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

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

Alcohols are a homologous series of organic compounds containing the hydroxyl (-OH) functional group. In IGCSE Chemistry, you need to know their general formula, naming, physical and chemical properties, preparation methods, and uses, especially ethanol as a biofuel. This revision guide covers all essential points with clear comparisons and example reactions.

醇是含有羟基 (-OH) 官能团的同系物。在 IGCSE 化学中，你需要掌握它们的通式、命名、物理和化学性质、制备方法以及用途，尤其是乙醇作为生物燃料的相关知识。本考点精讲涵盖所有要点，并附有清晰的对比和反应示例。

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. The functional group is the hydroxyl group, -OH, which is attached to a saturated carbon chain. The simplest alcohol is methanol (CH₃OH), followed by ethanol (C₂H₅OH).

醇类构成一个同系物，通式为 CₙH₂ₙ₊₁OH，其中 n 是碳原子数。官能团是连接在饱和碳链上的羟基 -OH。最简单的醇是甲醇 (CH₃OH)，其次是乙醇 (C₂H₅OH)。

As with all homologous series, successive members differ by a CH₂ unit, they have similar chemical properties due to the same functional group, and they show a gradual trend in physical properties such as boiling point.

与所有同系物一样，相邻成员之间相差一个 CH₂ 基团，由于具有相同的官能团，它们化学性质相似，物理性质（如沸点）呈现规律性变化。

Alcohol	Formula	n
Methanol	CH₃OH	1
Ethanol	C₂H₅OH	2
Propanol	C₃H₇OH	3
Butanol	C₄H₉OH	4

2. Naming Alcohols | 醇的命名

The IUPAC name of an alcohol ends in ‘-ol’. Choose the longest continuous carbon chain containing the -OH group, number the chain so that the -OH carbon gets the lowest possible number, and place the number before the ‘-ol’ suffix. For example, CH₃CH₂CH₂OH is propan-1-ol.

醇的 IUPAC 命名以 ‘-ol’ 结尾。选择包含 -OH 基团的最长连续碳链，从离 -OH 最近的一端开始编号，并将位次数字写在 ‘-ol’ 之前。例如 CH₃CH₂CH₂OH 命名为丙-1-醇。

If there are two -OH groups, the name ends in ‘-diol’, e.g. ethane-1,2-diol. For branched isomers, indicate alkyl substituents as usual. The position of the -OH group takes priority over alkyl groups and halogens when numbering.

如果有两个 -OH 基团，则以 ‘-二醇’ 结尾，如乙-1,2-二醇。对于支链异构体，按常规标明烷基取代基。编号时，-OH 基团的位置优先于烷基和卤素。

CH₃CH(OH)CH₃: propan-2-ol | 丙-2-醇
CH₃CH₂CH₂CH₂OH: butan-1-ol | 丁-1-醇
CH₃CH₂CH(OH)CH₃: butan-2-ol | 丁-2-醇

3. Physical Properties of Alcohols | 醇的物理性质

Alcohols have relatively high boiling points compared to alkanes of similar molar mass due to hydrogen bonding between -OH groups. Methanol and ethanol are volatile liquids at room temperature. As molar mass increases, boiling point increases and they become less volatile.

与相似摩尔质量的烷烃相比，醇的沸点较高，因为 -OH 基团之间能形成氢键。甲醇和乙醇在室温下是挥发性液体。随着摩尔质量的增加，沸点升高，挥发性降低。

Short-chain alcohols (methanol, ethanol, propanol) are completely miscible with water because the hydroxyl group can form hydrogen bonds with water molecules. Solubility in water decreases as the hydrocarbon chain lengthens, as the non-polar part becomes dominant.

短链醇（甲醇、乙醇、丙醇）与水完全混溶，因为羟基能与水分子形成氢键。随着碳链增长，非极性部分占主导，醇在水中的溶解度下降。

Alcohols are neutral in aqueous solution; they do not change the colour of litmus or universal indicator. This is because the -OH group is covalently bonded and does not dissociate to release OH⁻ ions.

醇的水溶液呈中性，不会改变石蕊或通用指示剂的颜色。这是因为 -OH 基团是以共价键结合的，不会电离释放 OH⁻ 离子。

4. Combustion of Alcohols | 醇的燃烧

Alcohols burn in excess oxygen to produce carbon dioxide and water, releasing a large amount of energy. This is a complete combustion reaction. The general equation for an alcohol CₙH₂ₙ₊₁OH is:

醇在过量氧气中燃烧生成二氧化碳和水，并释放大量能量。这是完全燃烧反应。醇 CₙH₂ₙ₊₁OH 燃烧的通式为：

CₙH₂ₙ₊₁OH + (1.5n + 0.5) O₂ → n CO₂ + (n+1) H₂O

For example, ethanol burns with a clean, blue flame: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O. Alcohols are used as fuels, either pure or blended with petrol (gasohol). Ethanol produced from sugar cane is a renewable biofuel.

例如，乙醇燃烧产生清洁的蓝色火焰：C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O。醇类可单独或与汽油混合（乙醇汽油）作为燃料。由甘蔗生产的乙醇是一种可再生的生物燃料。

If oxygen supply is limited, incomplete combustion may occur, producing carbon monoxide (poisonous) or carbon (soot). The flame becomes yellow and smoky.

