📚 IGCSE AQA Chemistry: Alcohols – Essential Revision Notes | IGCSE AQA 化学：醇 考点精讲
Alcohols are a key family of organic compounds in the AQA IGCSE Chemistry specification. They contain the hydroxyl functional group (–OH) and display a range of interesting physical and chemical properties. Understanding alcohols will help you explain topics like fuels, oxidation, and the production of ethanol, all of which appear regularly in examinations.
醇是 AQA IGCSE 化学考纲中重要的有机化合物类别。它们含有羟基官能团（–OH），表现出多种有趣的物理和化学性质。理解醇的知识能帮助你解释燃料、氧化反应以及乙醇的生产等主题，这些内容在考试中经常出现。
1. What Are Alcohols? | 什么是醇？
Alcohols are a homologous series of organic compounds that contain at least one hydroxyl group, –OH, attached to a carbon atom. This –OH group is the functional group responsible for the characteristic reactions of alcohols. The simplest and most important alcohols for IGCSE are methanol, ethanol, propanol, and butanol.
醇是一类同系列的有机化合物，其分子中至少有一个羟基（–OH）连接在碳原子上。这个 –OH 基团是决定醇类典型化学性质的官能团。在 IGCSE 范围里，最简单且最重要的醇包括甲醇、乙醇、丙醇和丁醇。
Alcohols are not hydrocarbons because they contain oxygen in addition to carbon and hydrogen. The presence of the polar –OH group makes them significantly different from alkanes and alkenes, especially in their physical properties and reactivity.
醇并不是烃，因为除了碳和氢之外，它们还含有氧元素。极性的 –OH 基团使得醇与烷烃和烯烃有很大的不同，尤其是在物理性质和反应活性方面。
2. General Formula and Homologous Series | 通式和同系物
highlights that an alcohol is essentially an alkane with one hydrogen atom replaced by an –OH group. For example, when n=1, methanol is CH₃OH; when n=2, ethanol is C₂H₅OH.
醇同系物的通式是 CₙH₂ₙ₊₁OH，其中 n 表示碳原子的个数。这个通式说明了醇可以看作是一个烷烃分子中的一个氢原子被 –OH 基团取代后的产物。例如，当 n=1 时，甲醇为 CH₃OH；当 n=2 时，乙醇为 C₂H₅OH。
All members of this series share the same functional group and follow a gradual trend in physical properties such as boiling point and viscosity. As the carbon chain length increases, alcohols become less soluble in water but their boiling points rise due to increased van der Waals forces.
该同系物中的所有成员都具有相同的官能团，并且在沸点、黏度等物理性质上呈现出逐渐变化的趋势。随着碳链长度的增加，醇的水溶性会降低，但由于范德华力的增强，沸点会升高。
3. Naming Alcohols | 醇的命名
Naming alcohols follows a systematic pattern. The name of an alcohol ends with the suffix '-ol'. The parent chain is the longest continuous carbon chain that includes the carbon attached to the –OH group. The position of the –OH group is indicated by a number placed before the '-ol' suffix, giving the lowest possible number to the hydroxyl carbon. For IGCSE, you are primarily expected to name and recognise the first four straight-chain alcohols: methanol, ethanol, propan-1-ol (often simply called propanol), and butan-1-ol.
醇的命名遵循系统的规则。醇的名称以“-ol”结尾。主链是包含连接 –OH 基团的碳原子在内的最长连续碳链。–OH 基团的位置用数字标注在“-ol”前面，并使该碳原子的编号尽可能小。在 IGCSE 考试中，主要要求掌握并识别前四种直链醇：甲醇、乙醇、丙-1-醇（常简称为丙醇）和丁-1-醇。
For example, propan-1-ol means the –OH group is on the first carbon of a three-carbon chain. A structural isomer, propan-2-ol, would have the –OH group on the middle carbon. Always remember that numbering starts from the end of the chain closest to the –OH group.
例如，丙-1-醇表示 –OH 基团连接在三碳链的第一个碳原子上。它的同分异构体丙-2-醇则让 –OH 连接在中间的碳原子上。务必牢记，编号应从最靠近 –OH 基团的链端开始。
4. Physical Properties and Hydrogen Bonding | 物理性质与氢键
Alcohols have relatively high melting and boiling points compared to alkanes of similar molecular mass. This is due to hydrogen bonding between alcohol molecules. The highly electronegative oxygen atom in the –OH group creates a significant dipole, allowing strong intermolecular attractions – hydrogen bonds – to form between the lone pair on oxygen of one molecule and the partially positive hydrogen of another molecule's –OH group.
