📚 Alcohols for IB WJEC Chemistry | IB WJEC 化学：醇 考点精讲
Alcohols are a fundamental class of organic compounds characterised by the presence of one or more hydroxyl (−OH) functional groups attached to a saturated carbon atom. In IB and WJEC Chemistry, you are expected to understand their structure, classification, naming, physical properties, key reactions, and methods of synthesis. This article will systematically cover each of these areas to help you prepare for examinations and develop a deep conceptual understanding of alcohol chemistry.
醇是一类基本的有机化合物，其特征是在饱和碳原子上连有一个或多个羟基(−OH)官能团。在IB和WJEC化学考试中，你需要掌握醇的结构、分类、命名、物理性质、关键反应以及合成方法。本文将系统地梳理这些知识点，帮助你在备考的同时建立对醇化学的深刻理解。
1. Definition and Functional Group | 定义与官能团
An alcohol is an organic molecule in which a hydroxyl group (–OH) is bonded to a saturated (sp³ hybridised) carbon atom. The general formula for a monohydric saturated alcohol is CₙH₂ₙ₊₁OH. The hydroxyl group is polar due to the high electronegativity difference between oxygen and hydrogen, which strongly influences both physical and chemical properties.
醇是指羟基(−OH)连接在饱和(sp³杂化)碳原子上的有机分子。饱和一元醇的通式为CₙH₂ₙ₊₁OH。由于氧和氢之间较大的电负性差异，羟基具有极性，这深刻影响了醇的物理和化学性质。
2. Classification: Primary, Secondary, Tertiary | 分类：伯醇、仲醇、叔醇
Alcohols are classified according to the number of carbon atoms directly attached to the carbon bearing the –OH group. A primary (1°) alcohol has the −OH group on a carbon atom that is bonded to only one other carbon atom (e.g., ethanol CH₃CH₂OH). A secondary (2°) alcohol has the hydroxyl carbon attached to two other carbons (e.g., propan-2-ol). A tertiary (3°) alcohol has the hydroxyl carbon attached to three other carbons (e.g., 2-methylpropan-2-ol).
醇按照直接与带–OH的碳原子相连的碳原子数进行分类。伯(1°)醇的–OH连在一个仅与一个其他碳原子相连的碳上(如乙醇 CH₃CH₂OH)。仲(2°)醇的羟基碳上连有两个其他碳(如丙-2-醇)。叔(3°)醇的羟基碳上连有三个其他碳(如2-甲基丙-2-醇)。
This classification is crucial because the behaviour of alcohols in oxidation reactions depends directly on whether they are primary, secondary, or tertiary. Primary alcohols can be oxidised to aldehydes and then to carboxylic acids; secondary alcohols oxidise to ketones; tertiary alcohols resist oxidation under normal conditions.
这个分类至关重要，因为醇在氧化反应中的行为直接取决于它们是伯醇、仲醇还是叔醇。伯醇可被氧化成醛，进而氧化成羧酸；仲醇被氧化成酮；叔醇在常规条件下难以被氧化。
3. Nomenclature | 命名法
In systematic IUPAC naming, select the longest carbon chain that contains the –OH group and replace the final '–e' of the corresponding alkane with '–ol'. Number the chain to give the hydroxyl carbon the lowest possible locant. Substituents are named and numbered as usual. For example, CH₃CH(OH)CH₃ is propan-2-ol, and CH₃CH₂CH₂OH is propan-1-ol. When multiple –OH groups are present, suffixes such as '–diol' or '–triol' are used (e.g., ethane-1,2-diol).
