📚 Alcohols | Key Points Review | 醇类考点精讲
Alcohols are a fundamental group of organic compounds in IB and OCR A-Level Chemistry, characterized by the presence of one or more hydroxyl (–OH) functional groups attached to a saturated carbon atom. Mastering their structure, properties, and reactions is essential for understanding organic synthesis, reaction mechanisms, and practical applications from fuels to pharmaceuticals.
醇是 IB 与 OCR A-Level 化学中一类基础的有机化合物，其特征是一个或多个羟基（–OH）连接在饱和碳原子上。掌握醇的结构、性质与反应对于理解有机合成、反应机理以及从燃料到药物的实际应用至关重要。
1. Classification and Nomenclature | 分类与命名
Alcohols are classified as primary (1°), secondary (2°), or tertiary (3°) based on the number of carbon atoms directly attached to the carbon bearing the –OH group. If the α‑carbon is bonded to one other carbon, it is primary; to two carbons, secondary; and to three carbons, tertiary.
依据与羟基（–OH）相连的 α‑碳原子上直接连接的碳原子数目，醇可分为一级（伯，1°）、二级（仲，2°）和三级（叔，3°）。α‑碳与 1 个碳相连为一级，与 2 个碳相连为二级，与 3 个碳相连为三级。
Systematic naming follows the suffix ‑ol. The longest carbon chain containing the –OH group is numbered to give the hydroxyl group the lowest possible locant. For example, CH₃CH₂CH₂OH is propan‑1‑ol, while CH₃CH(OH)CH₃ is propan‑2‑ol.
系统命名以 ‑醇 结尾。选择包含羟基的最长碳链，编号时使羟基位次最小。例如 CH₃CH₂CH₂OH 为 1‑丙醇，而 CH₃CH(OH)CH₃ 为 2‑丙醇。
When a molecule contains two –OH groups, it is called a diol (e.g. ethane‑1,2‑diol). Cyclic alcohols use the prefix ‘cyclo’ before the alkane name with the suffix ‑ol.
当分子中含有两个羟基时，称为二醇（如 1,2‑乙二醇）。环状醇在相应烷烃名称前加“环”字，并以“醇”结尾。
2. Physical Properties | 物理性质
The most important physical property of alcohols is their ability to form hydrogen bonds, both between alcohol molecules and with water. This gives short‑chain alcohols (methanol, ethanol, propan‑1‑ol) complete miscibility with water and significantly higher boiling points than alkanes of comparable molar mass.
醇最重要的物理特性是能够形成分子间氢键，既可发生于醇分子之间，也可与水分子形成氢键。这使得短链醇（甲醇、乙醇、1‑丙醇）与水完全互溶，其沸点远高于相对分子质量相近的烷烃。
As the hydrocarbon chain lengthens, the influence of the hydrophobic alkyl group increases, reducing water solubility. Beyond about four carbons, alcohols become only slightly soluble.
随着碳链增长，疏水性烷基的影响增大，水溶性下降。约四个碳原子以上的醇仅微溶于水。
Branching lowers boiling points because branched molecules have less surface contact and weaker van der Waals forces, although hydrogen bonding remains the dominant intermolecular force.
支链化会使沸点降低，因为支链分子接触面积减小、范德华力变弱，但氢键仍是主要的分子间作用力。
| Alcohol | Boiling point (°C) | Miscibility with water |
|---|---|---|
| Methanol | 65 | Completely miscible |
| Ethanol | 78 | Completely miscible |
| Propan‑1‑ol | 97 | Miscible |
| Butan‑1‑ol | 118 | 8.0 g/100 mL |
3. Preparation of Alcohols | 醇的制备方法
Alcohols can be synthesised through several key reactions. Hydration of alkenes is an industrial route: ethene reacts with steam over a phosphoric(V) acid catalyst (H₃PO₄) at 300°C and 60 atm to produce ethanol. An alternative in the laboratory is hydrolysis of haloalkanes by heating with aqueous sodium hydroxide.
