Alcohols | 醇

📚 Alcohols | 醇

In organic chemistry, alcohols form a family of compounds that contain the hydroxyl group, -OH. They are among the most important industrial chemicals, used as solvents, fuels, and starting materials for many useful products such as esters. In the OCR GCSE Chemistry specification, you need to understand the structure, naming, preparation, and key chemical reactions of alcohols, particularly methanol, ethanol, and propanol.

在有机化学中，醇是一类含有羟基（-OH）的化合物。它们是最重要的工业化学品之一，被用作溶剂、燃料以及许多有用产品（如酯）的起始原料。在 OCR GCSE 化学考试大纲中，你需要理解醇的结构、命名、制备和关键化学反应，尤其是甲醇、乙醇和丙醇。

1. What are Alcohols? | 什么是醇？

Alcohols are a homologous series of organic compounds in which one or more hydrogen atoms in an alkane have been replaced by the hydroxyl functional group, -OH. The simplest alcohols are methanol (CH₃OH), ethanol (C₂H₅OH), and propanol (C₃H₇OH). They are not hydrocarbons because they contain oxygen, but they are closely related to alkanes.

醇是一类同系有机化合物，其中烷烃中的一个或多个氢原子被羟基官能团 -OH 取代。最简单的醇是甲醇（CH₃OH）、乙醇（C₂H₅OH）和丙醇（C₃H₇OH）。它们不是烃，因为它们含有氧，但与烷烃密切相关。

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2. The Hydroxyl Functional Group | 羟基官能团

The functional group that defines alcohols is the hydroxyl group, written as -OH. This consists of an oxygen atom covalently bonded to a hydrogen atom, and is attached to a carbon chain. The -OH group is responsible for the characteristic properties of alcohols, such as their ability to form hydrogen bonds and their reactivity in oxidation and esterification.

定义醇的官能团是羟基，写作 -OH。它由一个氧原子与一个氢原子共价键合而成，并连接在碳链上。-OH 基团决定了醇的特征性质，例如形成氢键的能力，以及它们在氧化和酯化反应中的活性。

• The hydroxyl group is polar, making short-chain alcohols soluble in water.
• 羟基是极性的，使短链醇可溶于水。
• It is different from the hydroxide ion (OH⁻) found in alkalis; the -OH in alcohols is covalently bonded and does not dissociate.
• 它不同于碱中的氢氧根离子（OH⁻）；醇中的 -OH 是共价键合的，不会解离。

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3. Naming Alcohols | 醇的命名

The names of alcohols follow the standard IUPAC rules, using the suffix ‘-ol’. The main chain is numbered so that the carbon atom attached to the -OH group gets the lowest possible number. For the first three alcohols in the series, the names are methanol, ethanol, and propanol. If the -OH is not on a terminal carbon, a number is used, such as propan-2-ol.

醇的命名遵循标准的 IUPAC 规则，使用后缀“-ol”。主链编号应使连接 -OH 基团的碳原子获得尽可能小的编号。对于该系列前三种醇，名称为甲醇、乙醇和丙醇。如果 -OH 不在末端碳上，则使用数字表示，例如 propan-2-ol（2-丙醇）。

Number of C atoms	Molecular formula	Displayed formula	Name
1	CH₃OH	H₃C-OH	Methanol
2	C₂H₅OH	CH₃-CH₂-OH	Ethanol
3	C₃H₇OH	CH₃-CH₂-CH₂-OH	Propan-1-ol
3	C₃H₇OH	CH₃-CHOH-CH₃	Propan-2-ol

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

The alcohols form a homologous series with the general formula CₙH₂ₙ₊₁OH, where n is the number of carbon atoms. As we go up the series, each member differs by a CH₂ unit. The physical properties change gradually, such as increasing boiling point and decreasing solubility in water, due to the increasing length of the non-polar hydrocarbon chain.

醇构成一个同系物，通式为 CₙH₂ₙ₊₁OH，其中 n 是碳原子数。随着系列上升，每个成员相差一个 CH₂ 单元。物理性质逐渐变化，如沸点升高和水溶性降低，这是因为非极性烃链长度增加。

• Methanol, ethanol, propanol, butanol… follow the same pattern.
• 甲醇、乙醇、丙醇、丁醇……遵循相同的规律。
• They have similar chemical properties because they all contain the -OH group.
• 它们具有相似的化学性质，因为它们都含有 -OH 基团。

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5. Manufacture of Ethanol: Fermentation | 乙醇的制备：发酵

Ethanol for beverages and biofuel can be produced by fermentation of sugars using yeast. Yeast contains enzymes that catalyse the anaerobic respiration of glucose, producing ethanol and carbon dioxide. The reaction is carried out in aqueous solution at around 30-40°C; if the temperature is too high, the enzymes denature, and if oxygen is present, ethanoic acid forms instead.

