A-Level Chemistry: Alcohols – Key Concepts & Reactions | A-Level 化学：醇考点与反应精讲

📚 A-Level Chemistry: Alcohols – Key Concepts & Reactions | A-Level 化学：醇考点与反应精讲

Alcohols are a fundamental homologous series in A-Level Chemistry, featuring the hydroxyl (-OH) functional group. They appear in numerous exam questions covering nomenclature, physical properties, characteristic reactions and interconversions with other functional groups. This article consolidates all key concepts and reaction conditions you need to master for top marks.

醇是 A-Level 化学中一个基础的同系物，其特征官能团为羟基（-OH）。它们在考题中频繁出现，涉及命名、物理性质、特征反应以及与其他官能团之间的相互转化。本文整合了所有你需要掌握的核心概念和反应条件，助你夺取高分。

1. Structure and Classification | 结构与分类

Alcohols are organic compounds containing one or more hydroxyl (-OH) groups attached to a saturated carbon atom. The general formula for saturated monohydric alcohols is CₙH₂ₙ₊₁OH. Based on the number of carbon atoms directly bonded to the carbon bearing the -OH group, alcohols are classified as primary (1°), secondary (2°) or tertiary (3°).

醇是含有连接在饱和碳原子上的一个或多个羟基（-OH）的有机化合物。饱和一元醇的通式为 CₙH₂ₙ₊₁OH。根据与连有 -OH 基团的碳原子直接相连的碳原子数目，醇可分为伯醇（1°）、仲醇（2°）或叔醇（3°）。

In a primary alcohol the hydroxyl-bearing carbon is attached to only one other carbon (e.g. ethanol, CH₃CH₂OH). In a secondary alcohol it is attached to two other carbons (e.g. propan-2-ol, (CH₃)₂CHOH). In a tertiary alcohol it is attached to three other carbons (e.g. 2-methylpropan-2-ol, (CH₃)₃COH). This classification is central to predicting oxidation products and substitution mechanisms.

在伯醇中，带有羟基的碳只与另一个碳原子相连（如乙醇 CH₃CH₂OH）。在仲醇中，它与另外两个碳原子相连（如丙-2-醇 (CH₃)₂CHOH）。在叔醇中，它与另外三个碳原子相连（如2-甲基丙-2-醇 (CH₃)₃COH）。这种分类对于预测氧化产物和取代反应机理至关重要。

2. Nomenclature | 命名法

According to IUPAC, the longest continuous carbon chain carrying the -OH group is chosen as the parent. Number the chain so that the carbon attached to -OH receives the lowest possible locant. The name ends with the suffix ‘-ol’, preceded by the position number if necessary. If there are multiple -OH groups, suffixes like ‘-diol’ or ‘-triol’ are used.

根据 IUPAC 规则，选取包含 -OH 基团的最长连续碳链为主链。给主链编号时，使连接 -OH 的碳原子获得尽可能小的位次。名称以 ‘-ol’ 结尾，必要时在词尾前标出位次数字。如有多个 -OH 基团，则使用 ‘-diol’ 或 ‘-triol’ 等后缀。

Examples: CH₃CH₂OH is ethanol. CH₃CH₂CH₂OH is propan-1-ol, while CH₃CHOHCH₃ is propan-2-ol. (CH₃)₂CHCH₂OH is 2-methylpropan-1-ol. A diol such as HOCH₂CH₂OH is ethane-1,2-diol.

示例：CH₃CH₂OH 是乙醇。CH₃CH₂CH₂OH 是丙-1-醇，而 CH₃CHOHCH₃ 是丙-2-醇。(CH₃)₂CHCH₂OH 是 2-甲基丙-1-醇。二元醇如 HOCH₂CH₂OH 是乙-1,2-二醇。

3. Physical Properties | 物理性质

Compared with alkanes of similar relative molecular mass, alcohols have much higher boiling points. This is because they can form hydrogen bonds between their -OH groups, requiring extra energy to overcome. The O-H bond also makes short-chain alcohols completely miscible with water, as they can hydrogen-bond with water molecules.