按照系统命名法，选择含有–OH的最长碳链，将相应烷烃的词尾“烷”改为“醇”。从离羟基最近的一端开始给碳链编号，使羟基所连碳的位次最小。取代基按常规方式命名和编号。例如CH₃CH(OH)CH₃称为丙-2-醇，CH₃CH₂CH₂OH则为丙-1-醇。若含多个羟基，使用“二醇”“三醇”等后缀(如乙-1,2-二醇)。
4. Physical Properties and Hydrogen Bonding | 物理性质与氢键
Alcohols exhibit significantly higher boiling points than analogous alkanes or haloalkanes of similar molar mass. This is mainly because alcohol molecules can form intermolecular hydrogen bonds between the hydrogen of one –OH group and the lone pair of oxygen on another molecule. The energy required to overcome these hydrogen bonds leads to elevated boiling points.
与摩尔质量相近的烷烃或卤代烷相比，醇的沸点明显更高。这主要是因为醇分子间可以在一个–OH基的氢与另一个分子氧上的孤对电子之间形成氢键。克服这些氢键需要的能量更高，因而沸点升高。
Solubility in water is another key physical property. The short-chain alcohols (methanol, ethanol, propan-1-ol, propan-2-ol) are fully miscible with water due to hydrogen bonding between alcohol and water molecules. As the hydrophobic alkyl chain lengthens, the influence of the hydroxyl group diminishes, and solubility decreases; long-chain alcohols become virtually insoluble in water.
在水中的溶解性是另一项关键物理性质。短链醇(甲醇、乙醇、丙-1-醇、丙-2-醇)由于醇与水分子间形成氢键而能与水完全混溶。随着憎水烷基链增长，羟基的影响减弱，溶解度下降；长链醇几乎不溶于水。
5. Reaction with Reactive Metals | 与活泼金属的反应
Alcohols react with electropositive metals, such as sodium, to form alkoxide salts and hydrogen gas. The reaction is similar to that of water with sodium, but less vigorous. For example, ethanol reacts with sodium to produce sodium ethoxide and hydrogen:
醇能与钠等活泼金属反应，生成醇盐和氢气。反应类似于水与钠的反应，但较为平缓。例如乙醇与钠反应生成乙醇钠和氢气：
2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂
The alkoxide ion is a strong base and a good nucleophile, making this reaction of synthetic importance. It also provides a simple laboratory test for the presence of an –OH group: effervescence of hydrogen gas can be observed.
醇盐离子是强碱和良好的亲核试剂，这一反应具有合成价值。同时，它也为实验室检测羟基的存在提供了一个简单的方法：可以观察到氢气泡的产生。
6. Oxidation Reactions | 氧化反应
Oxidation of alcohols is one of the most frequently tested areas. Mild oxidising agents such as acidified potassium dichromate(VI) (K₂Cr₂O₇/H₂SO₄) are used. The characteristic colour change from orange (Cr₂O₇²⁻) to green (Cr³⁺) indicates oxidation has taken place.
醇的氧化是考试中最高频的考点之一。通常使用酸化重铬酸钾(K₂Cr₂O₇/H₂SO₄)等温和氧化剂。溶液由橙色(Cr₂O₇²⁻)变为绿色(Cr³⁺)的典型颜色变化表明反应发生了。
Primary alcohols are first oxidised to aldehydes, which can be isolated by distillation to prevent further oxidation. Under stronger conditions (heating under reflux with excess oxidant), the aldehyde is further oxidised to a carboxylic acid. Secondary alcohols are oxidised to ketones. Tertiary alcohols do not react with acidified dichromate under normal conditions due to the absence of a hydrogen atom on the carbon bearing the –OH group.