用于饮料和生物燃料的乙醇可通过酵母发酵糖类来生产。酵母含有酶，可以催化葡萄糖的无氧呼吸，产生乙醇和二氧化碳。反应在水溶液中进行，温度约为 30-40°C；温度过高会使酶变性，如果存在氧气，则会生成乙酸。

C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂

The ethanol concentration reaches about 10-15% before the yeast becomes inactive. Fractional distillation is then used to obtain more concentrated ethanol. Fermentation is a renewable process but relatively slow.

在酵母失活之前，乙醇浓度可达到约 10-15%。然后使用分馏法得到更浓的乙醇。发酵是一种可再生过程，但相对较慢。

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6. Manufacture of Ethanol: Hydration of Ethene | 乙醇的制备：乙烯水合

Industrially, a large amount of ethanol is produced by the direct hydration of ethene. Ethene (from crude oil cracking) is reacted with steam at a high temperature (300°C) and pressure (60-70 atm) in the presence of a phosphoric acid catalyst. This is a continuous process that produces purer ethanol more quickly, but it relies on non-renewable crude oil.

工业上，大量乙醇通过乙烯的直接水合制得。乙烯（来自原油裂解）在磷酸催化剂存在下，在高温（300°C）和高压（60-70 atm）下与水蒸气反应。这是一个连续过程，能更快地产生更纯的乙醇，但它依赖不可再生的原油。

C₂H₄ + H₂O ⇌ C₂H₅OH

The forward reaction is exothermic. A high pressure and a moderate temperature are used to favour the forward reaction while maintaining a reasonable rate. Unlike fermentation, this produces almost 100% ethanol and requires no further purification.

正向反应是放热的。使用高压和中等温度，以有利于正向反应同时保持合理的速率。与发酵不同，这种方法生产几乎 100% 的乙醇，无需进一步提纯。

Feature	Fermentation	Hydration of ethene
Raw material	Sugar cane, grains (renewable)	Ethene from crude oil (non-renewable)
Type of process	Batch, slow	Continuous, fast
Conditions	30–40°C, yeast, aqueous	300°C, 60–70 atm, H₃PO₄
Purity of product	~15%, needs distillation	Almost pure ethanol

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

Short-chain alcohols (methanol, ethanol, propanol) are colourless liquids at room temperature with characteristic smells. They have much higher boiling points than the corresponding alkanes because they can form strong hydrogen bonds between their -OH groups. They are also completely miscible with water due to hydrogen bonding between alcohol molecules and water molecules. As the alkyl chain length increases, solubility decreases because the non-polar part becomes dominant.

短链醇（甲醇、乙醇、丙醇）在室温下为无色液体，具有特殊气味。它们的沸点比相应烷烃高得多，因为它们可以在 -OH 基团之间形成强氢键。由于醇分子与水分子之间的氢键作用，它们与水完全混溶。随着烷基链增长，溶解度降低，因为非极性部分占主导。

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8. Chemical Reaction: Combustion | 化学反应：燃烧

Alcohols burn readily in air with a clean, pale blue flame, releasing a large amount of energy. This makes them useful as fuels. The complete combustion of an alcohol produces carbon dioxide and water. For example, the combustion of ethanol is:

醇在空气中容易燃烧，发出干净的淡蓝色火焰，释放大量能量。这使它们可用作燃料。醇的完全燃烧生成二氧化碳和水。例如，乙醇的燃烧方程式为：

C₂H₅OH + 3 O₂ → 2 CO₂ + 3 H₂O

Incomplete combustion can occur if the oxygen supply is limited, producing carbon monoxide or soot (carbon). Ethanol is increasingly used as a biofuel, often blended with petrol, because the carbon dioxide released during its combustion is offset by the CO₂ absorbed during the growth of the sugar crop, making it relatively carbon-neutral.

如果氧气供应不足，会发生不完全燃烧，产生一氧化碳或碳烟（碳）。乙醇日益被用作生物燃料，通常与汽油混合，因为其燃烧释放的二氧化碳可由糖料作物生长过程中吸收的 CO₂ 抵消，从而使其相对碳中和。

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9. Oxidation of Alcohols | 醇的氧化

Primary alcohols such as ethanol can be oxidised to form carboxylic acids. In the laboratory, this is carried out by heating the alcohol with an oxidising agent such as acidified potassium dichromate(VI) (K₂Cr₂O₇ in dilute sulfuric acid). The orange dichromate(VI) solution turns green as the Cr(VI) is reduced to Cr(III). The reaction must be heated under reflux to obtain a good yield of the acid.