醇可通过几种关键反应合成。烯烃的水合是一个工业路线：乙烯在 300°C 和 60 atm 下，经磷酸(V)（H₃PO₄）催化与水蒸气反应生成乙醇。实验室中则常用卤代烷与氢氧化钠水溶液加热水解来制备。
The hydration reaction follows Markownikoff’s rule when unsymmetrical alkenes are used: the –OH attaches to the more substituted carbon. For example, propene yields mainly propan‑2‑ol, not propan‑1‑ol.
当使用不对称烯烃时，水合反应遵循马氏规则：羟基加到含氢较多的碳上。例如丙烯水合主要得到 2‑丙醇，而非 1‑丙醇。
Biological preparation of ethanol by fermentation of glucose using yeast (zymase enzymes) operates under anaerobic conditions at around 35°C, producing dilute aqueous ethanol (up to ~15%) which can be concentrated by fractional distillation.
葡萄糖在酵母菌（酿酶）作用下厌氧发酵是乙醇的生物制备法，温度约 35°C，可制得稀乙醇溶液（最高约 15%），后续通过分馏浓缩。
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
4. Combustion of Alcohols | 醇的燃烧反应
Alcohols burn readily in excess oxygen to produce carbon dioxide and water, releasing large amounts of energy. This makes them valuable as fuels. Ethanol, for example, is used as a biofuel and as a petrol additive.
醇在过量氧气中容易燃烧，生成二氧化碳和水，放出大量能量，因而可作为有价值的燃料。例如乙醇被用作生物燃料及汽油添加剂。
The general equation for complete combustion of a saturated monoalcohol is: CnH2n+1OH + (3n/2)O₂ → nCO₂ + (n+1)H₂O. Because oxygen is already present in the molecule, alcohols require somewhat less oxygen per mole than the corresponding alkane.
饱和一元醇完全燃烧的通式为：CnH2n+1OH + (3n/2)O₂ → nCO₂ + (n+1)H₂O。由于分子内本身含氧，醇的耗氧量较相同碳数的烷烃稍低。
C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O
Incomplete combustion can produce carbon monoxide or soot (carbon) and is promoted by a limited supply of oxygen.
氧气不足时可能发生不完全燃烧，生成一氧化碳或碳黑。
5. Reaction with Sodium | 与金属钠的反应
Alcohols react with highly electropositive metals such as sodium, mirroring the behaviour of water but far less vigorously. The O–H bond breaks, generating an alkoxide ion and hydrogen gas.
醇可与钠等活泼金属反应，类似于水的反应但温和得多。O–H 键断裂，生成醇负离子和氢气。
2C₂H₅OH + 2Na → 2C₂H₅O⁻Na⁺ + H₂
The product is a white, crystalline solid called sodium ethoxide (CH₃CH₂ONa). The reaction can be used as a qualitative test for the –OH group, with effervescence indicating hydrogen evolution. The relative rates follow the order: water > primary alcohol > secondary alcohol > tertiary alcohol, as steric hindrance and electron‑donating alkyl groups weaken the O–H bond polarity.
产物为白色结晶固体乙醇钠（CH₃CH₂ONa）。该反应可用于羟基的定性检验，逸出的气泡表明生成氢气。反应速率顺序为：水 > 一级醇 > 二级醇 > 三级醇，因为位阻效应和给电子烷基削弱了 O–H 键的极性。
6. Oxidation of Alcohols | 醇的氧化反应
Oxidation is a key distinguishing reaction among alcohol classes. Common oxidising agents include acidified potassium dichromate(VI) (K₂Cr₂O₇/H₂SO₄). The orange Cr(VI) is reduced to green Cr(III), providing a visual colour change.
氧化反应是区分不同级醇的关键反应。常用氧化剂为酸性重铬酸钾（K₂Cr₂O₇/H₂SO₄）。反应中橙色的 Cr(VI) 被还原为绿色的 Cr(III)，产生明显颜色变化。
- Primary alcohols are oxidised first to aldehydes, which can be distilled off to prevent further oxidation; under reflux with excess oxidant, they are fully oxidised to carboxylic acids.
