📚 The Homologous Series of Alcohols | 醇类同系物
Alcohols are one of the most important homologous series in A-Level organic chemistry. They contain the hydroxyl functional group, -OH, and show a clear gradation in physical properties while sharing similar chemical reactions. Understanding their structure, naming, classification and reactivity is essential for mastering organic synthesis and reaction mechanisms.
醇是 A-Level 有机化学中最重要的同系物之一。它们含有羟基官能团 -OH，物理性质呈现明显的递变规律，同时具有相似的化学反应。掌握醇的结构、命名、分类和反应活性，对于理解有机合成和反应机理至关重要。
1. Definition and General Formula | 定义与通式
A homologous series is a family of organic compounds with the same functional group and similar chemical properties. Each successive member differs from the previous one by a -CH₂- unit. Alcohols are organic compounds in which a hydroxyl group, -OH, is bonded to a saturated carbon atom.
同系物是一类具有相同官能团和相似化学性质的有机化合物。相邻成员之间相差一个 -CH₂- 单元。醇是羟基 -OH 连接在饱和碳原子上的有机化合物。
For saturated monohydric alcohols, the general formula is CₙH₂ₙ₊₁OH, which can also be written as CₙH₂ₙ₊₂O. The first few members are methanol, ethanol, propan-1-ol and butan-1-ol.
饱和一元醇的通式为 CₙH₂ₙ₊₁OH，也可以写成 CₙH₂ₙ₊₂O。前几个成员为甲醇、乙醇、丙-1-醇和丁-1-醇。
- Methanol: CH₃OH | 甲醇：CH₃OH
- Ethanol: C₂H₅OH | 乙醇：C₂H₅OH
- Propan-1-ol: C₃H₇OH | 丙-1-醇：C₃H₇OH
- Butan-1-ol: C₄H₉OH | 丁-1-醇：C₄H₉OH
This homologous series is characterised by the presence of the polar -OH group, which dominates both physical properties and the main chemical reactions.
该同系物的特征在于极性 -OH 基团的存在，该基团主导了醇的物理性质和主要化学反应。
2. IUPAC Naming of Alcohols | 醇的 IUPAC 命名
According to IUPAC rules, the longest continuous carbon chain containing the -OH group is selected. The chain is numbered so that the hydroxyl group receives the lowest possible locant, and the suffix -ol is added to the parent alkane name.
根据 IUPAC 规则，应选择包含 -OH 基团的最长连续碳链。对碳链编号时使羟基获得最小的位次，并在母体烷烃名称后加上后缀 -ol。
If more than one -OH group is present, the suffixes -diol and -triol are used, with numerical prefixes indicating the positions. For example, ethane-1,2-diol contains two hydroxyl groups on adjacent carbon atoms.
如果含有多个 -OH 基团，则使用后缀 -diol 和 -triol，并用数字前缀标出位置。例如，乙-1,2-二醇中两个羟基位于相邻的碳原子上。
Examples include:
示例包括：
- Pentan-2-ol | 戊-2-醇
- 2-methylpropan-1-ol | 2-甲基丙-1-醇
- Butane-1,3-diol | 丁-1,3-二醇
When naming alcohols, the hydroxyl group takes priority over alkyl groups and halogen atoms, so it determines the numbering of the chain.
命名醇时，羟基的优先顺序高于烷基和卤素原子，因此它决定碳链的编号方式。
3. Classification: Primary, Secondary and Tertiary | 分类：伯醇、仲醇与叔醇
Alcohols are classified according to the number of carbon atoms directly bonded to the carbon bearing the -OH group. This classification is important because it determines the products of oxidation.
醇根据与连接 -OH 的碳原子直接相连的碳原子数目进行分类。这种分类很重要，因为它决定了氧化反应的产物。
A primary alcohol has the -OH group attached to a carbon atom that is bonded to only one other carbon atom, or to no carbon atom in the case of methanol. A secondary alcohol has the -OH group on a carbon bonded to two other carbon atoms, and a tertiary alcohol has the -OH group on a carbon bonded to three other carbon atoms.