与相对分子质量相近的烷烃相比，醇的沸点高得多。这是因为醇分子间的 -OH 基团能够形成氢键，需要额外的能量来破坏。O-H 键也使短链醇可以与水以任意比例互溶，因为它们能与水分子形成氢键。

As the hydrocarbon chain lengthens, the influence of the hydrophobic alkyl group increases, and solubility in water decreases. In infrared spectroscopy, the O-H stretching vibration gives a characteristic broad, strong absorption around 3230–3550 cm⁻¹; the C-O stretch appears in the fingerprint region (1000–1300 cm⁻¹).

随着碳氢链增长，疏水烷基的影响增大，在水中的溶解度降低。在红外光谱中，O-H 伸缩振动在约 3230–3550 cm⁻¹ 处产生一个宽而强的特征吸收；C-O 伸缩振动出现在指纹区（1000–1300 cm⁻¹）。

4. Preparation of Alcohols | 醇的制备

Ethanol can be produced industrially by hydration of ethene. Direct hydration uses steam and a phosphoric(V) acid catalyst at around 300 °C and 60 atm: CH₂=CH₂ + H₂O ⇌ CH₃CH₂OH. An indirect route treats ethene with concentrated sulfuric acid to form ethyl hydrogensulfate, followed by hydrolysis.

乙醇可通过乙烯的水合进行工业生产。直接水合法使用水蒸气和磷酸（V）催化剂，在约 300 °C、60 atm 下进行：CH₂=CH₂ + H₂O ⇌ CH₃CH₂OH。间接法先将乙烯与浓硫酸反应生成硫酸氢乙酯，再经水解。

Fermentation of sugars, catalysed by enzymes in yeast, is the traditional route to ethanol: C₆H₁₂O₆(aq) → 2 CH₃CH₂OH(aq) + 2 CO₂(g). The process is carried out under anaerobic conditions at 30–40 °C. It produces a dilute aqueous solution that is then concentrated by distillation. Ethanol obtained this way is a renewable fuel, but requires subsequent purification for many uses.

糖的发酵（由酵母中的酶催化）是制备乙醇的传统途径：C₆H₁₂O₆(aq) → 2 CH₃CH₂OH(aq) + 2 CO₂(g)。该过程在 30–40 °C 的厌氧条件下进行，得到稀溶液，再通过蒸馏浓缩。由此得到的乙醇是一种可再生燃料，但用于多种用途时需后续提纯。

5. Combustion | 燃烧

Alcohols burn readily in excess oxygen to give carbon dioxide and water, releasing large amounts of energy. For example, the complete combustion of ethanol: CH₃CH₂OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(g). Their high enthalpy of combustion makes alcohols useful as fuels and fuel additives.

醇在过量氧气中很容易燃烧，生成二氧化碳和水，并释放大量能量。例如，乙醇的完全燃烧：CH₃CH₂OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(g)。燃烧焓较高使得醇可用作燃料和燃料添加剂。

In a limited supply of oxygen, incomplete combustion can occur, producing carbon monoxide and/or soot (carbon). This is why alcohols should be burned in properly ventilated appliances.

在氧气不足时，可能发生不完全燃烧，产生一氧化碳和/或碳烟。因此醇类应当在通风良好的器具中燃烧。

6. Oxidation Reactions | 氧化反应

The oxidation behaviour of an alcohol depends critically on its class. Common oxidising agent: acidified potassium dichromate(VI) solution, K₂Cr₂O₇/H₂SO₄, which turns from orange (Cr₂O₇²⁻) to green (Cr³⁺).

醇的氧化行为关键取决于其类别。常用氧化剂：酸化重铬酸钾溶液 (K₂Cr₂O₇/H₂SO₄)，由橙色 (Cr₂O₇²⁻) 变为绿色 (Cr³⁺)。

Primary alcohols are oxidised initially to aldehydes and can be oxidised further to carboxylic acids. To isolate the aldehyde, the alcohol is warmed with oxidising agent and the aldehyde is distilled out immediately to prevent further oxidation. To obtain the carboxylic acid, the mixture is heated under reflux with excess oxidiser.