- 一级醇氧化首先生成醛，若需获得醛需及时蒸馏分离，以防继续氧化；在过量氧化剂和回流条件下，则完全氧化为羧酸。
CH₃CH₂OH + [O] → CH₃CHO + H₂O
CH₃CHO + [O] → CH₃COOH
- Secondary alcohols are oxidised to ketones, which resist further oxidation under normal conditions because breaking a C–C bond would be required.
- 二级醇氧化生成酮，酮在常规条件下不易进一步氧化，因为需要断裂碳–碳键。
CH₃CH(OH)CH₃ + [O] → CH₃COCH₃ + H₂O
- Tertiary alcohols do not undergo oxidation under these conditions because they lack a hydrogen atom on the α‑carbon.
- 三级醇在该条件下不氧化，因其 α‑碳上无氢原子可供脱除。
Potassium manganate(VII) (KMnO₄) can also be used; it changes from purple to colourless (Mn²⁺) in acid, or to a brown precipitate (MnO₂) in neutral/alkaline media.
也可使用高锰酸钾（KMnO₄）作为氧化剂；酸性条件下紫色变为无色 Mn²⁺，中性或碱性条件下变为棕色 MnO₂ 沉淀。
7. Esterification | 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 an equilibrium reaction, driven to the right by removal of water or use of an excess of one reagent.
醇与羧酸在浓硫酸催化下反应生成酯和水。此反应为可逆平衡反应，通过除去水或使某反应物过量可提高酯的产率。
CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O
Esters are characterised by their pleasant, fruity odours and are widely used as solvents, plasticizers, and in flavourings. The catalyst, H₂SO₄, also acts as a dehydrating agent, shifting equilibrium to the right.
酯具有令人愉悦的果香味，广泛用作溶剂、增塑剂和食用香料。催化剂浓硫酸同时起脱水作用，使平衡向右移动。
Acid anhydrides can also be used to form esters, giving a higher yield and no reversible water product. For example, ethanol reacts with ethanoic anhydride to form ethyl ethanoate and ethanoic acid.
酸酐也可与醇反应生成酯，产率更高且不生成可逆的水。例如乙醇与乙酸酐反应生成乙酸乙酯和乙酸。
8. Reaction with Hydrogen Halides | 与氢卤酸的反应
Alcohols undergo nucleophilic substitution with hydrogen halides (HCl, HBr, HI) or with phosphorus halides to form haloalkanes. For hydrogen halides, the reactivity order is HI > HBr > HCl, and tertiary alcohols react much faster than primary ones.
醇可与氢卤酸（HCl, HBr, HI）或卤化磷发生亲核取代反应，生成卤代烷。氢卤酸的反应活性顺序为 HI > HBr > HCl，三级醇反应速率远快于一级醇。
For HCl, a mixture of concentrated HCl and anhydrous ZnCl₂ (Lucas reagent) is often used for tertiary and secondary alcohols; primary alcohols react only upon prolonged heating. The reaction proceeds via an SN1 mechanism for tertiary alcohols and SN2 for primary ones.
使用 HCl 时，常需要浓盐酸与无水 ZnCl₂ 混合（Lucas 试剂）来引发二级和三级醇的反应；一级醇需长时间加热。三级醇按 SN1 机理进行，一级醇按 SN2 机理进行。
C₂H₅OH + HBr → C₂H₅Br + H₂O
Phosphorus tribromide (PBr₃) and phosphorus pentachloride (PCl₅) are alternative halogenating agents that give cleaner products, often with a visible fume of HCl in the case of PCl₅.
三溴化磷（PBr₃）和五氯化磷（PCl₅）也可用作卤化试剂，产率较高；后者反应时有 HCl 白雾生成。
9. Dehydration (Elimination) | 脱水（消除）反应
Alcohols can be dehydrated to alkenes by heating with a concentrated acid catalyst, usually H₂SO₄ or H₃PO₄. The elimination follows Zaitsev’s rule, where the more substituted, thermodynamically stable alkene predominates.
