GCSE OCR Chemistry: Aldehydes and Ketones Exam Essentials | GCSE OCR 化学：醛和酮考点精讲

📚 GCSE OCR Chemistry: Aldehydes and Ketones Exam Essentials | GCSE OCR 化学：醛和酮考点精讲

For GCSE OCR Chemistry, mastering the homologous series of aldehydes and ketones is crucial for Organic Chemistry. This concise guide breaks down functional groups, naming, properties, key reactions, and exam-style application to help you secure top marks.

在 GCSE OCR 化学中，掌握醛和酮这两个同系物对有机化学至关重要。这份精讲将官能团、命名、性质、核心反应以及考试应用一一拆解，助你稳拿高分。

1. Functional Groups: The Carbonyl C=O | 官能团：羰基 C=O

Both aldehydes and ketones contain the carbonyl group, a carbon atom doubly bonded to an oxygen atom, written as C=O. The chemistry of these compounds is dominated by this polar, unsaturated group.

醛和酮都含有羰基，即碳原子与氧原子通过双键连接，写作 C=O。这些化合物的化学性质主要由这个极性不饱和基团主导。

In an aldehyde, the carbonyl carbon is bonded to at least one hydrogen atom, with the general formula R–CHO (where R can be H or an alkyl group). The functional group is often displayed as –CHO.

在醛中，羰基碳至少与一个氢原子相连，通式为 R–CHO（R 可以是 H 或烷基）。官能团常表示为 –CHO。

In a ketone, the carbonyl carbon is bonded to two alkyl or aryl groups, with the general formula R–CO–R’. The functional group is written as –CO–.

在酮中，羰基碳与两个烷基或芳基相连，通式为 R–CO–R’。官能团写作 –CO–。

2. Homologous Series and General Formulae | 同系物与通式

Aldehydes and ketones form separate homologous series with the same general formula CₙH₂ₙO (for saturated, non-cyclic carbonyls). This makes them functional group isomers of each other.

醛和酮各自形成同系物，且具有相同的通式 CₙH₂ₙO（针对饱和、非环状羰基化合物）。这使得它们互为官能团异构体。

For example, propanal (an aldehyde) and propanone (a ketone) both have the molecular formula C₃H₆O, but differ in the position of the carbonyl group and the bonding of the carbonyl carbon.

例如，丙醛（一种醛）和丙酮（一种酮）分子式均为 C₃H₆O，区别在于羰基的位置和羰基碳的连接方式。

The simplest aldehyde is methanal (H–CHO), while the simplest ketone is propanone (CH₃–CO–CH₃).

最简单的醛是甲醛（H–CHO），而最简单的酮是丙酮（CH₃–CO–CH₃）。

3. Naming Aldehydes and Ketones | 醛和酮的命名

OCR GCSE expects you to name and draw aliphatic aldehydes and ketones with up to six carbon atoms. Use systematic IUPAC rules.

OCR GCSE 要求你能够命名和绘制最多含六个碳原子的脂肪族醛和酮。使用系统 IUPAC 规则。

For aldehydes: select the longest continuous carbon chain containing the –CHO group. Replace the final ‘e’ of the parent alkane with ‘al’. Number the chain starting from the carbonyl carbon, which is always carbon-1, so the position number is not needed in the name.

对于醛：选择包含 –CHO 的最长连续碳链。将母体烷烃末尾的 ‘e’ 替换为 ‘al’。从羰基碳开始编号，它总是 1 号碳，因此名称中无需标出位置数字。

Examples: HCHO is methanal; CH₃CHO is ethanal; CH₃CH₂CHO is propanal.

例子：HCHO 是甲醛；CH₃CHO 是乙醛；CH₃CH₂CHO 是丙醛。

For ketones: find the longest chain containing the carbonyl group, replace ‘e’ with ‘one’, and use a number to indicate the position of the carbonyl carbon, applied as the lowest locant possible.

对于酮：找到包含羰基的最长碳链，将 ‘e’ 替换为 ‘one’，并用数字标出羰基碳的位置，确保位号尽可能小。

Example: CH₃COCH₂CH₃ is butan-2-one (not butan-3-one). Propanone is correctly named without a number because the carbonyl can only be at the 2-position in a three-carbon chain.

例子：CH₃COCH₂CH₃ 是丁-2-酮（不是丁-3-酮）。丙酮无需用数字标出位置，因为在三碳链上羰基只能在 2 号位。

4. Physical Properties: Boiling Points and Solubility | 物理性质：沸点与溶解度

Aldehydes and ketones are polar molecules due to the electronegativity difference between carbon and oxygen in C=O. This gives rise to permanent dipole–dipole attractions between molecules.

醛和酮是极性分子，因为 C=O 中碳和氧的电负性不同。这使得分子间存在永久偶极-偶极引力。

Unlike alcohols, carbonyls cannot form hydrogen bonds between their own molecules because there is no hydrogen atom bonded to oxygen in the functional group. Therefore, their boiling points are lower than those of corresponding alcohols but higher than non-polar alkanes of similar relative molecular mass.

