IGCSE CCEA Chemistry: Aldehydes and Ketones – Key Points | IGCSE CCEA 化学：醛和酮考点精讲

📚 IGCSE CCEA Chemistry: Aldehydes and Ketones – Key Points | IGCSE CCEA 化学：醛和酮考点精讲

Aldehydes and ketones are two closely related families of organic compounds that both contain the carbonyl group, C=O. Their chemistry appears regularly in IGCSE CCEA examinations, with questions often focusing on structure, naming, preparation from alcohols, and the key reactions that allow us to distinguish between them. Mastering these fundamental ideas will help you answer typical short-answer and structured questions with confidence.

醛和酮是两类密切相关的有机化合物，都含有羰基 C=O。它们的化学性质在 IGCSE CCEA 考试中经常出现，题目常围绕结构、命名、由醇制备的方法以及能够区分它们的关键反应。掌握这些基本概念，将有助于你自信地解答典型的简答题和结构化问题。

1. Introduction to Aldehydes and Ketones | 醛和酮概述

Aldehydes and ketones are carbonyl compounds. In an aldehyde, the carbonyl group is at the end of the carbon chain, so the carbonyl carbon is bonded to at least one hydrogen atom. In a ketone, the carbonyl group is situated on a carbon atom that is attached to two other carbon atoms, never at the very end of the chain.

醛和酮都是羰基化合物。在醛中，羰基位于碳链的末端，因此羰基碳上至少连接着一个氢原子。在酮中，羰基位于一个与另外两个碳原子相连的碳原子上，从不会出现在碳链的绝对末端。

The simplest aldehyde is methanal, with the formula HCHO, commonly known as formaldehyde. The simplest ketone is propanone, CH₃COCH₃, commonly known as acetone. This fundamental difference in position of the carbonyl group leads to significant differences in their chemical behaviour, especially when reacting with mild oxidising agents.

最简单的醛是甲醛，分子式为 HCHO，俗称福尔马林。最简单的酮是丙酮，分子式为 CH₃COCH₃。羰基位置的这一根本性差异，导致它们的化学行为有很大不同，尤其是在与温和氧化剂反应时。

2. Functional Groups and General Formulas | 官能团与通式

The functional group of an aldehyde is –CHO, also written as –CH=O. It is a terminal group. The general molecular formula for saturated aliphatic aldehydes is CₙH₂ₙO, where n is the number of carbon atoms. Remember that the carbon of the –CHO group is counted as carbon number 1.

醛的官能团是 –CHO，也可写作 –CH=O，是一个末端基团。饱和脂肪族醛的通式为 CₙH₂ₙO，其中 n 为碳原子数。注意 –CHO 基团中的碳原子应被计为 1 号碳。

The functional group of a ketone is >C=O, often written as –CO–, situated between two carbon atoms. Saturated aliphatic ketones also follow the general formula CₙH₂ₙO, making them functional group isomers of aldehydes with the same number of carbon atoms. For example, propanal (CH₃CH₂CHO) and propanone (CH₃COCH₃) are isomers with molecular formula C₃H₆O.

酮的官能团是 >C=O，常写作 –CO–，位于两个碳原子之间。饱和脂肪族酮也遵循通式 CₙH₂ₙO，因此它们与碳原子数相同的醛互为官能团异构体。例如，丙醛 (CH₃CH₂CHO) 和丙酮 (CH₃COCH₃) 是同分异构体，分子式均为 C₃H₆O。

In both families, the carbonyl carbon is sp² hybridised, and the C=O bond is polar. The oxygen atom carries a partial negative charge, while the carbon carries a partial positive charge. This polarity influences both physical properties and many addition reactions.

