Aldehydes and Ketones: Key Points for A-Level OCR Chemistry

📚 Aldehydes and Ketones: Key Points for A-Level OCR Chemistry | 醛和酮考点精讲

Aldehydes and ketones are carbonyl compounds central to organic synthesis and analysis. For OCR A-Level Chemistry, you must master their structures, naming, physical properties, preparation, characteristic addition–elimination reactions, and the tests used to distinguish them. This article breaks down every essential point into clear, bilingual explanations, helping you tackle multiple-choice questions, structured problems, and practical assessments with confidence.

醛和酮是含有羰基的有机化合物，在有机合成与分析中处于核心地位。针对 OCR A-Level 化学考试，你需要掌握它们的结构、命名、物理性质、制备方法、特征性的亲核加成反应，以及用于区分它们的鉴别测试。本文以清晰的双语讲解逐个击破所有重要考点，帮助你自信应对选择题、结构化问题及实验评估。

1. The Carbonyl Group and Functional Group Recognition | 羰基与官能团识别

The carbonyl group consists of a carbon atom doubly bonded to an oxygen atom, written as C=O. In aldehydes, the carbonyl carbon is bonded to at least one hydrogen atom, giving the functional group –CHO. In ketones, the carbonyl carbon is bonded to two other carbon atoms, giving the functional group –CO–.

羰基由碳原子与氧原子双键相连构成，记作 C=O。在醛中，羰基碳上至少连接一个氢原子，官能团为 –CHO。在酮中，羰基碳上连接两个其他碳原子，官能团为 –CO–。

In displayed formulas, an aldehyde always has a hydrogen atom directly attached to the C=O carbon, whereas a ketone never does. This simple difference explains why aldehydes can be oxidised further to carboxylic acids while ketones cannot under mild conditions.

在结构式中，醛的 C=O 碳上永远直接连有氢原子，而酮则没有。这个简单的差异解释了为何醛可以进一步氧化成羧酸，而酮在温和条件下不能发生氧化。

Alkanal general formula: CₙH₂ₙO (n ≥ 1).
Alkanone general formula: CₙH₂ₙO (n ≥ 3).

一元饱和醛的通式：CₙH₂ₙO（n ≥ 1）。
一元饱和酮的通式：CₙH₂ₙO（n ≥ 3）。

2. IUPAC Naming of Aldehydes and Ketones | 醛和酮的系统命名

For aldehydes, identify the longest continuous carbon chain containing the –CHO group. Replace the final ‘e’ of the corresponding alkane with ‘al’. The carbonyl carbon is always carbon number 1, so you do not need to include its locant in the name. For example, HCHO is methanal, CH₃CHO is ethanal, CH₃CH₂CHO is propanal.

对于醛，选择包含 –CHO 的最长连续碳链，将相应烷烃名称末尾的 ‘e’ 替换为 ‘al’。羰基碳永远是 1 号碳，因此命名时无需标出位次。例如 HCHO 为 methanal，CH₃CHO 为 ethanal，CH₃CH₂CHO 为 propanal。

For ketones, select the longest chain containing the carbonyl group, replace the ‘e’ with ‘one’, and insert the lowest possible number before ‘one’ to indicate the position of the carbonyl group. For example, CH₃COCH₃ is propanone, CH₃COCH₂CH₃ is butanone, and CH₃CH₂COCH₂CH₃ is pentan-3-one.

对于酮，选择含羰基的最长碳链，将 ‘e’ 替换为 ‘one’，并在 ‘one’ 前使用尽可能小的数字标明羰基位置。例如 CH₃COCH₃ 为 propanone，CH₃COCH₂CH₃ 为 butanone，CH₃CH₂COCH₂CH₃ 为 pentan-3-one。

In molecules where a higher priority group is present (e.g. –COOH), the aldehyde or ketone may be named as a substituent using ‘oxo’ or ‘formyl’. However, at A-Level, you are mainly expected to name simple aldehydes and ketones.

若分子中存在优先度更高的基团（如 –COOH），醛或酮可能以 ‘oxo’ 或 ‘formyl’ 作为取代基命名。不过在 A-Level 阶段，主要要求掌握简单醛酮的命名。

3. Physical Properties and Solubility | 物理性质与溶解性

Short-chain aldehydes and ketones are polar molecules because of the significant difference in electronegativity between carbon and oxygen of the carbonyl group. However, they cannot form hydrogen bonds with themselves, as there is no hydrogen attached to a highly electronegative atom in the functional group. Consequently, their boiling points are higher than those of comparable alkanes but lower than those of corresponding alcohols, which can form intermolecular hydrogen bonds.

