📚 GCSE WJEC Chemistry: Aldehydes and Ketones – Key Points | GCSE WJEC 化学：醛和酮 考点精讲
Welcome to your essential revision guide on aldehydes and ketones for the GCSE WJEC Chemistry specification. These two carbonyl-containing homologous series may look similar, but their different structures lead to contrasting chemical behaviours – especially in oxidation reactions, a topic that appears regularly in exam papers. This article breaks down every key concept, from naming and drawing to test-tube identifications, so you can tackle questions with confidence.
欢迎来到 GCSE WJEC 化学中醛和酮的必备复习指南。这两个含羰基的同系列看起来相似，但它们不同的结构导致了不同的化学行为——尤其是在氧化反应中，这是一个经常出现在试卷中的主题。本文拆解了从命名和绘制到试管鉴别的每一个关键概念，让你能自信地应对考题。
1. Introduction to the Topic | 主题介绍
Aldehydes and ketones are organic compounds that share a common feature: the carbonyl group, a carbon atom double‑bonded to an oxygen atom. In the WJEC GCSE course, you need to recognise their functional groups, understand how to name simple members, and explain why one can be oxidised while the other cannot. You will also learn how to distinguish between them using simple chemical tests.
醛和酮是共享羰基这一共同特征的有机化合物，羰基是一个碳原子与氧原子双键键合。在 WJEC GCSE 课程中，你需要识别它们的官能团，理解如何命名简单的成员，并解释为什么一种能够被氧化而另一种不能。你还将学习如何使用简单的化学测试来区分它们。
These compounds are important building blocks in chemistry. For example, ethanal is used in the production of perfumes and solvents, while propanone (acetone) is a common nail varnish remover. Their distinct reactivity makes the study of aldehydes and ketones an excellent way to link organic structure to chemical properties.
这些化合物是化学中的重要构建模块。例如，乙醛用于制造香水和溶剂，而丙酮（丙酮）是一种常见的洗甲水。它们不同的反应活性使醛和酮的研究成为将有机结构与化学性质联系起来的绝佳途径。
2. The Carbonyl Functional Group | 羰基官能团
Both aldehydes and ketones contain the carbonyl functional group, written as C=O. In an aldehyde, the carbonyl carbon is bonded to at least one hydrogen atom, giving the characteristic group –CHO (where the carbon is double‑bonded to oxygen and also bonded to H). In a ketone, the carbonyl carbon is bonded to two carbon atoms, represented as –CO–.
醛和酮都含有羰基官能团，写作 C=O。在醛中，羰基碳原子至少与一个氢原子键合，形成特征基团 –CHO（碳与氧双键键合，并与 H 键合）。在酮中，羰基碳原子与两个碳原子键合，表示为 –CO–。
This simple structural difference is responsible for their different chemical fates: the hydrogen atom attached directly to the carbonyl carbon in an aldehyde makes it susceptible to oxidation, whereas a ketone lacks this hydrogen and resists oxidation under the same conditions.
这个简单的结构差异导致了它们不同的化学命运：醛中直接连在羰基碳上的氢原子使其易于氧化，而酮缺少这个氢，因此在相同条件下抵抗氧化。
The general molecular formula for open‑chain saturated aldehydes and ketones is the same: CₙH₂ₙO (n ≥ 1 for aldehydes, n ≥ 3 for ketones). They are functional group isomers when they share the same carbon number – for example, propanal (CH₃CH₂CHO) and propanone (CH₃COCH₃) both have the formula C₃H₆O.
开链饱和醛和酮的通式相同：CₙH₂ₙO（醛的 n ≥ 1，酮的 n ≥ 3）。当它们碳数相同时互为官能团异构体——例如，丙醛 (CH₃CH₂CHO) 和丙酮 (CH₃COCH₃) 的分子式都是 C₃H₆O。
3. Naming Aldehydes and Ketones | 醛和酮的命名
For aldehydes, the name is derived from the corresponding alkane by replacing the final ‘‑e’ with ‑al. The carbon of the –CHO group is always numbered as carbon 1, so no locant is needed. For example: CH₃CHO is ethanal, CH₃CH₂CHO is propanal, and CH₃CH₂CH₂CHO is butanal.
醛的命名来自相应的烷烃，将词尾的 “‑e” 替换为 ‑al。–CHO 基团的碳总是编号为碳 1，因此不需要位置编号。例如：CH₃CHO 是乙醛，CH₃CH₂CHO 是丙醛，CH₃CH₂CH₂CHO 是丁醛。
For ketones, the suffix is ‑one. The position of the carbonyl group is indicated by a number if there are five or more carbons. Propanone (CH₃COCH₃) needs no number because there is only one possible position for the carbonyl group. As the chain lengthens, numbering becomes necessary: pentan‑2‑one and pentan‑3‑one are position isomers.
