IGCSE WJEC Chemistry: Alcohols – Key Points | IGCSE WJEC 化学：醇考点精讲

📚 IGCSE WJEC Chemistry: Alcohols – Key Points | IGCSE WJEC 化学：醇考点精讲

Alcohols are an important family of organic compounds containing the hydroxyl (-OH) functional group. In the IGCSE WJEC Chemistry specification, students are expected to understand the structure, naming, properties, preparation, and reactions of alcohols, with a special focus on ethanol.

醇是含羟基 (-OH) 官能团的重要有机化合物家族。在 IGCSE WJEC 化学大纲中，学生需理解醇的结构、命名、性质、制备和反应，重点为乙醇。

1. Homologous Series and General Formula | 同系物与通式

Alcohols form a homologous series with the general formula CₙH₂ₙ₊₁OH, where n is the number of carbon atoms. They contain the hydroxyl group, -OH, attached to a saturated hydrocarbon chain.

醇构成通式为 CₙH₂ₙ₊₁OH 的同系物，其中 n 为碳原子数。它们含有连接在饱和烃链上的羟基 -OH。

The first three members are methanol (CH₃OH), ethanol (C₂H₅OH) and propanol (C₃H₇OH). Each successive member differs by a CH₂ unit.

前三名成员为甲醇 (CH₃OH)、乙醇 (C₂H₅OH) 和丙醇 (C₃H₇OH)。每一后续成员相差一个 CH₂ 单元。

All alcohols share similar chemical properties because of the presence of the -OH functional group, but physical properties show gradual trends.

由于均含 -OH 官能团，所有醇具有相似的化学性质，但物理性质呈现递变趋势。

2. Naming Alcohols | 醇的命名

Alcohols are named by replacing the final ‘-e’ of the corresponding alkane with ‘-ol’. The position of the -OH group is indicated by a number if the chain has three or more carbons.

醇的命名是将相应烷烃词尾的 ‘-e’ 替换为 ‘-ol’。若碳链有三个或更多碳原子，需用数字标明 -OH 的位置。

For example, propan-1-ol has the -OH on the first carbon, while propan-2-ol has it on the second carbon. Methanol and ethanol do not require numbers because the -OH can only be attached to one position.

例如，propan-1-ol 的 -OH 位于第一个碳，而 propan-2-ol 的 -OH 位于第二个碳。甲醇和乙醇不需要数字，因为 -OH 只能连在一个位置上。

Methane (CH₄) → Methanol (CH₃OH)
Ethane (C₂H₆) → Ethanol (C₂H₅OH)
Propane (C₃H₈) → Propan-1-ol and Propan-2-ol
Butane (C₄H₁₀) → Butan-1-ol, Butan-2-ol, etc.

You should be able to draw displayed formulas and identify primary (1°), secondary (2°), and tertiary (3°) alcohols from structures.

你应能画出展示结构式，并从结构中识别伯 (1°)、仲 (2°) 和叔 (3°) 醇。

3. Physical Properties | 物理性质

Alcohols have significantly higher boiling points than alkanes of similar molecular mass because the -OH group allows hydrogen bonding between molecules. This requires more energy to overcome during boiling.

醇的沸点明显高于类似分子量的烷烃，因为 -OH 基团使分子间形成氢键，沸腾时需更多能量来克服。

Methanol, ethanol and propanol are completely miscible with water due to hydrogen bonding with water molecules. As the hydrocarbon chain lengthens, the hydrophobic part dominates, and solubility decreases. Butanol is only sparingly soluble, and longer alcohols are immiscible.

甲醇、乙醇和丙醇因与水分子形成氢键而完全混溶。随着烃链增长，疏水部分占主导，溶解度下降。丁醇仅微溶，更长的醇则不混溶。

Viscosity and density also increase with increasing chain length, but all alcohols are less dense than water.