短链醛和酮是极性分子，因为羰基中碳和氧的电负性差异显著。但由于官能团中没有连接在高电负性原子上的氢，它们自身之间无法形成氢键。因此，它们的沸点高于相对分子质量相近的烷烃，但低于能形成分子间氢键的相应醇。

Lower aldehydes and ketones (methanal, ethanal, propanone) are miscible with water because the carbonyl oxygen can form hydrogen bonds with water molecules. As the hydrocarbon chain lengthens, solubility decreases rapidly due to the increasing non-polar character of the molecule.

低级醛和酮（甲醛、乙醛、丙酮）可与水混溶，因为羰基氧能与水分子形成氢键。随着烃链增长，分子的非极性增强，溶解度迅速下降。

Compound	Mᵣ	Boiling point / °C	Solubility in water
Propan-1-ol	60	97	Miscible
Propanal	58	49	Slightly soluble
Propanone	58	56	Miscible
Butane	58	0	Insoluble

4. Preparation of Aldehydes and Ketones | 醛和酮的制备

Aldehydes can be prepared in the laboratory by the controlled oxidation of primary alcohols using acidified potassium dichromate(VI). The aldehyde must be distilled off immediately as it is formed to prevent further oxidation to the carboxylic acid. Apparatus for distillation with addition of the oxidising agent is typical of this preparation.

实验室中，醛可通过伯醇被酸化重铬酸钾(VI)控制氧化来制备。生成的醛必须立即蒸馏分离出来，以防止继续氧化成羧酸。典型的制备装置是边滴加氧化剂边蒸馏。

Ketones are prepared by oxidising secondary alcohols with acidified potassium dichromate(VI) under reflux. Because ketones do not have a hydrogen on the carbonyl carbon, they cannot be easily oxidised further, so reflux conditions are safe to use. The ketone product can then be separated by distillation.

酮可通过仲醇在回流条件下被酸化重铬酸钾(VI)氧化制备。由于酮的羰基碳上没有氢原子，不易被继续氧化，因此可以安全使用回流条件。生成的酮随后通过蒸馏分离。

On an industrial scale, ethanal is produced by the Wacker process (oxidation of ethene) and propanone is produced via the cumene process. These are not core OCR content but provide interesting context.

工业上，乙醛通过瓦克法（乙烯氧化）生产，丙酮则通过异丙苯法生产。这些并非 OCR 核心考查内容，但可作为背景了解。

Primary alcohol + [O] → Aldehyde + H₂O

Secondary alcohol + [O] → Ketone + H₂O

5. Nucleophilic Addition Reactions: The Key Mechanism | 亲核加成反应：核心机理

Both aldehydes and ketones undergo nucleophilic addition reactions because the carbonyl carbon is electron-deficient due to the polar C=O bond. Oxygen is more electronegative than carbon, causing the π electrons to be pulled towards oxygen. This makes the carbon δ+, so it is susceptible to attack by nucleophiles.

醛和酮都能发生亲核加成反应，因为极性的 C=O 键使得羰基碳缺电子。氧的电负性大于碳，π 电子被拉向氧，使碳带部分正电荷 δ+，容易受到亲核试剂的进攻。

The mechanism involves the nucleophile approaching the planar carbonyl group and attacking the δ+ carbon. The π bond breaks heterolytically, with both electrons moving to oxygen, producing an alkoxide intermediate. This intermediate then abstracts a proton (usually from water or an acid) to form the final addition product. You must be able to draw this mechanism with curly arrows.

机理为：亲核试剂从平面的羰基一侧进攻 δ+ 碳，π 键异裂，两个电子都移到氧上，生成烷氧负离子中间体。该中间体随后夺取一个质子（通常来自水或酸），形成最终加成产物。你必须能用弯箭号画出该机理。

OCR expects you to apply this mechanism to the reactions with NaBH₄/LiAlH₄, HCN, and hydrazine derivatives. Always show the dipole on C=O, the lone pair on the nucleophile attacking the carbon, the breaking π bond with an arrow going to oxygen, and the protonation step.