与醇不同，羰基化合物自身分子间不能形成氢键，因为官能团中没有与氧相连的氢原子。因此，它们的沸点低于相应的醇，但高于相对分子质量相近的非极性烷烃。

Short-chain aldehydes and ketones (e.g., methanal, ethanal, propanone) are soluble in water because they can form hydrogen bonds with water molecules using the lone pairs on the carbonyl oxygen.

短链醛和酮（如甲醛、乙醛、丙酮）可溶于水，因为它们能利用羰基氧上的孤对电子与水分子形成氢键。

As carbon chain length increases, solubility in water decreases because the non-polar hydrocarbon portion becomes more dominant.

随着碳链增长，水溶性下降，因为非极性的碳氢部分占了主导。

5. Oxidation Reactions: Telling Aldehydes and Ketones Apart | 氧化反应：区分醛和酮

A key chemical difference assessed at GCSE is that aldehydes can be oxidised, while ketones cannot. This is because the aldehyde has a hydrogen atom attached to the carbonyl carbon that can be removed.

GCSE 阶段考察的一个关键化学差异是：醛可以被氧化，而酮则不能。这是因为醛的羰基碳上连有一个可被脱去的氢原子。

Oxidation of an aldehyde produces a carboxylic acid containing the –COOH functional group. The oxygen is supplied by the oxidising agent, and water is formed in the reaction.

醛被氧化生成含有 –COOH 官能团的羧酸。氧化剂提供氧，反应中生成水。

For ethanal: CH₃CHO + [O] → CH₃COOH (ethanoic acid). The [O] symbol represents nascent oxygen from an oxidising mixture like acidified potassium dichromate(VI).

以乙醛为例：CH₃CHO + [O] → CH₃COOH（乙酸）。[O] 代表来自酸性重铬酸钾(VI)等氧化剂的新生氧。

6. Common Oxidising Agents and Colour Changes | 常用氧化剂与颜色变化

The two oxidising agents you must know for OCR GCSE are acidified potassium dichromate(VI) solution (K₂Cr₂O₇/H⁺) and Fehling’s solution.

OCR GCSE 要求你掌握的两种氧化剂是酸化重铬酸钾(VI)溶液 (K₂Cr₂O₇/H⁺) 和斐林试剂。

With acidified potassium dichromate(VI): aldehyde + [O] → carboxylic acid. The orange solution turns green (Cr³⁺ ions formed). Ketones give no colour change, which acts as a test for aldehydes.

使用酸化重铬酸钾(VI)：醛 + [O] → 羧酸。橙色溶液变为绿色（生成 Cr³⁺ 离子）。酮不发生颜色变化，这可作为醛的检验方法。

With Fehling’s solution (a blue alkaline solution of copper(II) sulfate and tartrate ions): aldehyde is oxidised, and the blue Cu²⁺ ions are reduced to a brick-red precipitate of copper(I) oxide, Cu₂O. Ketones show no reaction.

使用斐林试剂（硫酸铜(II)与酒石酸根离子的蓝色碱性溶液）：醛被氧化，蓝色的 Cu²⁺ 离子被还原为砖红色的氧化亚铜 (Cu₂O) 沉淀。酮无反应。

7. The Triiodomethane (Iodoform) Test for Methyl Ketones | 甲基酮的碘仿试验

A specific test distinguishes ketones with a methyl group directly attached to the carbonyl carbon (CH₃–CO– group). This is the triiodomethane (iodoform) reaction.

碘仿试验可以特异性地区分羰基碳上直接连有甲基 (CH₃–CO–) 的酮。

The compound is warmed with alkaline iodine solution. A positive result gives a pale yellow precipitate of triiodomethane, CHI₃, with a characteristic antiseptic smell.

将样品与碱性碘溶液混合加热。阳性结果析出淡黄色的三碘甲烷 (CHI₃) 沉淀，带有特有的消毒剂气味。

Propanone (CH₃COCH₃) and butan-2-one (CH₃COCH₂CH₃) give a positive test. Aldehydes do not give this test unless they are ethanal (CH₃CHO), which also has the CH₃CO– grouping after oxidation under the reaction conditions.

丙酮 (CH₃COCH₃) 和丁-2-酮 (CH₃COCH₂CH₃) 呈阳性结果。醛类一般不反应，但乙醛 (CH₃CHO) 例外，因为在反应条件下它被氧化后也会形成 CH₃CO– 结构。

8. Reduction of Carbonyls: Forming Alcohols | 羰基的还原：生成醇

Both aldehydes and ketones can be reduced back to alcohols. This is the reverse of oxidation for aldehydes, but it represents the addition of hydrogen across the C=O double bond.

醛和酮都可以被还原成醇。这好比是醛的氧化逆反应，本质上是氢气加成到 C=O 双键上。

A suitable reducing agent is sodium borohydride, NaBH₄, used in aqueous or alcoholic solution. The symbol [H] represents the reducing agent, with the general equation: R–CO–R’ + 2[H] → R–CHOH–R’.

