📚 Alcohols | 醇 考点精讲
Alcohols are among the most important organic compounds in chemistry, featuring the hydroxyl (–OH) functional group attached to a saturated carbon atom. This article covers the essential knowledge required for CCEA A-Level and IB Chemistry, ranging from structure and nomenclature to key reactions and practical tests.
醇是化学中最重要的有机化合物之一，其官能团为连接在饱和碳原子上的羟基（–OH）。本文涵盖 CCEA A-Level 及 IB 化学所要求的基础知识，从结构与命名到关键反应与实验检验，助你全面掌握醇的考点。
1. Structure and Classification | 结构与分类
Alcohols contain the hydroxyl group –OH bonded to an sp³ hybridised carbon. They 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. In a primary alcohol, the –OH carbon is attached to one other carbon; in secondary, to two; in tertiary, to three.
醇含有与 sp³ 杂化碳原子相连的羟基 –OH。根据与羟基所在碳直接相连的碳原子数目，醇可分为伯醇（1°）、仲醇（2°）和叔醇（3°）。伯醇中 –OH 碳连有一个碳，仲醇连有两个，叔醇连有三个。
- Primary alcohol: R–CH₂–OH (e.g., ethanol CH₃CH₂OH)
- Secondary alcohol: R₂CH–OH (e.g., propan-2-ol (CH₃)₂CHOH)
- Tertiary alcohol: R₃C–OH (e.g., 2-methylpropan-2-ol (CH₃)₃COH)
- 伯醇：R–CH₂–OH（如乙醇 CH₃CH₂OH）
- 仲醇：R₂CH–OH（如丙-2-醇 (CH₃)₂CHOH）
- 叔醇：R₃C–OH（如 2-甲基丙-2-醇 (CH₃)₃COH）
2. Nomenclature | 命名
IUPAC names for alcohols are derived by replacing the final -e of the parent alkane with -ol and numbering the chain to give the –OH group the lowest possible locant. If more than one –OH group is present, use suffixes -diol, -triol, etc., and do not drop the -e of the alkane stem.
醇的 IUPAC 命名是将母体烷烃词尾的 -e 替换为 -醇，并对碳链编号以使 –OH 基团的位置数字最小。若存在多个 –OH，则使用 -二醇、-三醇等后缀，且不删除烷烃词干的 -e。
- CH₃OH: methanol (甲醇)
- CH₃CH₂CH₂OH: propan-1-ol (丙-1-醇)
- CH₃CHOHCH₃: propan-2-ol (丙-2-醇)
- HOCH₂CH₂OH: ethane-1,2-diol (乙-1,2-二醇)
3. Physical Properties | 物理性质
Alcohols have relatively high boiling points compared to alkanes of similar molecular mass due to intermolecular hydrogen bonding. The –OH group can both donate and accept hydrogen bonds, leading to strong association in the liquid state. Short-chain alcohols are miscible with water, but solubility decreases as the hydrophobic hydrocarbon chain lengthens.
与分子量相近的烷烃相比，醇的沸点较高，因为分子间存在氢键。–OH 基团既能提供又能接受氢键，使得液态时分子间缔合作用很强。短链醇可与水混溶，但随着疏水烃链增长，溶解度降低。
| Alcohol | Formula | bp (°C) | Solubility in water |
|---|---|---|---|
| Methanol | CH₃OH | 65 | Miscible |
| Ethanol | C₂H₅OH | 78 | Miscible |
| Propan-1-ol | C₃H₇OH | 97 | Miscible |
| Butan-1-ol | C₄H₉OH | 118 | Slightly soluble |
4. Preparation of Alcohols | 醇的制备
Alcohols can be synthesised by several routes. Hydration of alkenes using steam and an acid catalyst (e.g., H₃PO₄) produces alcohols following Markovnikov’s rule. Fermentation of sugars using yeast yields ethanol, though it gives a dilute solution. Reduction of carbonyl compounds (aldehydes to primary alcohols, ketones to secondary alcohols) using reducing agents like NaBH₄ or LiAlH₄ is also common. Additionally, hydrolysis of halogenoalkanes with aqueous NaOH or KOH produces alcohols.