醇在浓酸催化（通常是浓硫酸或浓磷酸）下加热脱水可生成烯烃。消除反应遵循扎伊采夫规则，生成取代更多、热力学更稳定的烯烃为主产物。
For ethanol, dehydration requires heating to 170°C in excess concentrated sulfuric acid, yielding ethene. If the temperature is lower (about 140°C), intermolecular dehydration produces diethyl ether instead (not an alkene), which is a common pitfall in exam questions.
乙醇脱水需在过量浓硫酸中加热至 170°C 才能生成乙烯。若温度较低（约 140°C），则发生分子间脱水生成乙醚（非烯烃），这是考试中的常见陷阱。
C₂H₅OH → C₂H₄ + H₂O (170°C, conc. H₂SO₄)
2C₂H₅OH → C₂H₅OC₂H₅ + H₂O (140°C)
The mechanism for tertiary alcohols is E1 (carbocation intermediate), whereas primary alcohols often proceed via E2 under these conditions. Aluminium oxide (Al₂O₃) at high temperature can also be used as a dehydration catalyst, offering better control.
三级醇脱水为 E1 机理（碳正离子中间体），一级醇在此条件下常经历 E2 机理。高温下氧化铝（Al₂O₃）也可用作脱水催化剂，条件更易控制。
10. Distinguishing Between Alcohol Classes | 鉴别醇的类别
Two standard laboratory tests differentiate the three classes of alcohols:
两种标准实验室方法可区分三级醇：
- Lucas test: The alcohol is shaken with Lucas reagent (ZnCl₂ / conc. HCl) at room temperature. Tertiary alcohols give an immediate cloudiness (formation of insoluble chloroalkane); secondary alcohols turn cloudy after heating; primary alcohols remain clear unless heated for a prolonged time.
- Lucas 试验：将醇与 Lucas 试剂（ZnCl₂/浓盐酸）在室温下摇动。三级醇立即出现混浊（生成不溶性氯代烷）；二级醇加热后变混浊；一级醇即使加热也需很长时间才变混浊。
- Oxidation test: Warm the alcohol with acidified potassium dichromate(VI). Primary and secondary alcohols turn the solution from orange to green; tertiary alcohols leave the orange colour unchanged. In addition, the product of a primary alcohol can be tested for an aldehyde (e.g. with Tollens’ reagent or Fehling’s solution) to confirm its class.
- 氧化试验：将醇与酸性重铬酸钾溶液温热。一级和二级醇使溶液由橙色变为绿色；三级醇不使其变色。此外，一级醇的氧化产物可用银镜反应（Tollens 试剂）或斐林试剂检测醛基，以进一步确认。
Both tests exploit the structural differences that influence the rate and outcome of nucleophilic substitution or oxidation.
两个试验均利用了结构差异对亲核取代或氧化反应速率与结果的影响。
11. Bioethanol and Sustainability | 生物乙醇与可持续性
Ethanol produced by fermentation of biomass (sugar cane, corn, etc.) is called bioethanol and is a renewable fuel. Its combustion is carbon‑neutral in principle: the CO₂ released was recently absorbed from the atmosphere by the growing plants. However, the full lifecycle must account for energy inputs in farming, distillation, and transport, which can reduce its overall carbon advantage.
由生物质（甘蔗、玉米等）发酵制取的乙醇称为生物乙醇，是一种可再生燃料。其燃烧原则上属于碳中和：释放的 CO₂ 是植物生长过程中刚从大气中吸收的。但全生命周期需考量种植、蒸馏和运输中的能量投入，这可能减少其净碳优势。
Blends of petrol with ethanol (e.g. E10 – 10% ethanol) reduce fossil fuel consumption and improve octane ratings. However, concerns about food‑versus‑fuel, deforestation, and water use must be critically evaluated.
乙醇与汽油的混合燃料（如 E10 – 含 10% 乙醇）可减少化石燃料消耗并提升辛烷值。然而，食物与燃料之争、砍伐森林与水资源消耗等问题需要辩证评估。
In the laboratory, ethanol can also be produced from ethene via hydration – a faster, continuous process that yields pure ethanol, but relies on non‑renewable crude oil fractions.
实验室中也可由乙烯水合制取乙醇——这是一种更快、连续化的过程，可获得纯乙醇，但依赖不可再生的石油馏分。
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