酮的命名后缀为 ‑one。如果碳链含有五个及以上碳原子，需用数字标明羰基的位置。丙酮 (CH₃COCH₃) 不需要数字，因为羰基只有一个可能的位置。随着碳链变长，编号变得必要：2‑戊酮和3‑戊酮就是位置异构体。
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Example: CH₃COCH₂CH₃ is butanone (the C=O must be on carbon 2, so the number is often omitted).
示例：CH₃COCH₂CH₃ 是丁酮（C=O 必须在碳 2 上，因此数字通常省略）。
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Example: CH₃COCH₂CH₂CH₃ is pentan‑2‑one.
示例：CH₃COCH₂CH₂CH₃ 是 2‑戊酮。
4. Physical Properties | 物理性质
Short‑chain aldehydes and ketones are soluble in water because the carbonyl oxygen can form hydrogen bonds with water molecules. As the hydrocarbon chain length increases, solubility decreases. Methanal (formaldehyde) and ethanal are gases at room temperature, but most other common aldehydes and ketones are volatile liquids.
短链醛和酮可溶于水，因为羰基氧能与水分子形成氢键。随着烃链增长，溶解度降低。甲醛（甲醛）和乙醛在室温下为气体，但大多数其他常见的醛和酮是挥发性液体。
Their boiling points are higher than those of alkanes of comparable molecular mass due to the polarity of the carbonyl group, which gives rise to permanent dipole–dipole forces. However, since they lack an –OH group, they cannot form strong intermolecular hydrogen bonds with each other, so their boiling points are significantly lower than those of the corresponding alcohols (e.g. propanal boils at 49°C, while propan‑1‑ol boils at 97°C).
它们的沸点高于相近分子质量的烷烃，这是因为羰基的极性产生了永久偶极‑偶极作用力。然而，由于缺少 –OH 基团，它们之间不能形成强烈的分子间氢键，因此它们的沸点显著低于相应的醇（例如丙醛的沸点为 49°C，而正丙醇的沸点为 97°C）。
5. Oxidation of Aldehydes | 醛的氧化
Aldehydes are easily oxidised to carboxylic acids. The oxidising agent commonly used in the laboratory is acidified potassium dichromate(VI), K₂Cr₂O₇ dissolved in dilute sulfuric acid. When heated with an aldehyde, the orange solution turns green because the Cr₂O₇²⁻ ions are reduced to Cr³⁺ ions.
醛容易被氧化成羧酸。实验室常用的氧化剂是酸化重铬酸钾(VI)，即将 K₂Cr₂O₇ 溶于稀硫酸。与醛共热时，橙色溶液变为绿色，因为 Cr₂O₇²⁻ 离子被还原为 Cr³⁺ 离子。
The essential conversion is represented by the equation:
关键的转化可用方程式表示：
RCHO + [O] → RCOOH
A specific example is the oxidation of ethanal to ethanoic acid:
一个具体的例子是乙醛被氧化成乙酸：
CH₃CHO + [O] → CH₃COOH
In the laboratory, the aldehyde is heated under reflux with the oxidising agent to ensure complete conversion to the carboxylic acid. This reaction confirms the presence of the reactive –CHO group.
在实验室中，醛与氧化剂在回流条件下加热，以确保完全转化为羧酸。该反应证实了 –CHO 活性基团的存在。
6. Why Ketones Resist Oxidation | 为什么酮不易被氧化
Ketones cannot be easily oxidised under the same conditions that oxidise aldehydes. The reason lies in the absence of a hydrogen atom directly bonded to the carbonyl carbon. Oxidation of the carbonyl carbon would require breaking a strong C–C bond, which is not energetically favourable under mild conditions.
酮无法在与氧化醛相同的条件下轻易被氧化。原因在于缺少直接连接在羰基碳上的氢原子。羰基碳的氧化需要断裂牢固的 C–C 键，这在温和条件下能量上是不利的。
When a ketone is heated with acidified potassium dichromate(VI), the orange colour persists – there is no colour change to green and no carboxylic acid is formed. This striking difference forms the basis of an important chemical test to distinguish between aldehydes and ketones.
当酮与酸化重铬酸钾(VI)共热时，橙色保持不变——没有颜色变为绿色，也没有羧酸形成。这一显著的差异构成了区分醛和酮的重要化学测试的基础。
Thus, the oxidation test with acidified K₂Cr₂O₇ is a quick way to identify an aldehyde: a colour change from orange to green indicates oxidation, while no change suggests a ketone (or a non‑oxidizable compound).