伯醇首先被氧化成醛，还可进一步氧化成羧酸。为得到醛，将醇与氧化剂温热并立即将生成的醛蒸馏出去，以防止继续氧化。为得到羧酸，则需用过量的氧化剂加热回流。

CH₃CH₂OH + [O] → CH₃CHO + H₂O (aldehyde); CH₃CHO + [O] → CH₃COOH (followed by oxidation to ethanoic acid). Secondary alcohols are oxidised to ketones; they cannot be oxidised further under the same conditions because there is no hydrogen atom available on the carbonyl carbon. Tertiary alcohols resist oxidation under these conditions – the dichromate solution remains orange.

CH₃CH₂OH + [O] → CH₃CHO + H₂O（生成醛）；再氧化为 CH₃COOH（乙酸）。仲醇被氧化为酮；因羰基碳上没有可被氧化的氢，它们在同等条件下不能被进一步氧化。叔醇在上述条件下不被氧化——重铬酸钾溶液保持橙色。

7. Elimination to Alkenes | 消去制烯

Alcohols undergo dehydration when heated with an excess of concentrated sulfuric acid or phosphoric acid. The -OH group and a hydrogen atom on an adjacent carbon are removed as water, forming a C=C double bond.

醇与过量浓硫酸或磷酸共热时发生脱水反应。 -OH 基团与相邻碳上的氢原子以水的形式脱去，形成碳碳双键。

The reaction of ethanol at 170 °C with concentrated H₂SO₄ produces ethene: CH₃CH₂OH → CH₂=CH₂ + H₂O. If a mixture of propan-1-ol and propan-2-ol is used, propene is the major product, following Zaitsev’s rule (the more substituted alkene is favoured). This elimination proceeds via carbocation formation; thus secondary and tertiary alcohols dehydrate more readily than primary alcohols.

乙醇在 170 °C 下与浓 H₂SO₄ 反应生成乙烯：CH₃CH₂OH → CH₂=CH₂ + H₂O。如果用丙-1-醇和丙-2-醇的混合物，主要产物是丙烯，遵循扎伊采夫规则（倾向于生成取代基较多的烯烃）。该消去反应经由碳正离子中间体进行，因此仲醇和叔醇比伯醇更易脱水。

8. Substitution with Hydrogen Halides | 与氢卤酸取代

Alcohols react with hydrogen halides to form halogenoalkanes and water. The reactivity of hydrogen halides is HI > HBr > HCl. For HCl, a zinc chloride catalyst (Lucas reagent) is often needed to speed up the reaction with primary and secondary alcohols.

醇与氢卤酸反应生成卤代烷和水。氢卤酸的反应活性顺序为 HI > HBr > HCl。对于 HCl，常需用氯化锌催化剂（卢卡斯试剂）来加速伯醇和仲醇的反应。

CH₃CH₂OH + HBr → CH₃CH₂Br + H₂O (heating under reflux). HBr can be generated in situ by using NaBr with concentrated H₂SO₄. Tertiary alcohols react rapidly with concentrated HCl at room temperature via an SN1 mechanism, while primary alcohols favour SN2. The different rates of reaction with Lucas reagent (cloudiness due to immiscible alkyl chloride) allow simple differentiation between 1°, 2° and 3° alcohols.

CH₃CH₂OH + HBr → CH₃CH₂Br + H₂O（加热回流）。HBr 可用 NaBr 与浓 H₂SO₄ 在体系中生成。叔醇在室温下与浓 HCl 迅速反应（SN1 机理），而伯醇倾向于 SN2 历程。利用卢卡斯试剂反应速率的不同（生成不可混溶的氯代烷而变浑浊），可简便区分伯、仲、叔醇。

9. Esterification | 酯化反应

Alcohols react with carboxylic acids in the presence of an acid catalyst (typically concentrated sulfuric acid) to form esters. The reaction is reversible, and heating under reflux is used to increase the rate.