如果氧气供应不足，可能发生不完全燃烧，生成一氧化碳（有毒）或碳（烟灰）。火焰变为黄色、带黑烟。

5. Oxidation of Alcohols | 醇的氧化

Primary alcohols can be oxidised to aldehydes and further to carboxylic acids. For IGCSE, you mainly need to know that ethanol is oxidised to ethanoic acid. The laboratory oxidising agent is acidified potassium dichromate(VI) (K₂Cr₂O₇/H₂SO₄), which changes colour from orange to green.

伯醇可以被氧化成醛，并进一步氧化成羧酸。IGCSE 阶段主要需要知道乙醇被氧化成乙酸。实验室常用的氧化剂是酸化重铬酸钾 (K₂Cr₂O₇/H₂SO₄)，反应中颜色由橙色变为绿色。

The oxidation of ethanol can be summarised: C₂H₅OH + 2[O] → CH₃COOH + H₂O, where [O] represents oxygen from the oxidising agent. This reaction requires heating under reflux if full oxidation to the acid is desired.

乙醇的氧化可概括为：C₂H₅OH + 2[O] → CH₃COOH + H₂O，其中 [O] 代表来自氧化剂的氧。若要完全氧化成酸，需要加热回流。

Tertiary alcohols are resistant to oxidation under these conditions. This difference can be used to distinguish between primary and tertiary alcohols. For IGCSE, focus on ethanol’s oxidation to ethanoic acid as the key example.

叔醇在此条件下不易被氧化。这一区别可用于区分伯醇和叔醇。IGCSE 重点掌握乙醇氧化为乙酸这一关键示例。

6. Preparation of Ethanol by Fermentation | 发酵法制备乙醇

Ethanol can be produced by anaerobic respiration of yeast (fermentation). Yeast contains the enzyme zymase, which converts glucose into ethanol and carbon dioxide. The conditions: aqueous solution, 30–40°C, absence of oxygen.

乙醇可通过酵母的无氧呼吸（发酵）制取。酵母含有酒化酶，可将葡萄糖转化为乙醇和二氧化碳。条件：水溶液、30–40°C、无氧环境。

C₆H₁₂O₆(aq) → 2C₂H₅OH(aq) + 2CO₂(g)

When ethanol concentration reaches about 15%, it kills the yeast, stopping fermentation. Fractional distillation is then used to concentrate ethanol from the mixture.

当乙醇浓度达到约 15% 时，酵母被杀死，发酵停止。然后通过分馏将乙醇从混合物中浓缩出来。

This method produces a renewable fuel and uses sugar cane, sugar beet, or other starch-rich crops that can be hydrolysed to glucose first. The CO₂ released is from the atmosphere via photosynthesis, so it is considered carbon neutral over the crop’s life cycle (though energy for processing may involve fossil fuels).

这种方法可制得可再生燃料，原料为甘蔗、甜菜或其他富含淀粉的作物（先水解成葡萄糖）。释放的 CO₂ 来自植物光合作用从大气中吸收的碳，因此在整个作物生命周期中被认为是碳中性的（尽管加工过程可能消耗化石能源）。

7. Industrial Preparation: Hydration of Ethene | 工业制备：乙烯水合法

Ethanol can also be manufactured by direct hydration of ethene using steam. This is a continuous process requiring a phosphoric acid catalyst, high temperature (300°C) and high pressure (60–70 atm).

乙醇也可通过乙烯与蒸汽的直接水合反应生产。这是一个连续过程，需要磷酸催化剂、高温 (300°C) 和高压 (60–70 atm)。

C₂H₄(g) + H₂O(g) ⇌ C₂H₅OH(g)

The reaction is reversible and the yield is relatively low, so unreacted ethene is recycled. The product is a mixture of ethanol and water, which is separated by fractional distillation.

该反应可逆，产率相对较低，因此未反应的乙烯需循环利用。产物为乙醇和水的混合物，通过分馏分离。

Hydration gives high-purity ethanol and is not dependent on agricultural crops, but uses non-renewable crude oil fractions (ethene from cracking) and large energy input, making it less sustainable than fermentation.

水合法可得到高纯度乙醇，不依赖于农作物，但使用不可再生的石油馏分（裂解得到的乙烯），且需要大量能量输入，因此可持续性不如发酵法。

Feature	Fermentation	Hydration of Ethene
Raw material	Renewable crops (sugar)	Non-renewable (crude oil)
Temperature	30–40°C	300°C
Pressure	Atmospheric	60–70 atm
Catalyst	Yeast (enzyme)	Phosphoric acid
Type	Batch process	Continuous process
Product purity	Dilute, needs distillation	High, still requires separation

8. Ethanol as a Biofuel | 乙醇作为生物燃料

Biofuels are fuels produced from biological material (biomass). Ethanol is a common biofuel. It can be blended with petrol up to 10% (E10) without engine modification, reducing fossil fuel use.