OCR 要求你将此机理应用于与 NaBH₄/LiAlH₄、HCN 以及肼衍生物的反应。务必标出 C=O 的偶极，亲核试剂上孤对电子进攻碳，π 键断裂的箭头指向氧，以及夺取质子的步骤。

6. Reduction of Aldehydes and Ketones | 醛和酮的还原

Aldehydes are reduced to primary alcohols and ketones to secondary alcohols. The most common reducing agent in organic synthesis is sodium tetrahydridoborate(III), NaBH₄, in aqueous or alcoholic solution. The mechanism follows typical nucleophilic addition: the hydride ion H⁻ (from NaBH₄) acts as a nucleophile, attacking the carbonyl carbon.

醛被还原为伯醇，酮被还原为仲醇。有机合成中最常用的还原剂是硼氢化钠 NaBH₄，在水或醇溶液中使用。机理遵循典型的亲核加成：来自 NaBH₄ 的氢负离子 H⁻ 作为亲核试剂进攻羰基碳。

The nucleophilic addition equation can be represented using [H] to denote reduction. For example: CH₃CHO + 2[H] → CH₃CH₂OH. Propanone + 2[H] → Propan-2-ol.

亲核加成方程式可用 [H] 表示还原。例如：CH₃CHO + 2[H] → CH₃CH₂OH。丙酮 + 2[H] → 2-丙醇。

Lithium tetrahydridoaluminate(III), LiAlH₄, is a more powerful reducing agent but is used in dry ether; it is less common at A-Level specifically for these conversions. The main focus is NaBH₄ because it is chemoselective – it reduces carbonyl groups but not carbon–carbon double bonds or esters under typical A-Level conditions.

四氢铝锂 LiAlH₄ 是更强的还原剂，但需在干醚中使用；A-Level 阶段更侧重 NaBH₄，因为它具有化学选择性——在典型条件下只还原羰基，而不还原碳碳双键或酯基。

7. Addition with Hydrogen Cyanide (HCN) | 与氰化氢的加成

Aldehydes and ketones react with hydrogen cyanide to form hydroxynitriles (also called cyanohydrins). This reaction is important because it extends the carbon chain by one carbon atom. The nitrile group –CN can later be hydrolysed to a carboxylic acid or reduced to an amine, providing versatile synthetic intermediates.

醛和酮与氰化氢反应生成羟基腈（也称氰醇）。该反应的重要意义在于使碳链增长一个碳原子。–CN 氰基后续可水解为羧酸或还原成胺，提供多用途的合成中间体。

The reaction uses HCN gas, or more safely, KCN in aqueous acid to generate HCN in situ. The mechanism is again nucleophilic addition: the cyanide ion CN⁻ attacks the carbonyl carbon, followed by protonation of the oxygen. OCR will ask you to draw the mechanism and name the product, e.g. ethanal + HCN gives 2-hydroxypropanenitrile.

反应使用 HCN 气体，或更安全地，用 KCN 在酸性水溶液中原位产生 HCN。机理同样是亲核加成：氰根离子 CN⁻ 进攻羰基碳，随后氧负离子质子化。OCR 会考查画出机理和命名产物，如乙醛 + HCN 生成 2-羟基丙腈。

CH₃CHO + HCN → CH₃CH(OH)CN

乙醛 + 氰化氢 → 2-羟基丙腈

You should note that the product has both a hydroxyl (–OH) and a nitrile (–CN) group. The carbon attached to both groups is now chiral if the starting carbonyl compound has two different R groups, raising the possibility of optical isomerism.

注意产物同时含有羟基（–OH）和氰基（–CN）。如果起始羰基化合物连有两个不同的 R 基团，那么同时连接 –OH 和 –CN 的碳原子为手性中心，存在光学异构的可能。

8. Testing for Carbonyls: 2,4-Dinitrophenylhydrazine (2,4-DNPH) | 羰基测试：2,4-二硝基苯肼

2,4-DNPH (Brady’s reagent) is used to detect the presence of a carbonyl group. When added to an aldehyde or ketone, a yellow or orange precipitate of the corresponding 2,4-dinitrophenylhydrazone is formed. This is a nucleophilic addition–elimination reaction (condensation) where water is eliminated to form a C=N bond.