醇与羧酸在酸催化剂（通常是浓硫酸）存在下反应生成酯。该反应是可逆的，通过加热回流来提高速率。

Example: CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O (ethyl ethanoate). The concentrated sulfuric acid acts as both a catalyst and a dehydrating agent to shift the equilibrium in favour of the ester. Esters have characteristic sweet smells and are widely used as solvents, plasticisers and flavourings.

示例：CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O（乙酸乙酯）。浓硫酸既作催化剂也作脱水剂，促使平衡向生成酯的方向移动。酯类具有特征性的甜香气味，广泛用作溶剂、增塑剂和食用香精。

10. Reaction with Sodium | 与金属钠反应

Alcohols behave as very weak acids and react with highly electropositive metals such as sodium. The metal displaces hydrogen gas and forms an alkoxide.

醇表现出极弱的酸性，能与电正性很高的金属（如钠）反应。金属置换出氢气并生成醇盐。

2 C₂H₅OH(l) + 2Na(s) → 2 C₂H₅ONa(s) + H₂(g). The reaction is less vigorous than the reaction of sodium with water because the O-H bond in an alcohol is less polarised and the alkoxide ion is a stronger base. This reaction demonstrates the acidic character of the -OH group and is a useful way to generate alkoxide nucleophiles for further synthesis.

2 C₂H₅OH(l) + 2Na(s) → 2 C₂H₅ONa(s) + H₂(g)。此反应不如钠与水反应剧烈，因为醇中 O-H 键的极性较弱，且醇盐离子碱性更强。该反应体现了 -OH 的酸性，也提供了一个制备醇盐亲核试剂以用于后续合成的途径。

11. Tests and Distinguishing Alcohols | 醇的鉴别与区分

Several tests can identify an alcohol and differentiate between primary, secondary and tertiary isomers.

Acidified potassium dichromate(VI): turns from orange to green with primary and secondary alcohols on warming, but remains orange with tertiary alcohols. For primary alcohols the green product is rapid; secondary alcohols also give a colour change.
Lucas test: involves shaking the alcohol with Lucas reagent (ZnCl₂ in concentrated HCl) at room temperature. Tertiary alcohols give immediate cloudiness, secondary alcohols become cloudy within 5–10 minutes, while primary alcohols show no reaction unless heated.
Iodoform test: alcohols containing a CH₃CH(OH)- group (ethanol, propan-2-ol, etc.) react with iodine in alkaline solution to form a yellow precipitate of triiodomethane (iodoform, CHI₃). This test distinguishes ethanol from propan-1-ol, and propan-2-ol from other secondary alcohols lacking that group.
Infrared spectroscopy: a broad O-H peak around 3230–3550 cm⁻¹ and a C-O stretch near 1000–1300 cm⁻¹ confirm the presence of the hydroxyl group.

多种测试可鉴定醇，并可区分伯、仲、叔醇的异构体。

酸化重铬酸钾：与伯醇、仲醇在温热后由橙色变为绿色，而叔醇保持橙色。伯醇的变色迅速；仲醇同样变色。
卢卡斯测试：将醇与卢卡斯试剂（ZnCl₂ 在浓 HCl 中）在室温下振摇。叔醇立即出现浑浊，仲醇在 5–10 分钟内变浑浊，伯醇除非加热否则无明显反应。
碘仿测试：含有 CH₃CH(OH)- 基团的醇（如乙醇、丙-2-醇等）与碘的碱性溶液反应生成黄色三碘甲烷（碘仿，CHI₃）沉淀。该测试能将乙醇与丙-1-醇区分，也能将丙-2-醇与其他不含该基团的仲醇区分。
红外光谱：在约 3230–3550 cm⁻¹ 处的宽 O-H 峰以及 1000–1300 cm⁻¹ 附近的 C-O 伸缩振动峰可确认羟基的存在。

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A-Level Chemistry: Alcohols – Key Concepts & Reactions | A-Level 化学：醇 考点与反应精讲