CH₃CH(OH)– group (or CH₃CO– group). When such an alcohol is warmed with iodine in aqueous sodium hydroxide, a pale yellow precipitate of iodoform (CHI₃) forms.

碘仿反应用于鉴别含有CH₃CH(OH)–基团（或CH₃CO–基团）的化合物。当此类醇与碘的氢氧化钠水溶液共热时，会生成淡黄色碘仿（CHI₃）沉淀。

CH₃CH(OH)R + 4I₂ + 6NaOH → CHI₃↓ + RCOONa + 5NaI + 5H₂O

Ethanol and propan-2-ol give positive iodoform tests, whereas propan-1-ol does not. This is a fast and reliable way to distinguish between isomeric alcohols.

乙醇和丙-2-醇的碘仿反应呈阳性，而丙-1-醇呈阴性。这是区分异构醇快速可靠的方法。

10. Complete Reaction Summary Table | 反应总览表

The following table consolidates all the key reactions for quick revision. You should be able to reproduce any of these equations under timed exam conditions.

下表汇总了所有关键反应，便于快速复习。你应当在限时考试条件下能够写出其中任何方程式。

Reaction Type	Reagent & Conditions	Product(s)	Observation
Combustion	O₂, ignition	CO₂ + H₂O	Blue flame, heat released
With sodium	Na metal, room temp	Alkoxide salt + H₂	Effervescence of H₂
Substitution	PCl₅ / PCl₃ / SOCl₂	Haloalkane	Misty HCl fumes (PCl₅)
Dehydration	Conc. H₂SO₄, 170°C	Alkene + H₂O	Gas bubbles of alkene
Oxidation (1°)	K₂Cr₂O₇/H⁺, reflux	Carboxylic acid	Orange → green
Oxidation (2°)	K₂Cr₂O₇/H⁺, reflux	Ketone	Orange → green
Oxidation (3°)	K₂Cr₂O₇/H⁺	No reaction	Stays orange
Esterification	RCOOH + H₂SO₄, reflux	Ester + H₂O	Sweet smell

11. Key Distinguishing Tests | 关键鉴别实验

CIE exams frequently ask how to distinguish between different alcohols or between alcohols and other functional groups. Here are the most reliable test-tube reactions.

CIE考试经常要求区分不同醇或醇与其他官能团。以下是最可靠的试管鉴别反应。

Test for –OH group: Add PCl₅ – appearance of misty white HCl fumes confirms an –OH group. Alternatively, add sodium metal – effervescence of H₂ confirms an alcoholic –OH (though carboxylic acids also react).
检验–OH基团：加入PCl₅，出现白色酸雾（HCl气体）即可确认–OH存在。或者加入金属钠，产生H₂气泡也可确认醇羟基（但羧酸也会反应）。
Distinguish 1°/2° from 3° alcohol: Warm with acidified K₂Cr₂O₇. Orange → green indicates 1° or 2°; no change indicates 3°.
区分伯/仲醇与叔醇：与酸化K₂Cr₂O₇共热。橙色变绿说明是伯醇或仲醇；不变色则为叔醇。
Distinguish 1° from 2° alcohol:Oxidation products differ – aldehyde (can be further oxidised, gives silver mirror with Tollens’ reagent) versus ketone (no silver mirror).
区分伯醇与仲醇：氧化产物不同——伯醇生成醛（可被继续氧化，与托伦试剂产生银镜），仲醇生成酮（无银镜）。
Iodoform test: Warm with I₂/NaOH – pale yellow precipitate confirms CH₃CH(OH)– group.
碘仿试验：与I₂/NaOH共热，淡黄色沉淀确认存在CH₃CH(OH)–基团。

12. Common Exam Pitfalls and Revision Checklist | 常见错误与复习清单

Students frequently lose marks on small but critical details. Here are the most common pitfalls and the correct responses.

学生常因细小但关键的问题失分。以下是常见错误及正确应对。

Pitfall: Writing the oxidation of ethanol directly to ethanoic acid without the aldehyde intermediate. Correction: Show the stepwise oxidation: ethanol → ethanal → ethanoic acid.
易错点：直接将乙醇写成氧化为乙酸而忽略醛中间体。更正：展示分步氧化：乙醇 → 乙醛 → 乙酸。
Pitfall: Using KMnO₄ when the question specifies K₂Cr₂O₇. Correction: Always match the reagent to the question. KMnO₄ (purple → colourless) is an alternative oxidant but the colour change is different.
易错点：题目指定K₂Cr₂O₇却写成KMnO₄。更正：试剂必须与题目一致。KMnO₄（紫色→无色）是替代氧化剂，但颜色变化不同。
Pitfall: Forgetting the temperature for dehydration. Correction: 170°C for alkene, 140°C for ether.
易错点：忘记脱水温度。更正：170°C生成烯烃，140°C生成醚。
Pitfall: Saying tertiary alcohols “cannot be oxidised” without mentioning the reason. Correction: The carbon bearing –OH has no hydrogen atom, so elimination to a carbonyl compound is impossible.
易错点：只说叔醇”不能被氧化”而不说明原因。更正：连接–OH的碳上没有氢原子，因此无法消除生成羰基化合物。
Pitfall: Confusing distillation and reflux conditions. Correction: Distillation removes the aldehyde product as it forms; reflux keeps everything boiling so complete oxidation to the acid occurs.
易错点：混淆蒸馏与回流条件。更正：蒸馏将生成的醛及时移出；回流使体系持续沸腾，实现完全氧化到酸。

Final checklist before the exam: (1) Can you write balanced equations for all reactions? (2) Can

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Complete Guide to Typical Reactions of Alcohols | A-Level化学：醇的典型反应归纳