伯醇的 -OH 连接在只与一个其他碳原子相连的碳原子上；甲醇是该碳原子上没有其他碳原子的特殊情况。仲醇的 -OH 连接在与两个其他碳原子相连的碳上，叔醇的 -OH 连接在与三个其他碳原子相连的碳上。
- Primary: ethanol, CH₃CH₂OH | 伯醇：乙醇，CH₃CH₂OH
- Secondary: propan-2-ol, CH₃CH(OH)CH₃ | 仲醇：丙-2-醇，CH₃CH(OH)CH₃
- Tertiary: 2-methylpropan-2-ol, (CH₃)₃COH | 叔醇：2-甲基丙-2-醇，(CH₃)₃COH
The carbon bearing the -OH group is often called the carbinol carbon. In primary and secondary alcohols, this carbon still carries hydrogen atoms, which allows oxidation to occur.
连接 -OH 的碳原子通常称为醇碳。在伯醇和仲醇中，该碳原子上仍带有氢原子，因此可以发生氧化反应。
4. Physical Properties and Hydrogen Bonding | 物理性质与氢键
Alcohols are polar molecules because the -OH group has a highly polar O-H bond. The oxygen atom is more electronegative than hydrogen, producing a permanent dipole. Alcohol molecules can form hydrogen bonds with each other through the -OH groups.
醇是极性分子，因为 -OH 基团具有强极性的 O-H 键。氧原子的电负性比氢大，产生永久偶极。醇分子之间可以通过 -OH 基团形成氢键。
These intermolecular hydrogen bonds are much stronger than the van der Waals forces found in alkanes of similar relative molecular mass. As a result, alcohols have significantly higher boiling points than their corresponding alkanes.
这些分子间氢键远强于相对分子质量相近的烷烃中的范德华力。因此，醇的沸点显著高于相应的烷烃。
Short-chain alcohols such as methanol, ethanol and propan-1-ol are completely miscible with water. This is because alcohol molecules can form hydrogen bonds with water molecules. As the hydrocarbon chain becomes longer, the non-polar alkyl group interferes with hydrogen bonding, so solubility decreases.
短链醇如甲醇、乙醇和丙-1-醇能与水完全混溶。这是因为醇分子可以和水分子形成氢键。随着烃链增长，非极性烷基会干扰氢键的形成，因此溶解度下降。
5. Trends in Boiling Point and Solubility | 沸点与溶解度递变规律
Within the alcohol homologous series, boiling points increase as the number of carbon atoms increases. This is because longer chains have more electrons and greater surface contact, leading to stronger van der Waals forces.
在醇同系物中，沸点随着碳原子数的增加而升高。这是因为碳链越长，电子数越多，分子接触面积越大，范德华力越强。
Branched-chain alcohols have lower boiling points than their straight-chain isomers. Branching reduces the surface area available for intermolecular contact, weakening the van der Waals forces.
支链醇的沸点低于其直链异构体。支链会减少分子间接触的表面积，削弱范德华力。
The solubility of alcohols in water decreases steadily as the non-polar hydrocarbon chain lengthens. The hydrophobic alkyl portion becomes more dominant, so larger alcohols behave more like alkanes and are less water-soluble.
醇在水中的溶解度随着非极性烃链的增长而逐渐降低。疏水烷基部分逐渐占主导地位，因此较大的醇更接近烷烃的性质，水溶性较差。
6. Combustion of Alcohols | 醇的燃烧
Alcohols are flammable and burn in a plentiful supply of oxygen to produce carbon dioxide and water. Complete combustion is exothermic, releasing a large amount of energy, which is why ethanol is used as a biofuel.