伯醇如乙醇可以被氧化生成羧酸。在实验室中，这是通过将醇与氧化剂（如酸化重铬酸钾(VI)，即在稀硫酸中的 K₂Cr₂O₇）一起加热来进行的。橙色的重铬酸钾(VI)溶液变为绿色，因为 Cr(VI) 被还原为 Cr(III)。反应必须在回流条件下加热，以获得高产率的酸。

CH₃CH₂OH + 2[O] → CH₃COOH + H₂O

In this equation, [O] represents the oxygen supplied by the oxidising agent. Further oxidation beyond the acid is not possible under these conditions. Propan-1-ol is oxidised to propanoic acid. This reaction is important in the laboratory and can also occur when alcoholic drinks are exposed to air for a long time, turning them sour.

在这个方程式中，[O] 表示氧化剂提供的氧。在这些条件下，不可能再进一步氧化成其他产物。丙-1-醇被氧化成丙酸。这个反应在实验室中很重要，当含酒精饮料长时间暴露在空气中时也会发生，使其变酸。

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10. Esterification Reaction | 酯化反应

Alcohols react with carboxylic acids to form esters in a condensation reaction, with concentrated sulfuric acid acting as a catalyst. An ester has the functional group -COO-. The reaction is reversible, and water is also produced. For example, ethanol reacts with ethanoic acid to form ethyl ethanoate, a sweet-smelling ester used in flavourings and solvents.

醇与羧酸在浓硫酸催化下发生缩合反应生成酯。酯的官能团是 -COO-。该反应可逆，同时生成水。例如，乙醇与乙酸反应生成乙酸乙酯，这是一种有甜味的酯，用于调味剂和溶剂。

CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O

The acid catalyst speeds up the reaction but does not change the position of equilibrium. Esters formed from small alcohols and acids are volatile liquids with pleasant, fruity smells. This reaction is a key way of linking alcohols to other functional groups in organic chemistry.

酸催化剂加快了反应速率，但不改变平衡位置。由小分子醇和酸生成的酯是具有宜人水果香味的挥发性液体。这个反应是将醇与有机化学中其他官能团联系起来的关键途径。

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11. Uses of Alcohols | 醇的用途

Alcohols have a wide range of applications. Ethanol is the most well-known and is used in alcoholic drinks, as a solvent in perfumes and medicines, as a fuel (both straight and blended with petrol), and as a feedstock for making other chemicals including esters and ethanoic acid. Methanol is used as an industrial solvent and as a starting material for producing polymers and other chemicals; it is toxic and should never be consumed. Propan-2-ol is commonly used in antiseptic wipes and cleaning products.

醇有着广泛的用途。乙醇最为人所知，用于酒精饮料、香水和医药中的溶剂、燃料（纯乙醇或与汽油混合），以及作为制造其他化学品（包括酯和乙酸）的原料。甲醇用作工业溶剂和生产聚合物及其他化学品的起始原料；它有毒，绝不可饮用。丙-2-醇常用于消毒湿巾和清洁产品。

• Solvents: alcohol-based inks, paints, and varnishes.
• 溶剂：醇基油墨、油漆和清漆。
• Biofuels: ethanol can reduce reliance on fossil fuels.
• 生物燃料：乙醇可以减少对化石燃料的依赖。
• Chemical industry: making esters, acetic acid, and detergents.
• 化学工业：制造酯、醋酸和洗涤剂。

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12. Alcohols and Sustainability | 醇与可持续发展

The use of ethanol as a biofuel presents a debate about sustainability. On one hand, ethanol from fermentation is a renewable energy source that can help lower net CO₂ emissions. On the other hand, growing crops for fuel can compete with food production, lead to deforestation, and require significant amounts of water and fertilisers. Understanding these trade-offs is part of making informed decisions about chemistry’s role in society, which is a key theme in the OCR GCSE course.

乙醇作为生物燃料的使用引发了关于可持续性的讨论。一方面，发酵制得的乙醇是可再生的能源，有助于减少净二氧化碳排放。另一方面，种植燃料作物可能与粮食生产竞争，导致森林砍伐，并需要大量的水和化肥。理解这些权衡是做出关于化学在社会中作用的明智决策的一部分，这也是 OCR GCSE 课程的一个重要主题。

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