常用的还原剂是硼氢化钠 NaBH₄（溶于水或醇中）。用 [H] 表示还原剂，通式为：R–CO–R’ + 2[H] → R–CHOH–R’。

Aldehydes are reduced to primary alcohols: CH₃CHO + 2[H] → CH₃CH₂OH (ethanol). Ketones are reduced to secondary alcohols: CH₃COCH₃ + 2[H] → CH₃CHOHCH₃ (propan-2-ol).

醛被还原为伯醇：CH₃CHO + 2[H] → CH₃CH₂OH（乙醇）。酮被还原为仲醇：CH₃COCH₃ + 2[H] → CH₃CHOHCH₃（丙-2-醇）。

9. Comparing Aldehydes, Ketones and Carboxylic Acids | 醛、酮与羧酸的比较

Confusion often arises in exams between these three oxygen-containing families. A systematic comparison will safeguard your answers.

考试中常混淆这三类含氧有机物。系统对比能确保你的答案准确无误。

Property	Aldehyde	Ketone	Carboxylic Acid
Functional group	–CHO (terminal)	–CO– (internal)	–COOH (terminal)
Oxidation with K₂Cr₂O₇/H⁺	Yes → carboxylic acid	No reaction	Already fully oxidised; resists further oxidation
Fehling’s test	Brick‑red ppt	No change	No change
Bromine water	No decolourisation (saturated)	No decolourisation (saturated)	No decolourisation
Reduction product	1° alcohol	2° alcohol	1° alcohol (with strong reducing agent)

Property 性质，Aldehyde 醛，Ketone 酮，Carboxylic Acid 羧酸，Functional group 官能团，Oxidation with K₂Cr₂O₇/H⁺ 酸性重铬酸钾氧化，Fehling’s test 斐林试验，Bromine water 溴水，Reduction product 还原产物，No reaction 不反应，Brick‑red ppt 砖红色沉淀，No change 无变化，No decolourisation 不褪色。

10. Drawing Structures and Displayed Formulae | 绘制结构式和展示式

OCR exam papers frequently ask for displayed formulae. Always show every bond, including the C=O double bond. For an aldehyde, you must draw the group as –C(H)=O, with the hydrogen explicitly shown on the carbonyl carbon.

OCR 试卷常要求画展示式。务必画出所有键，包括 C=O 双键。对于醛，必须将基团画为 –C(H)=O，明确显示出连在羰基碳上的氢原子。

Example for ethanal: CH₃–C(H)=O is one correct representation. A common mistake is to forget the hydrogen on the aldehyde carbon, making it look like a ketone.

乙醛的正确表示之一为 CH₃–C(H)=O。常见错误是遗漏醛碳上的氢原子，导致画出来像个酮。

For ketones, the carbonyl carbon is connected to two carbon atoms. Display correctly as –C(=O)– between two carbon skeletons.

对于酮，羰基碳连接两个碳原子，正确绘制为夹在两个碳骨架间的 –C(=O)–。

11. Predicting Products and Exam Short-Answer Tips | 产物的预测与简答题技巧

When given an unfamiliar aldehyde or ketone, count carbons carefully, locate the C=O, and apply the logic: aldehyde → oxidise to acid; ketone → no oxidation. For reduction, both give alcohols with the same carbon skeleton.

遇到不熟悉的醛或酮时，仔细数碳原子，定位 C=O，按逻辑处理：醛 → 氧化成酸；酮 → 不氧化。还原时，两者都生成碳骨架相同的醇。

Write balanced equations using [O] or [H] as appropriate, and ensure the carbon chain length and functional groups are accurate. OCR allows the use of molecular or structural formulae.

书写方程式时用 [O] 或 [H] 表示氧化或还原，确保碳链长度和官能团正确。OCR 接受分子式或结构式。

In tests: for distinguishing, always state the reagent, the condition (warm if needed), and the observation. E.g., ‘Add acidified potassium dichromate(VI), warm; with propanal solution turns green, with propanone it stays orange.’

在鉴别题中：务必写明试剂、条件（如需加热）和观察结果。例如：“加入酸化重铬酸钾(VI)，微热；丙醛溶液变绿，丙酮溶液保持橙色。”

12. Checking Understanding: Quick Quiz | 理解自查：快速问答

Test yourself: 1) Name CH₃CH₂CH₂CHO. 2) Write the product of oxidation of this compound. 3) How could you distinguish propanal from propanone using Fehling’s solution?

自测：1) 命名 CH₃CH₂CH₂CHO。2) 写出该化合物氧化的产物。3) 如何用斐林试剂区分丙醛和丙酮？

Answers: 1) Butanal. 2) Butanoic acid, CH₃CH₂CH₂COOH. 3) Warm with Fehling’s solution; propanal gives a brick‑red precipitate, propanone shows no change. If these are not instantly clear, revisit the corresponding sections above.

答案：1) 丁醛。2) 丁酸，CH₃CH₂CH₂COOH。3) 与斐林试剂共热；丙醛产生砖红色沉淀，丙酮无变化。如果不能立刻作答，请回看上方对应小节。

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GCSE OCR Chemistry: Aldehydes and Ketones Exam Essentials | GCSE OCR 化学：醛和酮 考点精讲