与分子量相近的烷烃相比，醇具有较高的熔点和沸点。这是因为醇分子之间能形成氢键。–OH 基团中电负性很强的氧原子产生显著的偶极，使得一个分子中氧原子上的孤对电子与另一个分子中 –OH 基团部分带正电的氢原子之间形成强烈的分子间作用力——氢键。
Short-chain alcohols (methanol, ethanol, propanol) are completely miscible with water because they can form hydrogen bonds with water molecules. As the hydrocarbon chain becomes longer, the non-polar hydrocarbon part dominates, solubility in water decreases, and the alcohol behaves more like an alkane.
短链醇（甲醇、乙醇、丙醇）能与水以任意比例混溶，因为它们能与水分子形成氢键。随着碳氢链变长，非极性的碳氢部分逐渐占据主导，在水中的溶解度随之下降，醇的性质也变得更像烷烃。
5. Combustion of Alcohols | 醇的燃烧
Alcohols burn readily in a plentiful supply of oxygen to produce carbon dioxide and water vapour. The combustion is highly exothermic, which makes short-chain alcohols useful as fuels. For example, the complete combustion of ethanol is represented by the equation:
醇在充足的氧气中很容易燃烧，生成二氧化碳和水蒸气。燃烧反应是高度放热的，因此短链醇可用作燃料。例如，乙醇完全燃烧的化学方程式为：
C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O (complete combustion)
Methanol and ethanol are often used as petrol additives or alternative fuels because they burn relatively cleanly. It is important to learn how to write balanced equations for the combustion of alcohols and to understand that incomplete combustion can produce toxic carbon monoxide and soot.
甲醇和乙醇常被用作汽油添加剂或替代燃料，因为它们燃烧相对清洁。重要的是学会书写醇燃烧的配平方程式，并理解不完全燃烧会产生有毒的一氧化碳和炭黑。
6. Reaction with Sodium | 与钠的反应
Alcohols react with the reactive metal sodium to produce an alkoxide (a sodium salt of the alcohol) and hydrogen gas. The reaction is less vigorous than the reaction of sodium with water, but it still produces effervescence. For ethanol, the reaction is:
醇能与活泼金属钠发生反应，生成醇钠（醇的钠盐）和氢气。该反应不如钠与水的反应那样剧烈，但仍会有气泡产生。以乙醇为例，其反应方程式为：
2C₂H₅OH + 2Na → 2C₂H₅O⁻ Na⁺ + H₂
(Often written with the ionic sodium salt: 2C₂H₅ONa + H₂.) This reaction is a classic test for the –OH group in organic compounds, although in IGCSE the production of hydrogen gas (pops with a lighted splint) is the key observation. Note that this reaction is a displacement of hydrogen from the hydroxyl group, not from the C–H bonds.
（通常会写出离子型钠盐：2C₂H₅ONa + H₂。）这个反应是检验有机化合物中 –OH 基团的经典方法，不过在 IGCSE 考试中，观察到的关键现象是生成氢气（点燃的木条会发出爆鸣声）。请注意，该反应是羟基中氢原子的置换，而不是来自于 C–H 键。
7. Oxidation of Alcohols | 醇的氧化
Alcohols can be oxidised by chemical oxidising agents such as acidified potassium dichromate(VI) (K₂Cr₂O₇ with dilute sulfuric acid) or by the oxygen in air, often with the help of microbes. The product of oxidation depends on the structure of the alcohol and the reaction conditions.