在这两类化合物中，羰基碳都是 sp² 杂化的，且 C=O 键具有极性。氧原子带部分负电荷，碳原子带部分正电荷。这种极性既影响了物理性质，也影响了许多加成反应。

3. Naming Aldehydes and Ketones | 命名规则

For IGCSE CCEA, systematic naming follows IUPAC rules. For aldehydes, identify the longest continuous carbon chain that contains the –CHO group; replace the final ‘-e’ of the corresponding alkane with ‘-al’. The carbon of the aldehyde group is always carbon number 1, so no number is needed for the –CHO position. Substituents are numbered accordingly. Examples: HCHO is methanal; CH₃CHO is ethanal; CH₃CH₂CHO is propanal.

在 IGCSE CCEA 中，系统命名遵循 IUPAC 规则。对于醛，选择含有 –CHO 的最长连续碳链；将相应烷烃词尾的“-e”替换为“-al”。醛基的碳始终是 1 号碳，因此 –CHO 的位置无需编号。取代基按此编号。例如：HCHO 为 methanal；CH₃CHO 为 ethanal；CH₃CH₂CHO 为 propanal。

For ketones, find the longest chain containing the carbonyl group, replace ‘-e’ with ‘-one’, and indicate the position of the carbonyl carbon by the lowest possible number. The simplest ketone, propanone, does not require a number because the carbonyl can only be at position 2 in a three-carbon chain. But butanone (CH₃COCH₂CH₃) has the carbonyl on carbon 2, so the name can be written as butan-2-one or simply butanone. You should be able to name ketones like pentan-2-one and pentan-3-one.

对于酮，找到包含羰基的最长碳链，将“-e”替换为“-one”，并用尽可能小的数字标出羰基碳的位置。最简单的酮——丙酮，不需要标数字，因为在三碳链中羰基只能在 2 号位。但丁酮 (CH₃COCH₂CH₃) 的羰基位于 2 号碳，因此可命名为 butan-2-one 或简称 butanone。你应该能命名 pentan-2-one 和 pentan-3-one 等酮。

Common names like formaldehyde, acetaldehyde, and acetone are also frequently used in exam questions and should be remembered alongside the IUPAC names.

甲醛、乙醛和丙酮等俗名在考题中也经常出现，应与 IUPAC 名称一起记住。

4. Physical Properties | 物理性质

Short-chain aldehydes and ketones are polar molecules. The polarity of the carbonyl group gives rise to permanent dipole-dipole attractions between molecules, so their boiling points are higher than those of alkanes with similar relative molecular masses. However, since aldehydes and ketones cannot form intermolecular hydrogen bonds with themselves (they lack –OH or –NH groups), their boiling points are lower than the corresponding alcohols.

短链醛和酮是极性分子。羰基的极性使分子间产生永久偶极-偶极作用力，因此它们的沸点高于相对分子质量相近的烷烃。但由于醛和酮自身不能形成分子间氢键（缺少 –OH 或 –NH 基团），它们的沸点低于相应的醇。

The lower members, such as methanal, ethanal and propanone, are soluble in water because the carbonyl oxygen can form hydrogen bonds with water molecules. As the hydrocarbon chain lengthens, solubility decreases rapidly because the non-polar part of the molecule dominates.

低级成员如甲醛、乙醛和丙酮可溶于水，因为羰基氧能与水分子形成氢键。随着碳链增长，溶解度迅速下降，因为分子的非极性部分占了主导。

Both aldehydes and ketones have distinctive smells. Methanal has a pungent odour, while ethanal and propanone are quite volatile and have characteristic sweet, fruity or solvent-like smells. This volatility makes them useful as solvents, but also means they must be handled with care in the laboratory.

醛和酮都有特殊的气味。甲醛有刺激性气味，而乙醛和丙酮挥发性较强，具有特征性的甜味、果香味或类似溶剂的气味。这种挥发性使它们可用作溶剂，但也意味着在实验室操作时需小心处理。

5. Preparation: Oxidation of Alcohols | 制备：醇的氧化

In the IGCSE CCEA syllabus, a key preparation route for both aldehydes and ketones is the controlled oxidation of alcohols. A primary alcohol can be oxidised to an aldehyde using an oxidising agent such as acidified potassium dichromate(VI), K₂Cr₂O₇/H⁺, with gentle heating and immediate distillation of the aldehyde as it forms. This prevents further oxidation to the carboxylic acid.