醇可通过多种途径合成。烯烃在酸催化剂（如磷酸）作用下与水加成可制备醇，反应遵循马氏规则。用酵母发酵糖类可制得乙醇，但得到的是稀溶液。使用还原剂（如 NaBH₄ 或 LiAlH₄）还原羰基化合物（醛→伯醇，酮→仲醇）也很常见。此外，卤代烷在 NaOH 或 KOH 水溶液中水解也能生成醇。
- CH₂=CH₂ + H₂O → CH₃CH₂OH (industrial ethene hydration)
- C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂ (fermentation)
- CH₃CHO + 2[H] → CH₃CH₂OH (reduction of ethanal)
- CH₃CH₂Br + NaOH(aq) → CH₃CH₂OH + NaBr (nucleophilic substitution)
5. Reactions of Alcohols: Overview | 醇的反应概览
The chemistry of alcohols is dominated by the polar O–H bond and the C–O bond. Reactions can replace the –OH group (substitution), oxidise the carbon bearing –OH, or eliminate water to form alkenes. The type of alcohol (1°, 2°, 3°) strongly influences the oxidation products.
醇的化学性质主要由极性的 O–H 键和 C–O 键决定。反应可取代 –OH 基团（取代反应）、氧化与 –OH 相连的碳，或消除水生成烯烃。醇的类型（伯、仲、叔）对氧化产物有决定性影响。
6. Oxidation of Alcohols | 醇的氧化
Primary alcohols are oxidised first to aldehydes, and with further oxidation to carboxylic acids. Controlled oxidation can be achieved by distilling off the aldehyde as it forms (using acidified potassium dichromate(VI) with excess alcohol, gentle heating). Reflux with excess oxidising agent gives the carboxylic acid. Secondary alcohols oxidise to ketones, which are not easily oxidised further. Tertiary alcohols resist oxidation under normal conditions because they lack a hydrogen atom on the carbon bearing the –OH.
伯醇首先被氧化为醛，进一步氧化可生成羧酸。控制氧化可采用蒸馏的方法将生成的醛及时移出（用酸化重铬酸钾(VI)、过量醇、温和加热）。与过量氧化剂回流则得到羧酸。仲醇氧化为酮，酮不易继续氧化。叔醇在通常条件下不能被氧化，因为连有 –OH 的碳上不再有氢原子。
1° RCH₂OH → RCHO → RCOOH
2° R₂CHOH → R₂C=O
3° R₃COH → no reaction
Common oxidising agent: acidified potassium dichromate(VI), K₂Cr₂O₇/H₂SO₄, colour change from orange to green (Cr³⁺).
常用氧化剂：酸化重铬酸钾(VI)，K₂Cr₂O₇/H₂SO₄，颜色由橙色变为绿色（Cr³⁺）。
7. Esterification | 酯化
Alcohols react with carboxylic acids in the presence of a strong acid catalyst (e.g., concentrated H₂SO₄) to form esters and water. This is a reversible condensation reaction. The ester functional group –COOR is responsible for the fragrant odours of many fruits and is used in flavourings and perfumes.
醇与羧酸在强酸催化剂（如浓硫酸）存在下反应生成酯和水。这是一个可逆的缩合反应。酯官能团 –COOR 是许多水果芳香气息的来源，常用于调味剂和香料。
RCOOH + R′OH ⇌ RCOOR′ + H₂O
Example: ethanoic acid + ethanol ⇌ ethyl ethanoate + H₂O
示例：乙酸 + 乙醇 ⇌ 乙酸乙酯 + 水
Acid anhydrides can also be used to prepare esters more efficiently, avoiding the water by‑product.
也可用酸酐制备酯，反应更高效且不产生水副产物。
8. Dehydration to Alkenes | 脱水生成烯烃
Alcohols undergo elimination when heated with concentrated sulphuric acid or passed over heated alumina (Al₂O₃). Water is removed, producing an alkene. The reaction follows Zaitsev’s rule, favouring the more substituted alkene. Primary alcohols require higher temperatures (around 170°C with H₂SO₄).