醇可以被化学氧化剂氧化，例如酸化重铬酸钾(VI)（K₂Cr₂O₇ 与稀硫酸），也可以被空气中的氧气氧化，通常需要微生物的帮助。氧化的产物取决于醇的结构和反应条件。
For ethanol, oxidation with acidified potassium dichromate(VI) first forms ethanal (an aldehyde, CH₃CHO), which upon further oxidation produces ethanoic acid (CH₃COOH). This two-step process is shown below:
对乙醇而言，与酸化重铬酸钾(VI)反应首先生成乙醛（一种醛，CH₃CHO），进一步氧化则生成乙酸（CH₃COOH）。这两个步骤的过程如下所示：
CH₃CH₂OH + [O] → CH₃CHO + H₂O
CH₃CHO + [O] → CH₃COOH
The oxidising agent is itself reduced: the orange dichromate(VI) ions (Cr₂O₇²⁻) are converted to green chromium(III) ions (Cr³⁺). This colour change is a key observation when testing for an alcohol that can be oxidised. Primary alcohols (where the –OH group is at the end of the chain) are oxidised first to aldehydes and then to carboxylic acids, while secondary alcohols form ketones.
氧化剂本身也会被还原：橙色的重铬酸根离子（Cr₂O₇²⁻）转变为绿色的铬(III)离子（Cr³⁺）。这个颜色变化是检验可被氧化的醇类物质时的关键现象。伯醇（–OH 在碳链末端）先被氧化成醛，再被氧化成羧酸，而仲醇则被氧化成酮。
8. Microbial Oxidation – Making Vinegar | 微生物氧化——酿造醋
If an alcoholic drink like wine or beer is left exposed to air, naturally occurring bacteria (such as Acetobacter) can oxidise ethanol into ethanoic acid. This is the cause of 'spoiled' wine turning into vinegar. The overall change is an oxidation reaction using atmospheric oxygen:
如果红酒或啤酒等酒精饮料暴露在空气中，自然界中的细菌（如醋酸杆菌）可以将乙醇氧化成乙酸。这就是酒“变质”为醋的原因。总反应是消耗氧气的氧化反应：
C₂H₅OH + O₂ → CH₃COOH + H₂O
In the AQA IGCSE specification, this is often linked to the manufacture of ethanoic acid and demonstrates the real-world application of alcohol oxidation. It is a slow, low-temperature process compared to laboratory oxidation with dichromate.
在 AQA IGCSE 考纲中，这一过程常与乙酸的制备联系起来，展示了醇氧化反应的实际应用。与实验室中用重铬酸盐氧化的条件相比，这是一个缓慢的低温过程。
9. Production of Ethanol by Fermentation | 发酵法制备乙醇
Ethanol is produced on an industrial scale by the fermentation of carbohydrates such as glucose, using yeast. The yeast provides enzymes that catalyse the anaerobic breakdown of sugar into ethanol and carbon dioxide. The equation for the fermentation of glucose is:
工业上通过发酵碳水化合物（如葡萄糖）来大规模生产乙醇，使用酵母为菌种。酵母提供的酶催化糖在无氧条件下分解为乙醇和二氧化碳。葡萄糖发酵的化学方程式为：
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
This process is carried out at around 30–40°C in the absence of oxygen. If the temperature is too high, the yeast enzymes denature; if oxygen is present, the yeast switches to aerobic respiration and produces water and carbon dioxide instead of ethanol. Fermentation produces an aqueous solution containing a maximum of about 15% ethanol, because higher concentrations kill the yeast.
该过程在约 30–40°C、无氧环境下进行。温度过高会使酵母的酶变性；如果存在氧气，酵母会转为有氧呼吸，生成水和二氧化碳而不生成乙醇。发酵得到的产物是浓度最高约 15% 的乙醇水溶液，因为更高的浓度会杀死酵母。
To obtain pure ethanol, fractional distillation is used. The mixture is heated to about 78°C, the boiling point of ethanol, so that ethanol vapour can be condensed and collected while water and other impurities remain.
要获得纯乙醇，需要采用分馏法。将混合物加热到约 78°C（乙醇的沸点），使乙醇蒸气冷凝并收集，而水和其他杂质留在烧瓶中。
10. Production of Ethanol by Direct Hydration | 直接水化法制备乙醇
Another industrial method to manufacture ethanol is the direct hydration of ethene. Ethene (C₂H₄) reacts with steam in the presence of a phosphoric acid catalyst at high temperature (about 300°C) and high pressure (60-70 atmospheres). The reaction is an addition reaction:
另一种工业制备乙醇的方法是乙烯的直接水化法。乙烯（C₂H₄）与蒸汽在磷酸催化剂的存在下，在高温（约 300°C）和高压（60-70 个大气压）下反应。这是一个加成反应：
C₂H₄ + H₂O ⇌ C₂H₅OH
The ethene used is obtained from crude oil cracking, so this is a non-renewable route. In contrast, fermentation uses renewable plant materials. You are expected to compare the two methods in terms of raw materials, energy use, reaction conditions, and yield.
