📚 The Homologous Series of Alcohols | 醇的同系物
Alcohols constitute a vital homologous series in organic chemistry, defined by the presence of a hydroxyl (–OH) functional group bonded to a saturated carbon atom. They exhibit a gradual change in physical properties and undergo characteristic chemical reactions that are central to the Cambridge International A-Level Chemistry syllabus.
醇是有机化学中一类至关重要的同系物，其特征是一个羟基 (–OH) 官能团连接在饱和碳原子上。它们表现出逐步递变的物理性质，并能发生多种特征化学反应，这些反应是剑桥国际 A-Level 化学考纲的核心内容。
1. Introduction to Alcohols as a Homologous Series | 醇作为同系物的简介
A homologous series is a family of compounds with the same general formula, similar chemical properties, and a regular gradation in physical properties. Alcohols fit this definition perfectly: successive members differ by a –CH₂– unit, and they all contain the reactive –OH group attached to an alkyl chain.
同系物是指具有相同通式、相似化学性质以及物理性质呈现规律变化的一族化合物。醇完全符合这一定义：相邻成员之间相差一个 –CH₂– 单元，且都含有连接在烷基链上的活泼 –OH 基团。
The simplest alcohol is methanol (CH₃OH), followed by ethanol (C₂H₅OH), propan-1-ol (C₃H₇OH), and so on. As the hydrocarbon chain lengthens, physical properties such as boiling point and solubility change in a predictable manner.
最简单的醇是甲醇 (CH₃OH)，其次是乙醇 (C₂H₅OH)、丙‑1‑醇 (C₃H₇OH) 等等。随着烃链增长，沸点和溶解度等物理性质会以可预测的方式变化。
2. General Formula and Functional Group | 通式与官能团
The general molecular formula for saturated alcohols is CₙH₂ₙ₊₁OH, where n is the number of carbon atoms. This can also be written as CₙH₂ₙ₊₂O, emphasising the presence of one oxygen atom. The functional group is the hydroxyl group, –OH, which is covalently bonded to an sp³ hybridised carbon.
饱和一元醇的通式为 CₙH₂ₙ₊₁OH，其中 n 为碳原子数。该通式也可写作 CₙH₂ₙ₊₂O，以突出一个氧原子的存在。官能团是羟基 (–OH)，它共价连接在一个 sp³ 杂化的碳原子上。
The oxygen atom in the –OH group is electronegative, creating a polar C–O and O–H bond. This polarity is responsible for the hydrogen bonding capability and much of the reactivity of alcohols.
–OH 基团中的氧原子电负性较强，使得 C–O 键和 O–H 键具有极性。这种极性赋予了醇形成氢键的能力以及其主要反应活性。
3. IUPAC Nomenclature of Alcohols | 醇的 IUPAC 命名
The systematic naming follows IUPAC rules: select the longest carbon chain containing the –OH group, replace the final ‘e’ of the parent alkane with ‘ol’, and number the chain to give the hydroxyl group the lowest possible number. Substituents are named and numbered as usual.
系统命名遵循 IUPAC 规则：选择含有 –OH 的最长碳链，将母体烷烃名称末尾的“e”替换为“ol”，并对碳链进行编号，使羟基所在碳原子的编号尽可能小。取代基则按常规方式命名和编号。
For example, CH₃CH₂CH₂OH is propan-1-ol, and CH₃CH(OH)CH₃ is propan-2-ol. When more than one –OH group is present, suffixes such as ‘diol’ or ‘triol’ are used with appropriate locants.
例如，CH₃CH₂CH₂OH 是丙‑1‑醇，CH₃CH(OH)CH₃ 是丙‑2‑醇。当存在多个 –OH 时，使用“二醇”、“三醇”等后缀，并标出它们的位置。
Branched alcohols are named by identifying the principal chain and substituents; e.g. (CH₃)₂CHCH₂OH is 2‑methylpropan-1‑ol. This systematic approach allows unambiguous identification of any alcohol.
支链醇的命名需要确定主链和取代基；例如 (CH₃)₂CHCH₂OH 为 2‑甲基丙‑1‑醇。这种系统命名方法能够准确无误地识别各种醇。
4. Classification of Alcohols | 醇的分类
Alcohols 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 a secondary alcohol, to two; and in a tertiary alcohol, to three.