在 IGCSE CCEA 大纲中，醛和酮的一个关键制备途径是醇的控制氧化。伯醇可以用酸性重铬酸钾(VI)溶液 (K₂Cr₂O₇/H⁺) 氧化成醛，需要在温和加热下进行，并立即将生成的醛蒸馏出来。这样可以防止醛进一步氧化成羧酸。

The reaction for ethanol is: CH₃CH₂OH + [O] → CH₃CHO + H₂O. The oxidising agent changes colour from orange to green as dichromate(VI) ions, Cr₂O₇²⁻, are reduced to chromium(III) ions, Cr³⁺.

乙醇的反应为：CH₃CH₂OH + [O] → CH₃CHO + H₂O。氧化剂由橙色变为绿色，因为重铬酸根离子 Cr₂O₇²⁻ 被还原为铬(III)离子 Cr³⁺。

Secondary alcohols are oxidised to ketones under similar conditions. For example, propan-2-ol (CH₃CH(OH)CH₃) gives propanone (CH₃COCH₃). Ketones are resistant to further oxidation under these mild conditions, so immediate distillation is less critical. Tertiary alcohols are not oxidised by dichromate(VI) because they lack a hydrogen atom on the carbon bearing the –OH group.

仲醇在类似条件下被氧化成酮。例如，2-丙醇 (CH₃CH(OH)CH₃) 生成丙酮 (CH₃COCH₃)。在这些温和条件下，酮不易被进一步氧化，因此无需像制备醛那样必须立即蒸馏。叔醇不会被重铬酸钾(VI)氧化，因为其连接 –OH 的碳原子上没有氢原子。

6. Chemical Reactions: Oxidation of Aldehydes | 醛的氧化反应

A key chemical property that distinguishes aldehydes from ketones is that aldehydes are easily oxidised to carboxylic acids, whereas ketones are not oxidised under similar mild conditions. The carbonyl carbon in an aldehyde still carries a hydrogen atom that can be removed, while in a ketone there are two carbon groups, making such oxidation much more difficult.

区分醛和酮的一个关键化学性质是，醛容易被氧化成羧酸，而酮在类似的温和条件下则不被氧化。醛中的羰基碳上仍带有一个可被脱去的氢原子，而酮中有两个碳基团，使得这种氧化变得困难得多。

When an aldehyde is warmed with acidified potassium dichromate(VI), it is oxidised to the corresponding carboxylic acid. For example, ethanal (CH₃CHO) becomes ethanoic acid (CH₃COOH). There is a colour change from orange to green, and this reaction can be used as a test. However, it is not specific because primary alcohols also give the same colour change.

当醛与酸性重铬酸钾(VI)一起加热时，会被氧化成相应的羧酸。例如，乙醛 (CH₃CHO) 会变成乙酸 (CH₃COOH)。溶液由橙色变为绿色，该反应可用作一种检验方法。但它不够专一，因为伯醇也会产生同样的颜色变化。

Ketones do not undergo this oxidation under the same conditions because breaking a C–C bond is required to oxidise them, which dichromate(VI) cannot do under mild heating. Therefore, a ketone left with acidified dichromate(VI) will leave the mixture orange, and no carboxylic acid is formed.

酮在相同条件下不发生此氧化反应，因为氧化酮需要断裂 C–C 键，而重铬酸钾(VI)在温和加热下无法做到这一点。因此，若酮与酸性重铬酸钾(VI)混合，混合物仍为橙色，不会生成羧酸。

7. Distinguishing Between Aldehydes and Ketones: Fehling’s and Tollen’s Tests | 醛酮鉴别：斐林与托伦斯试验

Two specific and sensitive tests for aldehydes are Fehling’s test and Tollen’s test. Both rely on the ability of aldehydes to be oxidised to carboxylate ions under alkaline conditions, while ketones do not react.