所用的乙烯来自于原油的裂解，因此这是一种不可再生的途径。相比之下，发酵法使用的是可再生植物原料。考试中会要求你从原料、能源消耗、反应条件和产率等方面对这两种方法进行比较。
| Fermentation 发酵法 | Direct hydration 直接水化法 |
|---|---|
| Renewable raw materials (sugar cane, etc.) | Non-renewable (ethene from crude oil) |
| Low temperature, atmospheric pressure | High temperature, high pressure |
| Slow, batch process | Fast, continuous process |
| Low concentration ethanol (needs fractional distillation) | Pure ethanol produced directly |
Understanding the advantages and disadvantages of both processes is essential for evaluative questions in the IGCSE exam.
理解这两种工艺的优缺点对于 IGCSE 考试中的评价性问题至关重要。
11. Key Comparisons and Summary | 关键对比与总结
The table below summarises the main reactions of ethanol that you must know for the AQA IGCSE Chemistry specification.
下表总结了 AQA IGCSE 化学考纲中必须掌握的乙醇的主要反应。
| Reaction 反应 | Conditions 条件 | Products 产物 |
|---|---|---|
| Combustion 燃烧 | Plentiful air / O₂ | CO₂ + H₂O |
| With sodium 与钠 | Room temperature, sodium metal | C₂H₅ONa + H₂ |
| Oxidation by dichromate 重铬酸盐氧化 | Acidified K₂Cr₂O₇, heat | Ethanal then ethanoic acid |
| Microbial oxidation 微生物氧化 | Air, bacteria, warm | Ethanoic acid |
| Fermentation 发酵 | Yeast, 30–40°C, anaerobic | Ethanol + CO₂ |
| Hydration of ethene 乙烯水化 | 300°C, 60–70 atm, H₃PO₄ catalyst | Ethanol |
Remembering the colour change of dichromate from orange to green, the production of hydrogen gas with sodium, and the conditions for both methods of ethanol manufacture will stand you in excellent stead for any question on alcohols.
记住重铬酸盐由橙色变为绿色的颜色变化、与钠反应放出氢气以及乙醇两种制备方法的条件，将会帮助你顺利回答任何关于醇的问题。
12. Exam Tips and Common Pitfalls | 应试技巧与常见错误
When writing balanced equations, ensure you carefully count carbon, hydrogen and oxygen atoms. A common mistake is forgetting that ethanol already contains an oxygen atom, so it requires less O₂ for combustion than a corresponding alkane. For instance, ethanol (C₂H₆O) needs 3 O₂ to combust completely, whereas ethane (C₂H₆) needs 3.5 O₂.
在书写配平方程式时，务必仔细计算碳、氢和氧原子的数量。一个常见的错误是忽略了乙醇本身就含有一个氧原子，因此燃烧时所需的氧气比相应的烷烃要少。例如，乙醇（C₂H₆O）完全燃烧需要 3 个 O₂，而乙烷（C₂H₆）则需要 3.5 个 O₂。
Another pitfall is confusing addition and fermentation conditions. Direct hydration uses high temperature and pressure with a catalyst and is an addition reaction; fermentation is a biochemical process at mild temperatures. Always be specific about the catalyst: phosphoric acid (H₃PO₄) for hydration, no catalyst (except enzymes from yeast) for fermentation.
另一个易错点是混淆加成反应和发酵的条件。直接水化法需要高温、高压并使用催化剂，属于加成反应；而发酵是一个温和温度下的生化过程。一定要明确催化剂：水化法用磷酸（H₃PO₄），发酵法则没有催化剂（除了酵母本身提供的酶）。
Finally, be able to interpret structural and displayed formulae. The –OH group must be drawn as O–H with a single bond. In propan-1-ol, the OH is at the end; in propan-2-ol, the OH is on the middle carbon. A labelled diagram in an answer can often earn additional marks.
最后，要能够解释结构式和显示式。–OH 基团必须画成 O–H 单键连接。丙-1-醇中 OH 在端点；丙-2-醇中 OH 在中间的碳原子上。在答案中画出带标注的结构图通常能获得额外的分数。
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