根据与羟基所在碳原子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，带 –OH 的碳与一个其他碳相连；仲醇中与两个碳相连；叔醇中与三个碳相连。
For example, ethanol (CH₃CH₂OH) is primary, propan-2-ol (CH₃CH(OH)CH₃) is secondary, and 2‑methylpropan‑2‑ol ((CH₃)₃COH) is tertiary. This classification is crucial because the behaviour of alcohols under oxidation depends strongly on whether they are primary, secondary, or tertiary.
例如，乙醇 (CH₃CH₂OH) 属于伯醇，丙‑2‑醇 属于仲醇，而 2‑甲基丙‑2‑醇 ((CH₃)₃COH) 是叔醇。这种分类至关重要，因为醇在氧化反应中的行为与其属于伯、仲还是叔醇密切相关。
5. Physical Properties: Boiling Point and Solubility | 物理性质：沸点与溶解度
The boiling points of alcohols are significantly higher than those of analogous alkanes or ethers of similar relative molecular mass. This is due to intermolecular hydrogen bonding. As the carbon chain lengthens, boiling points increase due to greater London dispersion forces, but the influence of the –OH group becomes less dominant.
醇的沸点明显高于相对分子质量相近的烷烃或醚，这是分子间氢键所致。随着碳链增长，伦敦色散力增大，沸点升高，但 –OH 基团的影响逐渐减弱。
| Compound | Formula | Mᵣ | Boiling point (°C) |
|---|---|---|---|
| Ethane | C₂H₆ | 30 | −89 |
| Methanol | CH₃OH | 32 | 65 |
| Ethanol | C₂H₅OH | 46 | 78 |
| Propane | C₃H₈ | 44 | −42 |
| Propan-1-ol | C₃H₇OH | 60 | 97 |
Lower alcohols (methanol, ethanol, propan‑1‑ol) are completely miscible with water in all proportions. This high solubility is due to the ability of the –OH group to form hydrogen bonds with water molecules. As the non-polar hydrocarbon chain lengthens (butanol onwards), solubility decreases sharply because the hydrophobic alkyl portion dominates.
低级醇（甲醇、乙醇、丙‑1‑醇）能与水以任意比例混溶。如此高的溶解性源于 –OH 基团能与水分子形成氢键。随着非极性的烃链增长（从丁醇开始），疏水烷基部分占据主导，溶解度急剧下降。
6. Hydrogen Bonding in Alcohols | 醇中的氢键
Hydrogen bonding arises between the slightly positive hydrogen atom of one –OH group and the lone pair of electrons on the oxygen atom of a neighbouring alcohol molecule. This strong intermolecular force requires more energy to overcome during boiling, giving alcohols higher boiling points than analogous hydrocarbons or ethers that lack hydrogen bond donors.
一个 –OH 基团上带部分正电荷的氢原子与相邻醇分子中氧原子的孤对电子之间会形成氢键。这种较强的分子间作用力需要更多能量来克服，因此醇的沸点高于相应的烃或醚（它们缺少氢键给体）。
Ethers, such as methoxymethane (CH₃OCH₃), have the same molecular formula as ethanol but a much lower boiling point (–24 °C) because they cannot form hydrogen bonds. The difference highlights the importance of the –OH group for physical properties.
像甲氧基甲烷 (CH₃OCH₃) 这样的醚与乙醇分子式相同，但沸点却低得多（−24 °C），因为它们无法形成氢键。这一差异突显了 –OH 基团对物理性质的重要性。
7. Combustion of Alcohols | 醇的燃烧
Alcohols burn readily in excess oxygen to produce carbon dioxide and water, releasing large amounts of energy. This makes them useful as fuels. The general combustion equation for a saturated alcohol is:
醇在过量氧气中极易燃烧，生成二氧化碳和水，并释放大量能量，因此可用作燃料。饱和醇燃烧的通用方程式为：
CₙH₂ₙ₊₁OH + (3n/2)O₂ → nCO₂ + (n+1)H₂O
For example, the complete combustion of ethanol: C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l). Ethanol is increasingly blended with gasoline to reduce carbon emissions and is used in spirit burners.