两种针对醛的特异性灵敏试验是斐林试验和托伦斯试验。两者都基于醛在碱性条件下能被氧化成羧酸根离子，而酮不发生反应。

In Fehling’s test, an aldehyde is warmed with Fehling’s solution, which contains Cu²⁺ ions complexed with tartrate in an alkaline medium. The aldehyde reduces the blue Cu²⁺ ions to a brick-red precipitate of copper(I) oxide, Cu₂O. The general ionic equation is: RCHO + 2Cu²⁺ + 5OH⁻ → RCOO⁻ + Cu₂O + 3H₂O. A ketone will leave the solution unchanged, remaining clear blue.

在斐林试验中，将醛与斐林试剂（含有碱性介质中与酒石酸根络合的 Cu²⁺ 离子）一起加热。醛将蓝色的 Cu²⁺ 离子还原为砖红色的氧化亚铜沉淀 Cu₂O。净离子方程式为：RCHO + 2Cu²⁺ + 5OH⁻ → RCOO⁻ + Cu₂O + 3H₂O。酮则不会使溶液发生变化，溶液仍为澄清的蓝色。

Tollen’s reagent contains the diamminesilver(I) ion, [Ag(NH₃)₂]⁺. When warmed with an aldehyde, Ag⁺ is reduced to metallic silver, which often deposits as a shiny silver mirror on the inner surface of a clean test tube. The equation is: RCHO + 2[Ag(NH₃)₂]⁺ + 3OH⁻ → RCOO⁻ + 2Ag + 4NH₃ + 2H₂O. Ketones give no silver mirror.

托伦斯试剂含有二氨合银(I)离子 [Ag(NH₃)₂]⁺。与醛一起加热时，Ag⁺ 被还原为金属银，常在干净试管内壁形成光亮的银镜。反应方程式为：RCHO + 2[Ag(NH₃)₂]⁺ + 3OH⁻ → RCOO⁻ + 2Ag + 4NH₃ + 2H₂O。酮不会生成银镜。

These tests are often used in structured exam questions to identify an unknown compound as an aldehyde. It is important to note that both tests require the aldehyde to have a hydrogen atom attached to the carbonyl carbon; thus they are positive for all aldehydes, including aromatic aldehydes, but negative for ketones.

这些试验常用于结构化考题中，以鉴别未知化合物是否为醛。需要注意的是，这两种试验都要求醛的羰基上连有氢原子，因此对所有醛（包括芳香醛）都呈阳性，而对酮则呈阴性。

8. Reduction Reactions | 还原反应

Both aldehydes and ketones can be reduced to alcohols. The standard reducing agent is sodium tetrahydridoborate(III), NaBH₄, often represented as [H] in equations. Aldehydes are reduced to primary alcohols, while ketones are reduced to secondary alcohols.

醛和酮都可以被还原成醇。标准的还原剂是四氢合硼(III)酸钠 NaBH₄，在方程式中常用 [H] 表示。醛被还原为伯醇，酮则被还原为仲醇。

For example, reduction of ethanal (CH₃CHO) yields ethanol (CH₃CH₂OH), a primary alcohol. Reduction of propanone (CH₃COCH₃) yields propan-2-ol (CH₃CH(OH)CH₃), a secondary alcohol. These reactions are addition of hydrogen across the C=O bond, and are important in organic synthesis.

例如，乙醛 (CH₃CHO) 的还原生成乙醇 (CH₃CH₂OH)，一种伯醇。丙酮 (CH₃COCH₃) 的还原生成 2-丙醇 (CH₃CH(OH)CH₃)，一种仲醇。这些反应是氢气加成到 C=O 键上，在有机合成中非常重要。

The reaction conditions usually involve dissolving the carbonyl compound in water or ethanol and adding NaBH₄ at room temperature. Because NaBH₄ is a relatively mild reducing agent, it does not reduce C=C double bonds in the same molecule, providing a useful selectivity in molecules containing both functional groups.