2,4-二硝基苯肼（布拉迪试剂）用于检测羰基的存在。与醛或酮混合时，会生成相应的 2,4-二硝基苯腙黄色或橙色沉淀。这是一个亲核加成–消除反应（缩合），消除一分子水形成 C=N 键。

The test is positive for both aldehydes and ketones but negative for carboxylic acids, esters, and amides (which do not undergo this reaction under the same conditions). Therefore, 2,4-DNPH is a general test for the carbonyl group in aldehydes and ketones.

此试验对醛和酮均呈阳性，但对羧酸、酯和酰胺呈阴性（它们在相同条件下不发生该反应）。因此，2,4-DNPH 是醛和酮中羰基的通用检验方法。

The solid derivative can be filtered, purified by recrystallisation, and its melting point determined. The melting point can then be compared to a database of known 2,4-dinitrophenylhydrazones to identify the original aldehyde or ketone. This is a classic A-Level practical context.

所得固体衍生物可过滤、重结晶提纯，并测定其熔点。通过与已知 2,4-二硝基苯腙数据库的熔点比对，可鉴定原始的醛或酮。这是经典的 A-Level 实验考查情境。

9. Distinguishing Aldehydes from Ketones: Tollens’ Reagent and Fehling’s Solution | 鉴别醛和酮：托伦斯试剂与斐林溶液

Aldehydes are easily oxidised to carboxylic acids; ketones resist mild oxidation. This difference is exploited in two key tests. Tollens’ reagent contains the diamminesilver(I) ion [Ag(NH₃)₂]⁺. When warmed with an aldehyde, the silver(I) ion is reduced to metallic silver, forming a silver mirror on the inside of a clean test tube. Ketones give no reaction.

醛易氧化成羧酸，而酮在温和氧化条件下不反应。利用这一差异可进行两个关键试验。托伦斯试剂含有二氨合银(I)离子 [Ag(NH₃)₂]⁺。与醛一起温热时，银(I)被还原为金属银，在洁净试管内壁形成银镜。酮无反应。

The reaction can be represented as: RCHO + 2[Ag(NH₃)₂]⁺ + 3OH⁻ → RCOO⁻ + 2Ag + 4NH₃ + 2H₂O. This is an oxidation–reduction (redox) process, not an addition. Aldehydes reduce the Tollens’ reagent, hence they are reducing agents.

反应可表示为：RCHO + 2[Ag(NH₃)₂]⁺ + 3OH⁻ → RCOO⁻ + 2Ag + 4NH₃ + 2H₂O。此反应为氧化还原过程，而非加成。醛能还原托伦斯试剂，因此它们是还原剂。

Fehling’s solution contains blue Cu²⁺ complexed with tartrate ions in alkaline medium. With an aliphatic aldehyde, the blue Cu²⁺ is reduced to a brick-red precipitate of Cu₂O. Aromatic aldehydes (like benzaldehyde) give a negative result. Ketones do not react. Benedict’s solution, a similar test reagent using citrate complex, is also covered in some specifications.

斐林溶液含有在碱性酒石酸根离子中配合的蓝色 Cu²⁺。与脂肪族醛共热时，蓝色 Cu²⁺ 被还原为砖红色的 Cu₂O 沉淀。芳香醛（如苯甲醛）得到阴性结果。酮不反应。本尼迪特试剂是一种类似的柠檬酸盐配合物测试试剂，也在部分考纲中出现。

Summary: Tollens’ – aldehyde positive (silver mirror), ketone negative; Fehling’s – aliphatic aldehyde positive (brick-red), aromatic aldehyde and ketone negative. Be ready to interpret these results in exam questions.

小结：托伦斯试验——醛呈阳性（银镜），酮呈阴性；斐林试验——脂肪醛呈阳性（砖红色），芳香醛和酮呈阴性。在考题中做好解读实验结果的准备。

10. The Triiodomethane (Iodoform) Reaction | 碘仿反应

The triiodomethane test detects the presence of a methyl carbonyl group (CH₃CO–) or a methyl alcohol group that can be oxidised to that substructure (CH₃CH(OH)–). Only ethanal (CH₃CHO) and methyl ketones (e.g., propanone CH₃COCH₃) give a positive result among carbonyl compounds. This is a useful distinguishing test.