醇易燃，在充足的氧气中燃烧生成二氧化碳和水。完全燃烧是放热反应，释放大量能量，这就是乙醇用作生物燃料的原因。
The general equation for complete combustion of a saturated monohydric alcohol is:
饱和一元醇完全燃烧的通式为：
CₙH₂ₙ₊₁OH + (3n/2) O₂ → n CO₂ + (n+1) H₂O
For example, ethanol burns according to:
例如，乙醇燃烧的方程式为：
C₂H₅OH + 3 O₂ → 2 CO₂ + 3 H₂O
In limited oxygen, incomplete combustion may occur, producing carbon monoxide or carbon. This is a safety concern when alcohol burners are used in poorly ventilated areas.
在氧气不足时可能发生不完全燃烧，生成一氧化碳或碳。在通风不良的地方使用酒精灯时，这是一个安全问题。
7. Reaction with Sodium | 与钠的反应
Alcohols react with reactive metals such as sodium to form an alkoxide and hydrogen gas. The reaction is similar to the reaction of sodium with water, but it is much less vigorous.
醇与钠等活泼金属反应生成醇钠和氢气。该反应与钠和水的反应相似，但剧烈程度要小得多。
The general equation is:
通式为：
2 ROH + 2 Na → 2 RO⁻Na⁺ + H₂
For ethanol, the equation is:
对于乙醇，方程式为：
2 C₂H₅OH + 2 Na → 2 C₂H₅O⁻Na⁺ + H₂
This reaction provides evidence that the -OH group contains a hydrogen atom that can be displaced. The ionic product is called sodium ethoxide when ethanol is used.
该反应证明 -OH 基团中含有一个可被置换的氢原子。当使用乙醇时，生成的离子产物称为乙醇钠。
8. Oxidation of Alcohols | 醇的氧化反应
Oxidation of alcohols is one of the most frequently examined reactions at A-Level. The oxidising agent is usually acidified potassium dichromate(VI), K₂Cr₂O₇, in dilute sulfuric acid. During the reaction, the orange dichromate(VI) ion, Cr₂O₇²⁻, is reduced to the green chromium(III) ion, Cr³⁺.
醇的氧化是 A-Level 中最常考查的反应之一。氧化剂通常为稀硫酸酸化的重铬酸钾(VI)，K₂Cr₂O₇。反应中，橙色的重铬酸根离子 Cr₂O₇²⁻ 被还原为绿色的铬(III) 离子 Cr³⁺。
Primary alcohols are first oxidised to aldehydes. The aldehyde can be isolated by distillation to prevent further oxidation. If the reaction mixture is heated under reflux with excess oxidising agent, the aldehyde is oxidised further to a carboxylic acid.
伯醇首先被氧化为醛。通过蒸馏分离出醛，可以防止其进一步氧化。如果使用过量氧化剂并加热回流，醛会进一步氧化为羧酸。
RCH₂OH + [O] → RCHO + H₂O
RCHO + [O] → RCOOH
Secondary alcohols are oxidised to ketones. Ketones resist further oxidation under these conditions because breaking a C-C bond would be required.
仲醇被氧化为酮。酮在这些条件下难以被进一步氧化，因为进一步氧化需要断裂 C-C 键。
R₁R₂CHOH + [O] → R₁R₂C=O + H₂O
Tertiary alcohols are not readily oxidised by acidified dichromate(VI) because the carbon bearing the -OH group has no hydrogen atom attached to it. Therefore, the orange colour of the oxidising agent remains unchanged.
叔醇不易被酸化重铬酸钾(VI) 氧化，因为连接 -OH 的碳原子上没有氢原子。因此，氧化剂的橙色保持不变。
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 reaction is called esterification and is reversible.
醇在强酸催化剂（通常为浓硫酸）存在下与羧酸反应生成酯和水。该反应称为酯化反应，是可逆的。
The general equation is:
通式为：
RCOOH + R'OH ⇌ RCOOR' + H₂O
For example, ethanol reacts with ethanoic acid to form ethyl ethanoate:
例如，乙醇与乙酸反应生成乙酸乙酯：
CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O
Concentrated sulfuric acid acts as a catalyst and a dehydrating agent, removing water to shift the equilibrium towards ester formation. Esters often have pleasant, fruity smells and are used in flavourings and perfumes.