粘度和密度也随链长增加而增大，但所有醇的密度都小于水。

4. Preparation of Ethanol: Fermentation | 乙醇的制备：发酵

Ethanol for beverages and biofuels is produced by fermentation of sugars using yeast. Yeast contains the enzyme zymase, which catalyses the anaerobic conversion of glucose into ethanol and carbon dioxide.

用于饮料和生物燃料的乙醇通过酵母发酵糖类制得。酵母中含有酿酶，可在无氧条件下催化葡萄糖转化为乙醇和二氧化碳。

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

Reaction conditions: aqueous sugar solution, yeast, temperature 30–40 °C, and anaerobic (air-locked) conditions to prevent oxidation of ethanol to ethanoic acid.

反应条件：葡萄糖水溶液、酵母、温度 30–40 °C、厌氧环境（加空气阀）以防止乙醇氧化成乙酸。

The fermentation mixture produces a dilute ethanol solution (about 10–15%) because higher concentrations kill the yeast. Fractional distillation is then used to obtain pure ethanol (95.6% due to azeotrope, but further drying can yield anhydrous ethanol).

发酵混合物生成稀乙醇溶液（约 10–15%），因为更高浓度会杀死酵母。之后用分馏获得纯净乙醇（95.6%，因共沸物形成，但进一步干燥可得无水乙醇）。

Fermentation uses renewable resources (sugarcane, beet, corn) and operates at relatively low temperatures, but it is a batch process that is slow and produces CO₂.

发酵使用可再生资源（甘蔗、甜菜、玉米）且操作温度较低，但它是批次工艺，速度慢并产生 CO₂。

5. Preparation of Ethanol: Hydration of Ethene | 乙醇的制备：乙烯水合

Industrially, ethanol can also be synthesized by the direct catalytic hydration of ethene (from crude oil) with steam. This is a continuous process with high atom economy.

工业上也可通过乙烯（来自原油）与水蒸气的直接催化水合合成乙醇。这是一连续过程，原子经济性高。

C₂H₄ + H₂O ⇌ C₂H₅OH

Conditions: phosphoric acid (H₃PO₄) catalyst, temperature around 300 °C, and pressure 60–70 atm. The reaction is reversible, so unreacted ethene is recycled to achieve a high overall yield.

条件：磷酸催化剂、温度约 300 °C、压力 60–70 大气压。反应可逆，因此未反应的乙烯会被循环利用以实现高总产率。

This method produces highly pure ethanol and can run continuously, but it relies on non-renewable petroleum feedstock and requires high energy input for temperature and pressure.

该方法生成高纯度乙醇且可连续操作，但依赖不可再生的石油原料，且高温高压需大量能耗。

6. Combustion of Alcohols | 醇的燃烧

Alcohols burn in a plentiful supply of oxygen to produce carbon dioxide and water, releasing a great deal of heat energy. This makes them excellent fuels.

醇在充足氧气中燃烧生成二氧化碳和水，并释放大量热能，因此它们是优良的燃料。

C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O

The more carbon atoms in an alcohol, the more oxygen is required for complete combustion. You should be able to write balanced equations for the combustion of other alcohols, such as methanol and propanol.

醇中碳原子越多，完全燃烧需要的氧气就越多。你应能写出其他醇（如甲醇、丙醇）燃烧的配平方程式。

Incomplete combustion (limited oxygen) can produce carbon monoxide or carbon (soot), which is less efficient and more polluting.

不完全燃烧（氧气不足）会产生一氧化碳或碳（黑烟），效率较低且污染更大。

Ethanol is often blended with petrol (E10 contains 10% ethanol) to reduce fossil fuel consumption and lower harmful emissions.

乙醇常与汽油混合（E10 含 10% 乙醇）以减少化石燃料消耗并降低有害排放。

7. Reaction with Sodium | 与钠的反应

Alcohols react with reactive metals like sodium, but the reaction is less vigorous than that of sodium with water. The -OH hydrogen is displaced, forming a sodium alkoxide and hydrogen gas.