醇与浓硫酸共热或通过灼热的氧化铝（Al₂O₃）时发生消除反应，脱去水生成烯烃。反应遵循扎伊采夫规则，更有利于生成取代较多的烯烃。伯醇需要较高温度（浓硫酸条件下约 170°C）。
CH₃CH₂OH → CH₂=CH₂ + H₂O
Tertiary alcohols dehydrate most readily under mild conditions.
叔醇在温和条件下最容易脱水。
9. Reaction with Sodium | 与钠反应
Alcohols react with reactive metals such as sodium to produce alkoxides and hydrogen gas, similar to the reaction of water with sodium but less vigorous. This reaction demonstrates the weakly acidic nature of the –OH proton.
醇可与活泼金属（如钠）反应生成醇钠和氢气，类似于水与钠的反应但较为缓和。该反应体现了 –OH 质子的弱酸性。
2 ROH + 2 Na → 2 RO⁻Na⁺ + H₂
Sodium ethoxide is a useful base and nucleophile in organic synthesis.
乙醇钠是有机合成中常用的碱和亲核试剂。
10. Halogenation | 卤代
Alcohols can be converted to halogenoalkanes by several methods. Reaction with hydrogen halides (HX) or with phosphorus halides (PCl₅, PBr₃, P/I₂) replaces the –OH with a halogen. Tertiary alcohols react readily with concentrated HCl at room temperature via an SN1 mechanism, while primary alcohols require heating or a catalyst such as ZnCl₂.
醇可通过多种方法转化为卤代烷。与氢卤酸（HX）或卤化磷（PCl₅、PBr₃、P/I₂）反应可将 –OH 替换为卤素。叔醇在室温下即可与浓盐酸通过 SN1 机理顺利反应，而伯醇需要加热或使用催化剂如 ZnCl₂。
- ROH + PCl₅ → RCl + POCl₃ + HCl
- 3 ROH + PBr₃ → 3 RBr + H₃PO₃
- ROH + HI → RI + H₂O (often generated in situ from P, I₂)
11. Tests for Alcohols | 醇的检验
The presence of the –OH group can be confirmed using sodium metal, which gives effervescence of hydrogen gas. To distinguish among primary, secondary, and tertiary alcohols, the Lucas test uses Lucas reagent (ZnCl₂ in concentrated HCl). Tertiary alcohols give immediate turbidity, secondary alcohols react within a few minutes, and primary alcohols show no reaction at room temperature. Oxidation tests with acidified dichromate(VI) also give clear distinctions: primary and secondary alcohols turn orange to green; tertiary alcohols do not react.
可使用金属钠检验 –OH 基团的存在，会产生氢气气泡。为区分伯、仲、叔醇，可使用卢卡斯试剂（ZnCl₂ 的浓盐酸溶液）。叔醇立即出现浑浊，仲醇几分钟内反应，伯醇室温下无反应。用酸化重铬酸钾(VI)进行氧化测试也能清楚区分：伯醇和仲醇使橙色变为绿色，叔醇不反应。
Iodoform test (I₂/NaOH): ethanol and alcohols containing the CH₃CHOH– group give a pale yellow precipitate of triiodomethane (CHI₃) with a characteristic antiseptic smell. This is a test for methyl ketones or ethanol specifically.
碘仿试验（I₂/NaOH）：乙醇及含有 CH₃CHOH– 基团的醇会生成淡黄色沉淀三碘甲烷（CHI₃），具有特殊消毒水气味。这是专门针对甲基酮或乙醇的检验。
12. Uses and Importance | 用途与重要性
Methanol is used as a solvent and as a feedstock for making methanal (formaldehyde) and other chemicals. Ethanol is a biofuel, widely used in alcoholic beverages, as a solvent, and in hand sanitisers. Higher alcohols find applications as solvents, plasticisers, and in detergents and cosmetics. Understanding alcohol chemistry is fundamental for tackling organic synthesis and practical assessments in CCEA and IB examinations.
甲醇用作溶剂及制备甲醛等化学品的原料。乙醇是一种生物燃料，广泛用于酒精饮料、溶剂和洗手液。高级醇可用作溶剂、增塑剂以及洗涤剂和化妆品的成分。掌握醇的化学是应对 CCEA 和 IB 考试中有机合成与实验评估的基础。
Published by TutorHao | Chemistry Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导