伯醇首先被氧化成醛，可通过蒸馏将醛分离出来以防进一步氧化。在较剧烈的条件下(过量氧化剂加热回流)，醛进一步氧化成羧酸。仲醇氧化生成酮。叔醇由于带–OH的碳上没有可被脱去的氢，在常规条件下不与酸化重铬酸盐反应。
The oxidation of primary alcohols to aldehydes and carboxylic acids can be summarised:
伯醇氧化为醛和羧酸的反应可概括为：
RCH₂OH + [O] → RCHO + H₂O
RCHO + [O] → RCOOH
7. Esterification | 酯化反应
Alcohols react with carboxylic acids in the presence of an acid catalyst (usually concentrated sulfuric acid) to form esters and water. This reversible condensation reaction is known as Fischer esterification. For instance, ethanol and ethanoic acid produce ethyl ethanoate:
醇在酸催化剂(通常为浓硫酸)作用下与羧酸反应，生成酯和水。这种可逆的缩合反应称为费歇尔酯化。例如，乙醇与乙酸反应生成乙酸乙酯：
CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O
Esters have distinctive sweet, fruity smells and are widely used as solvents, flavourings, and in polyester production. The reaction is important both synthetically and analytically; formation of a fragrant ester can confirm the presence of an alcohol or acid.
酯具有特征性的甜果香气，广泛用作溶剂、食用香精以及聚酯的生产。该反应在合成和分析上都很重要；生成有香味的酯可以确证醇或酸的存在。
8. Dehydration: Alcohol to Alkene | 脱水反应：醇消除成烯
Alcohols undergo dehydration (elimination of water) when heated with a concentrated acid catalyst, typically concentrated phosphoric(V) acid or concentrated sulfuric acid, yielding alkenes. The reaction proceeds via a protonated alcohol intermediate and follows Zaitsev's rule, where the more substituted alkene is the major product. For example, ethanol dehydrates to ethene:
醇在浓酸催化剂(通常是浓磷酸或浓硫酸)加热下发生脱水(消去水)反应，生成烯烃。反应通过质子化醇中间体进行，并遵循札依采夫规则，即取代基多的烯烃为主要产物。例如，乙醇脱水生成乙烯：
C₂H₅OH → C₂H₄ + H₂O
This reaction is particularly useful for converting bioethanol into ethene, which is a key starting material for the polymer industry. Tertiary alcohols dehydrate more readily than secondary, which in turn dehydrate more readily than primary.
该反应在将生物乙醇转化为乙烯方面特别有用，乙烯是聚合物工业的关键原料。叔醇比仲醇更容易脱水，伯醇最难。
9. Reaction with Hydrogen Halides | 与氢卤酸反应
Alcohols react with hydrogen halides (HCl, HBr, HI) to form haloalkanes. The reaction is typically carried out by heating the alcohol with sodium bromide and concentrated sulfuric acid (generating HBr in situ), or by passing dry hydrogen halide gas through the alcohol. The hydroxyl group is protonated and then replaced by the halide ion through an Sₙ¹ or Sₙ² mechanism, depending on the alcohol structure.
醇与氢卤酸(HCl, HBr, HI)反应生成卤代烷。实验中通常将醇与溴化钠和浓硫酸一起加热(原位生成HBr)，或者将干燥的卤化氢气体通入醇中。羟基先被质子化，然后根据醇的结构以Sₙ¹或Sₙ²机理被卤离子取代。
Tertiary alcohols react rapidly via Sₙ¹ at room temperature, while primary alcohols require heating and proceed mainly via Sₙ². This reaction is a convenient way to prepare haloalkanes and also provides evidence for the reactivity order of alcohols.
叔醇室温下即可通过Sₙ¹机理快速反应，伯醇则需要加热且主要以Sₙ²历程进行。该反应是制备卤代烷的便捷方法，同时也为醇的反应活性顺序提供了证据。
10. Iodoform Reaction (Triiodomethane Test) | 碘仿反应（碘仿试验）
The iodoform test is used to identify the presence of a CH₃CH(OH)– group (i.e., a methyl group next to the carbon bearing –OH) or compounds that can be oxidised to this structure. When a secondary alcohol with a methyl group on the hydroxyl carbon (such as ethanol or any secondary alcohol of the form CH₃CH(OH)R) reacts with iodine in the presence of base, a pale yellow precipitate of triiodomethane (iodoform, CHI₃) is formed. This reaction is also positive for acetaldehyde and methyl ketones.