因此，使用酸化 K₂Cr₂O₇ 的氧化测试是快速识别醛的方法：橙色变为绿色表明发生氧化，而无变化则提示可能是酮（或其他不易氧化的化合物）。
7. The Fehling’s Test for Aldehydes | 斐林试剂检测醛
Fehling’s solution is a specific, mild oxidising agent used to test for the presence of aldehydes. It consists of two separate solutions mixed just before use: Fehling’s A (copper(II) sulfate solution, blue) and Fehling’s B (a mixture of sodium hydroxide and sodium potassium tartrate). The tartrate forms a complex with Cu²⁺, keeping it dissolved in the alkaline mixture.
斐林试剂是一种用于检测醛的特定而温和的氧化剂。它由使用前混合的两种溶液组成：斐林 A（硫酸铜(II)溶液，蓝色）和斐林 B（氢氧化钠与酒石酸钾钠的混合物）。酒石酸根与 Cu²⁺ 形成配合物，使其溶解在碱性混合液中。
When a few drops of an aldehyde are added to Fehling’s solution and the mixture is warmed in a water bath, the blue solution gradually produces a brick‑red precipitate of copper(I) oxide, Cu₂O. The aldehyde itself is oxidised to the corresponding carboxylic acid (in salt form under alkaline conditions). Ketones give no precipitate; the blue colour remains unchanged.
当滴加少量醛到斐林试剂中并在水浴中温热时，蓝色溶液逐渐产生砖红色的氧化亚铜 (Cu₂O) 沉淀。醛自身被氧化为相应的羧酸（在碱性条件下以盐的形式存在）。酮不产生沉淀，蓝色保持不变。
| Reagent | Observation with aldehyde | Observation with ketone |
|---|---|---|
| Fehling’s solution 斐林试剂 |
Blue solution → brick‑red precipitate 蓝色溶液 → 砖红色沉淀 |
No change – remains blue 无变化——保持蓝色 |
This test is particularly useful in identifying reducing sugars as well, since sugars with an aldehyde group also produce a positive result.
该测试在鉴定还原糖时也特别有用，因为具有醛基的糖也会产生阳性结果。
8. The Tollens’ Test (Silver Mirror Test) | 多伦试剂检测（银镜反应）
Tollens’ reagent is another mild oxidising agent used to distinguish aldehydes from ketones. It is prepared by adding sodium hydroxide to silver nitrate solution to form silver oxide, then adding dilute ammonia until the precipitate just dissolves, forming the complex ion [Ag(NH₃)₂]⁺.
多伦试剂是另一种用于区分醛和酮的温和氧化剂。制备方法是向硝酸银溶液中加入氢氧化钠生成氧化银沉淀，然后加入稀氨水直至沉淀恰好溶解，形成配合离子 [Ag(NH₃)₂]⁺。
A clean test tube is crucial for success. The aldehyde is added to Tollens’ reagent and warmed. If an aldehyde is present, the Ag⁺ ions are reduced to metallic silver, which deposits as a shiny mirror on the inner surface of the test tube. Ketones do not produce a silver mirror.
洁净的试管对于实验成功至关重要。将醛加入多伦试剂并温热。若存在醛，Ag⁺ 离子被还原为金属银，在试管内壁沉积形成光亮的银镜。酮不会产生银镜。
The overall equation for the reaction with ethanal can be written as:
与乙醛反应的总方程式可写作：
CH₃CHO + 2[Ag(NH₃)₂]⁺ + 3OH⁻ → CH₃COO⁻ + 2Ag↓ + 4NH₃ + 2H₂O
Key comparison: Fehling’s test gives a red precipitate, while Tollens’ test gives a silver mirror. Both are specific for aldehydes and are commonly referenced in WJEC exam questions.
关键对比：斐林测试产生红色沉淀，而多伦测试产生银镜。两者都对醛具有特异性，在 WJEC 考试题目中常被提及。
9. Oxidation of Alcohols – Review Link | 醇的氧化——回顾链接
Understanding how aldehydes and ketones are made helps reinforce their reactivity. Primary alcohols can be oxidised by acidified dichromate first to aldehydes, and then further to carboxylic acids. To obtain the aldehyde, it must be distilled out of the reaction mixture as soon as it forms, preventing further oxidation. Secondary alcohols are oxidised to ketones, which do not oxidise further under these conditions. Tertiary alcohols do not undergo oxidation with dichromate.
理解醛和酮的制备方法有助于巩固它们的反应性。伯醇可被酸化重铬酸盐氧化，首先生成醛，然后进一步氧化为羧酸。要获得醛，必须在它生成后立即从反应混合物中蒸馏出来，以防止进一步氧化。仲醇被氧化为酮，酮在此条件下不会继续被氧化。叔醇不会与重铬酸盐发生氧化反应。
| Alcohol type | Oxidation product 1 | Further oxidation |
|---|---|---|
| Primary (1°) – e.g. ethanol 伯醇，如乙醇 |
Aldehyde (
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