反应条件通常是将羰基化合物溶于水或乙醇，在室温下加入 NaBH₄。由于 NaBH₄ 是一种较温和的还原剂，它不会还原同一分子中的 C=C 双键，为同时含有两种官能团的分子提供了有用的选择性。

9. Addition Reactions with Hydrogen Cyanide | 与氰化氢的加成反应

A characteristic reaction of both aldehydes and ketones is nucleophilic addition. One important example for IGCSE CCEA is the addition of hydrogen cyanide, HCN, to the carbonyl group. The product is a hydroxynitrile (also called cyanohydrin).

醛和酮的一个特征反应是亲核加成。IGCSE CCEA 中的一个重要例子是氰化氢 HCN 与羰基的加成。产物是羟腈（也称氰醇）。

For example, ethanal reacts with HCN to form 2-hydroxypropanenitrile: CH₃CHO + HCN → CH₃CH(OH)CN. Propanone reacts similarly to form 2-hydroxy-2-methylpropanenitrile: CH₃COCH₃ + HCN → CH₃C(OH)(CN)CH₃. The reaction is carried out using potassium cyanide and dilute sulfuric acid to generate HCN in situ, as HCN is a toxic gas.

例如，乙醛与 HCN 反应生成 2-羟基丙腈：CH₃CHO + HCN → CH₃CH(OH)CN。丙酮发生类似反应，生成 2-羟基-2-甲基丙腈：CH₃COCH₃ + HCN → CH₃C(OH)(CN)CH₃。该反应使用氰化钾和稀硫酸来原位生成 HCN，因为 HCN 是有毒气体。

This reaction lengthens the carbon chain by one carbon atom and introduces two functional groups (–OH and –CN). The nitrile group can be hydrolysed to a carboxylic acid, so this sequence is useful in synthesis. Exam questions may ask you to draw the product or outline the mechanism, but for IGCSE, recognition of the product and its uses is usually sufficient.

这个反使碳链增长了一个碳原子，并引入了两个官能团（–OH 和 –CN）。腈基可以水解成羧酸，因此该反应序列在合成中非常有用。考题可能会要求你画出产物或概述机理，但在 IGCSE 中，通常能识别产物及其用途就足够了。

10. Summary of Key Differences | 醛酮关键对比总结

The following table summarises the main differences between aldehydes and ketones, which are frequently tested in CCEA IGCSE Chemistry.

下表总结了醛和酮的主要区别，这些内容在 CCEA IGCSE 化学考试中经常出现。

Property / Test	Aldehyde	Ketone
Functional group position	End of chain, –CHO	Internal, >C=O
Oxidation with Cr₂O₇²⁻/H⁺	Oxidised to carboxylic acid; orange → green	No reaction; remains orange
Fehling’s test	Positive; brick-red Cu₂O precipitate	Negative; blue solution remains
Tollen’s test	Positive; silver mirror formed	Negative; no silver mirror
Reduction with NaBH₄	Reduced to primary alcohol	Reduced to secondary alcohol

When answering exam questions, always link the observation to the chemical change. For instance, a compound that turns Fehling’s solution brick-red and gives a silver mirror is definitely an aldehyde; one that does neither is likely a ketone. Remember that methanal is also an aldehyde and will give positive results in both tests.

回答考题时，始终将观察到的现象与化学变化联系起来。例如，能使斐林试剂变为砖红色并产生银镜的化合物肯定是醛；两种现象都不出现的则很可能是酮。记住甲醛也是醛，在这两种试验中都会给出阳性结果。

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IGCSE CCEA Chemistry: Aldehydes and Ketones – Key Points | IGCSE CCEA 化学：醛和酮 考点精讲