碘仿试验用于检测含有CH₃CH(OH)–基团(即带–OH的碳旁边有一个甲基)或能被氧化成此结构的化合物。当连有甲基的羟基碳上的仲醇(如乙醇或任何CH₃CH(OH)R结构的仲醇)在碱存在下与碘反应，会生成淡黄色的三碘甲烷(碘仿，CHI₃)沉淀。该反应对乙醛和甲基酮也呈阳性。
CH₃CH₂OH + 4I₂ + 6NaOH → CHI₃ + HCOONa + 5NaI + 5H₂O
This test is particularly used to distinguish ethanol from methanol and other primary alcohols lacking a methyl group on the α‑carbon.
该试验尤其用于区分乙醇与甲醇以及其他α‑碳上无甲基的伯醇。
11. Methods of Preparation | 制备方法
Several methods exist for the laboratory and industrial preparation of alcohols. Nucleophilic substitution of haloalkanes by aqueous hydroxide ions is a common method: heating a haloalkane with NaOH(aq) yields the corresponding alcohol. For example:
醇的实验和工业生产方法有多种。卤代烷与氢氧根离子水溶液发生亲核取代是常用的方法：将卤代烷与NaOH水溶液共热得到相应的醇。例如：
C₂H₅Br + NaOH(aq) → C₂H₅OH + NaBr
Hydration of alkenes using steam and an acid catalyst (such as phosphoric acid on a solid support) is the principal industrial route to ethanol and other alcohols. This addition reaction follows Markovnikov's rule for unsymmetrical alkenes.
烯烃在酸催化剂(如负载磷酸)存在下水蒸气加成是工业上生产乙醇及其他醇的主要途径。该加成反应对于不对称烯烃遵循马氏规则。
Fermentation of sugars by yeast is a biological method that produces ethanol. This process is central to the production of alcoholic beverages and biofuels. The reaction can be summarised as:
酵母发酵糖类是生产乙醇的生物方法。这一过程是酒精饮料和生物燃料生产的核心。反应可概括为：
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
Methanol is manufactured industrially by the catalytic reaction of carbon monoxide with hydrogen (synthesis gas) under high pressure and temperature.
甲醇工业上通过一氧化碳与氢气(合成气)在高压高温下催化反应制得。
12. Identification and Analysis | 鉴定与分析
Alcohols can be distinguished through a combination of chemical tests and spectroscopic techniques. The use of acidified dichromate allows you to differentiate between primary/secondary and tertiary alcohols based on colour change. The Lucas test (reaction with ZnCl₂ in conc. HCl) can further classify low-molar-mass alcohols by the rate of cloudiness due to haloalkane formation: tertiary alcohols react immediately, secondary within minutes, and primary only upon heating.
醇可以通过化学测试和光谱技术相结合的途径进行区分。利用酸化重铬酸盐，根据颜色变化可以区分伯醇/仲醇与叔醇。卢卡斯试剂(氯化锌的浓盐酸溶液)通过生成卤代烷出现浑浊的速率为低分子量醇提供进一步分类：叔醇立即反应变浑浊，仲醇几分钟内，伯醇则需要加热。
Infrared spectroscopy shows a broad O–H stretching absorption around 3200–3600 cm⁻¹ (broad for hydrogen‑bonded alcohols) and a C–O stretch near 1000–1300 cm⁻¹. In mass spectrometry, alcohols often fragment via α‑cleavage, and the molecular ion peak may be weak. ¹H NMR and ¹³C NMR provide detailed information about the chemical environment of the hydroxyl proton and the carbon skeleton.
红外光谱中，醇在3200–3600 cm⁻¹处(氢键醇呈宽峰)显示O–H伸缩振动宽峰，C–O伸缩振动在1000–1300 cm⁻¹左右。在质谱中，醇常通过α‑断裂产生碎片，分子离子峰可能较弱。核磁共振氢谱和碳谱可以提供羟基氢和碳骨架所处化学环境的详细信息。
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