生物燃料是由生物质材料生产的燃料。乙醇是一种常见的生物燃料，可与汽油按最高 10% (E10) 的比例混合而无需改造发动机，从而减少化石燃料的使用。

Advantages: ethanol is renewable, burns more cleanly (less particulates), and is theoretically carbon neutral because the CO₂ released during combustion was recently absorbed by the plants used to make it.

优点：乙醇可再生，燃烧更清洁（微粒少），在理论上是碳中性的，因为燃烧释放的 CO₂ 是最近被用于制乙醇的植物所吸收的。

Disadvantages: large land areas are needed to grow crops, which may reduce land available for food production (food vs fuel debate); fertilisers and machinery used in farming may cause pollution and CO₂ emissions; distillation requires energy, often from fossil fuels.

缺点：种植作物需要大面积土地，可能减少粮食作物用地（粮食与燃料之争）；农业中使用的化肥和机械会造成污染和 CO₂ 排放；蒸馏需要能量，通常来自化石燃料。

9. Reaction with Sodium | 与钠的反应

Alcohols react with reactive metals like sodium to produce a salt (sodium alkoxide) and hydrogen gas. This reaction is slower than that of sodium with water, but bubbles of hydrogen are observed.

醇能与钠等活泼金属反应，生成盐（醇钠）和氢气。该反应比钠与水的反应慢，但可观察到氢气气泡产生。

2C₂H₅OH + 2Na → 2C₂H₅O⁻Na⁺ + H₂

The product is sodium ethoxide (an ionic compound). The reaction shows that the -OH hydrogen is slightly acidic, but alcohols are far weaker acids than water. This is a typical test for an -OH group in organic compounds.

产物为乙醇钠（离子化合物）。该反应表明羟基上的氢具有微弱的酸性，但醇的酸性远弱于水。这是检验有机物中 -OH 基团的一个典型反应。

10. Esterification Reaction | 酯化反应

Alcohols react with carboxylic acids in the presence of an acid catalyst (usually concentrated H₂SO₄) to form esters and water. This is a reversible reaction called esterification. Esters have sweet, fruity odours and are used as solvents and flavourings.

醇与羧酸在酸催化剂（通常为浓硫酸）存在下反应，生成酯和水。这是一个可逆反应，称为酯化反应。酯具有甜果香味，常用作溶剂和调味剂。

Ethanol + Ethanoic acid ⇌ Ethyl ethanoate + Water

C₂H₅OH + CH₃COOH ⇌ CH₃COOC₂H₅ + H₂O

Concentrated sulfuric acid acts as a catalyst and also removes water, shifting the equilibrium to the right. The reaction mixture is heated under reflux. The ester is separated by distillation and purified by washing with sodium carbonate solution to remove unreacted acid.

浓硫酸起催化作用，同时吸水使平衡右移。反应混合物需加热回流。通过蒸馏分离酯，并用碳酸钠溶液洗涤以除去未反应的酸，从而进行提纯。

11. Comparing Alcohols with Alkanes and Water | 醇与烷烃、水的比较

Alcohols share some properties with alkanes (combustibility, covalent bonding) and some with water (hydrogen bonding, ability to dissolve ionic compounds to some extent). However, alcohols are distinct: they are neutral, have higher boiling points than corresponding alkanes but lower than water, and are good solvents for both polar and non-polar substances.

醇兼有烷烃的某些性质（可燃性、共价键）和水的某些性质（氢键，能在一定程度上溶解离子化合物）。但醇有其独特性：呈中性，沸点高于相应烷烃但低于水，既是极性也是非极性物质的良好溶剂。

Property	Alkane (ethane)	Alcohol (ethanol)	Water
Boiling point (°C)	-89	78	100
Solubility in water	Insoluble	Miscible	—
Reaction with Na	No reaction	H₂ evolved, slow	H₂ evolved, vigorous
Combustion	CO₂ + H₂O	CO₂ + H₂O	Does not burn

12. Summary of Key Reagents and Conditions | 主要试剂和条件总结

For exams, you must remember the reagents, conditions, and observations for the main reactions of ethanol: oxidation with acidified potassium dichromate (orange to green, heat), combustion (clean blue flame, CO₂ + H₂O), esterification (carboxylic acid + alcohol + conc. H₂SO₄ catalyst, heat under reflux), and reaction with sodium (slow effervescence, colourless gas that pops with a lighted splint).

考试需要记住乙醇主要反应的试剂、条件和现象：用酸化重铬酸钾氧化（加热，橙色变绿），燃烧（清洁蓝色火焰，CO₂ + H₂O），酯化（羧酸 + 醇 + 浓硫酸催化剂，加热回流），与钠反应（缓慢冒泡，无色气体，遇点燃木条有爆鸣声）。

Also be ready to compare the two methods of manufacturing ethanol—fermentation and hydration of ethene—including raw materials, conditions, sustainability, and yield issues. Understanding the concept of carbon neutrality in biofuels is crucial for evaluation questions.

还要能比较乙醇的两种生产方法——发酵法和乙烯水合法，包括原料、条件、可持续性和产率问题。理解生物燃料的碳中性概念对于评价类题目至关重要。

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

更多咨询请联系16621398022（同微信）

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