例如，乙醇完全燃烧：C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l)。乙醇越来越多地与汽油混合以降低碳排放，并用于酒精灯。
The high enthalpy of combustion, particularly for smaller alcohols, means they are efficient liquid fuels. Methanol is used in racing cars, while ethanol is a renewable fuel obtained by fermentation.
低级醇的燃烧焓很高，因此它们是高效的液体燃料。甲醇用于赛车燃料，而乙醇则是通过发酵获得的可再生燃料。
8. Reaction with Sodium | 与钠的反应
Alcohols react with sodium metal to form an alkoxide and hydrogen gas, analogous to the reaction of sodium with water, but less vigorous. The reaction is a redox process in which the –OH proton is reduced to H₂, while sodium is oxidised to Na⁺.
醇与金属钠反应生成醇钠和氢气，类似于钠与水的反应，但不如后者剧烈。该反应是一个氧化还原过程：–OH 上的质子被还原成 H₂，而钠被氧化成 Na⁺。
2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂(g)
Effervescence from the evolved hydrogen is observed. The product is sodium ethoxide, an ionic solid containing the C₂H₅O⁻ ion, which is a strong base and a useful nucleophile in organic synthesis.
可观察到因氢气放出而产生的气泡。产物为乙醇钠，是一种含有 C₂H₅O⁻ 离子的离子型固体，该离子既是强碱，又是在有机合成中有用的亲核试剂。
Longer-chain alcohols react similarly but more slowly due to the increasing non-polar character of the alkyl group, which reduces the effective contact with sodium.
长链醇也能发生类似反应，但速率较慢，因为烷基的非极性特征增加，减少了与钠的有效接触。
9. Oxidation of Alcohols | 醇的氧化
The oxidation behaviour of alcohols depends on their class. The oxidising agent most commonly used is acidified potassium dichromate(VI), K₂Cr₂O₇/H⁺, which changes colour from orange (Cr₂O₇²⁻) to green (Cr³⁺) when reduced.
醇的氧化行为取决于其类别。最常用的氧化剂是酸化重铬酸钾 (K₂Cr₂O₇/H⁺)，该试剂被还原时颜色由橙色 (Cr₂O₇²⁻) 变为绿色 (Cr³⁺)。
Primary alcohols can be oxidised in two stages. Gentle oxidation with distillation yields an aldehyde; stronger oxidation under reflux gives a carboxylic acid. The aldehyde can be isolated because its lower boiling point allows it to be removed from the reaction mixture before further oxidation.
伯醇可分两步氧化。温和氧化并采用蒸馏可得到醛；在回流条件下强烈氧化则得到羧酸。醛可以被分离出来，因为其沸点较低，能在进一步氧化前从反应混合物中蒸出。
CH₃CH₂OH + [O] → CH₃CHO + H₂O
CH₃CHO + [O] → CH₃COOH
Secondary alcohols are oxidised to ketones. For example, propan‑2‑ol gives propanone. Ketones resist further oxidation under normal conditions because breaking a C–C bond would be required.
仲醇被氧化成酮。例如，丙‑2‑醇生成丙酮。酮在通常条件下难以进一步氧化，因为这需要断裂 C–C 键。
CH₃CH(OH)CH₃ + [O] → CH₃COCH₃ + H₂O
Tertiary alcohols do not react with acidified dichromate; the solution remains orange. This is because the carbon bearing the –OH has no hydrogen atom to release, making oxidation impossible without breaking carbon–carbon bonds.
叔醇不与酸化重铬酸钾反应，溶液保持橙色。这是因为连接 –OH 的碳原子上没有氢原子可以脱去，在不破坏碳碳键的情况下无法发生氧化。
10. 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 reversible condensation reaction is known as Fischer esterification.
醇与羧酸在强酸催化剂（通常为浓硫酸）存在下反应，生成酯和水。这一可逆的缩合反应被称为 Fischer 酯化反应。
CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O
In this example, ethanoic acid and ethanol form ethyl ethanoate, a sweet‑smelling compound widely used as a solvent and in flavourings. The reaction is typically heated under reflux to improve the rate, and the equilibrium can be driven by removing water or using an excess of one reactant.
在此例中，乙酸与乙醇生成乙酸乙酯，这是一种具有芳香气味的化合物，广泛用作溶剂和
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