浓硫酸既作催化剂又作脱水剂，通过除去水使平衡向生成酯的方向移动。酯通常具有令人愉快的水果香味，用于调味剂和香水中。
10. Dehydration to Alkenes | 脱水生成烯烃
Alcohols can be dehydrated to form alkenes. This is an elimination reaction in which water is removed from the alcohol molecule. Ethanol can be dehydrated by passing its vapour over hot aluminium oxide, or by heating with excess concentrated sulfuric acid at about 170 °C.
醇可以脱水生成烯烃。这是一个消除反应，从醇分子中脱去水。乙醇可以通过将蒸气通过热的氧化铝，或与过量浓硫酸在约 170 °C 下加热来脱水。
C₂H₅OH → CH₂=CH₂ + H₂O
For unsymmetrical alcohols, more than one alkene may be formed. The major product is usually the more substituted alkene, in accordance with Zaitsev's rule. For example, butan-2-ol gives but-2-ene as the main product rather than but-1-ene.
对于不对称醇，可能生成不止一种烯烃。主要产物通常是取代较多的烯烃，符合扎伊采夫规则。例如，丁-2-醇脱水的主要产物是丁-2-烯，而不是丁-1-烯。
This reaction is useful in polymer chemistry because ethene produced from ethanol can be used to make poly(ethene).
该反应在聚合物化学中很有用，因为由乙醇制得的乙烯可用于制造聚乙烯。
11. Halogenation Reactions | 卤代反应
Alcohols can be converted into halogenoalkanes by substitution reactions. The -OH group is replaced by a halogen atom. This is an important step in organic synthesis because halogenoalkanes are versatile intermediates.
醇可以通过取代反应转化为卤代烃。-OH 基团被卤素原子取代。这是有机合成中的重要步骤，因为卤代烃是用途广泛的中间体。
With phosphorus(V) chloride, PCl₅, alcohols react vigorously at room temperature to give a chloroalkane, phosphoryl chloride and hydrogen chloride:
醇与五氯化磷 PCl₅ 在室温下剧烈反应，生成氯代烷、三氯氧磷和氯化氢：
ROH + PCl₅ → RCl + POCl₃ + HCl
Alcohols can also react with hydrogen halides, such as HBr or HI, to form bromoalkanes or iodoalkanes. Tertiary alcohols react fastest with hydrogen halides because the intermediate tertiary carbocation is most stable.
醇也可以与氢卤酸（如 HBr 或 HI）反应生成溴代烷或碘代烷。叔醇与氢卤酸反应最快，因为中间体叔碳正离子最稳定。
12. Summary and Exam Tips | 总结与应试提示
Alcohols form a homologous series with the functional group -OH and the general formula CₙH₂ₙ₊₁OH. Their key properties arise from hydrogen bonding, which raises boiling points and increases water solubility for small alcohols.
醇是以 -OH 为官能团、通式为 CₙH₂ₙ₊₁OH 的同系物。它们的关键性质源于氢键，这提高了沸点，并使小分子醇具有较高的水溶性。
The most important reactions are combustion, reaction with sodium, oxidation, esterification, dehydration and halogenation. When answering exam questions, always link the type of alcohol to the expected oxidation product and use full structural or displayed formulae where required.
最重要的反应包括燃烧、与钠反应、氧化、酯化、脱水和卤代。在回答考试题目时，应始终将醇的类型与预期的氧化产物联系起来，并在需要时使用完整结构式或显示式。
Be prepared to explain why tertiary alcohols resist oxidation and why primary alcohols can give two different products depending on distillation or reflux conditions.
要能够解释为什么叔醇不易被氧化，以及为什么伯醇在蒸馏或回流条件下会生成两种不同的产物。
Published by TutorHao | Chemistry Revision Series | aleveler.com
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