醇与钠等活泼金属反应，但不如钠与水反应剧烈。羟基中的氢被取代，生成醇钠和氢气。

2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂

Observation: effervescence as colourless hydrogen gas is produced, and the sodium dissolves. The solution remains alkaline because alkoxide ions hydrolyse to form OH⁻ in water.

现象：有无色氢气气泡产生，钠溶解。由于醇钠离子在水中水解生成 OH⁻，溶液保持碱性。

This reaction indicates the presence of the -OH group and is a useful test for alcohols, though the similar reaction with carboxylic acids produces CO₂ with carbonates, providing differentiation.

该反应表明了 -OH 基团的存在，是醇的有用检验，但需注意羧酸与碳酸盐反应产生 CO₂，可加以区分。

8. Oxidation of Alcohols | 醇的氧化

The oxidation behaviour of alcohols depends on whether the -OH is attached to a primary, secondary, or tertiary carbon. The usual oxidising agent is acidified potassium dichromate(VI), K₂Cr₂O₇/H⁺.

醇的氧化行为取决于 -OH 连在伯、仲还是叔碳上。常用氧化剂是酸化重铬酸钾(VI)，K₂Cr₂O₇/H⁺。

Type of alcohol	Product(s)	Colour change
Primary (1°)	Aldehyde → Carboxylic acid	Orange → green
Secondary (2°)	Ketone	Orange → green
Tertiary (3°)	No reaction	Remains orange

Primary alcohol example: ethanol is first oxidised to ethanal (CH₃CHO) and then to ethanoic acid (CH₃COOH). With careful control (distillation of aldehyde as it forms), you can isolate the aldehyde.

伯醇示例：乙醇首先被氧化为乙醛 (CH₃CHO)，然后进一步氧化为乙酸 (CH₃COOH)。通过控制条件（氧化生成后立即将醛蒸出），可分离出醛。

CH₃CH₂OH + [O] → CH₃CHO + H₂O

CH₃CHO + [O] → CH₃COOH

Secondary alcohol example: propan-2-ol oxidises to propanone (CH₃COCH₃), the simplest ketone. No further oxidation occurs under these conditions.

仲醇示例：propan-2-ol 氧化为丙酮 (CH₃COCH₃)，这是最简单的酮。在此条件下不再继续氧化。

CH₃CHOHCH₃ + [O] → CH₃COCH₃ + H₂O

Tertiary alcohols, such as 2-methylpropan-2-ol, resist oxidation because they lack a hydrogen atom on the carbon carrying the -OH. The orange dichromate solution remains unchanged, which distinguishes them from primary and secondary alcohols.

叔醇（如 2-甲基-2-丙醇）难以氧化，因为连有 -OH 的碳原子上没有可被脱去的氢。橙色重铬酸盐溶液不变色，这可区分叔醇与伯、仲醇。

9. Esterification | 酯化反应

Alcohols react with carboxylic acids in the presence of a strong acid catalyst (usually concentrated sulfuric acid) to form esters and water. This is a reversible condensation reaction.

醇与羧酸在强酸催化剂（通常是浓硫酸）存在下反应生成酯和水。这是一个可逆的缩合反应。

Alcohol + Carboxylic acid ⇌ Ester + Water

For example, ethanol reacts with ethanoic acid to form ethyl ethanoate, a common solvent with a pleasant, fruity smell.

例如，乙醇与乙酸反应生成乙酸乙酯，一种具有宜人果香的常用溶剂。

CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O

To name an ester, the first part comes from the alcohol (e.g., ethyl) and the second part from the carboxylic acid (e.g., ethanoate). The reaction requires heating and is usually catalysed by concentrated H₂SO₄. The acid also acts as a dehydrating agent, shifting the equilibrium to the right by absorbing water.

命名酯时，第一部分来自醇（如 ethyl），第二部分来自羧酸（如 ethanoate）。反应需加热，通常以浓

Published by TutorHao | IGCSE Chemistry Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）

IGCSE WJEC Chemistry: Alcohols – Key Points | IGCSE WJEC 化学：醇 考点精讲