碘仿反应用于检测甲基羰基（CH₃CO–）或可被氧化为该结构的甲基醇基（CH₃CH(OH)–）的存在。在羰基化合物中，只有乙醛（CH₃CHO）和甲基酮（如丙酮 CH₃COCH₃）呈阳性。这是一个有用的区分测试。

The reagent is iodine in aqueous sodium hydroxide (I₂/NaOH). The reaction involves halogenation of the methyl group next to the carbonyl, followed by cleavage of the C–C bond to form the pale yellow solid triiodomethane (iodoform, CHI₃), which has a distinctive antiseptic smell.

试剂为碘的氢氧化钠水溶液（I₂/NaOH）。反应包括羰基旁甲基的卤代，随后 C–C 键断裂，生成淡黄色固体三碘甲烷（碘仿，CHI₃），具有独特的消毒水味。

Propanone + 3I₂ + 4NaOH → CHI₃ + CH₃COONa + 3NaI + 3H₂O. Ethanal similarly yields CHI₃ and a methanoate ion. Ketones without a CH₃CO– group, such as butanone (CH₃CH₂COCH₃), does contain a methyl ketone group? Wait: butanone is CH₃CH₂COCH₃, which has a CH₃CO– attached to an ethyl group, so it does give a positive iodoform test. Let’s clarify: any ketone with a methyl group on one side of the carbonyl gives a positive test. So butanone gives iodoform. Propanal (CH₃CH₂CHO) does not give iodoform. So methyl ketones and ethanal are the only carbonyl positives.

丙酮 + 3I₂ + 4NaOH → CHI₃ + CH₃COONa + 3NaI + 3H₂O。乙醛类似地生成 CHI₃ 和甲酸根离子。不含 CH₃CO– 基团的酮，如丁酮（CH₃CH₂COCH₃）确实含有甲基酮基团，实际上它会产生阳性碘仿反应。说明：任何羰基一侧连有甲基的酮均呈阳性。所以丁酮能反应。丙醛（CH₃CH₂CHO）不反应。因此只有甲基酮和乙醛是羰基化合物中的阳性对象。

11. Summary Flowchart for Identification | 鉴别流程总结

In exam questions, you may be asked to identify unknown carbonyl compounds using a logical sequence of tests. A typical approach is:

在考试中，可能会要求你利用逻辑顺序测试来鉴别未知羰基化合物。典型步骤如下：

Test with 2,4-DNPH: positive orange/yellow precipitate confirms a carbonyl (aldehyde or ketone).
Test with Tollens’ or Fehling’s: positive result (silver mirror or brick-red) indicates an aldehyde; negative confirms a ketone (or aromatic aldehyde for Fehling’s).
If Tollens’ positive and you need to further distinguish ethanal from other aldehydes, use the iodoform test: yellow precipitate and antiseptic smell confirm ethanal or a methyl ketone; but a non-methyl ketone would have been ruled out by Tollens’. So if iodoform positive and Tollens’ positive, it is ethanal. If Tollens’ negative and iodoform positive, it is a methyl ketone.

2,4-DNPH 测试：产生橘黄色沉淀确认羰基存在（醛或酮）。
托伦斯或斐林测试：阳性（银镜或砖红色）表明是醛；阴性确认是酮（或斐林测试下的芳香醛）。
若托伦斯阳性且需进一步区分乙醛与其他醛，使用碘仿测试：黄色沉淀和消毒水味确认乙醛或甲基酮；但非甲基酮已被托伦斯排除。因此若碘仿阳性且托伦斯阳性，则为乙醛。若托伦斯阴性而碘仿阳性，则为甲基酮。

You should always be careful with phenylethanone and benzaldehyde, which are aromatic carbonyls that behave differently in Fehling’s test. OCR tends to focus on aliphatic examples unless otherwise stated.

始终注意苯乙酮和苯甲醛这类芳香族羰基物在斐林测试中的不同表现。OCR 除非特别说明，一般以脂肪族实例为重点。

12. Exam Tips and Common Mistakes | 考试技巧与常见错误

When drawing mechanisms, never forget the dipole on C=O and the curly arrows. The arrow from the nucleophile must point to the δ+ carbon, not the oxygen. The arrow for the π bond breaking must start from the bond and point directly at the oxygen atom. The intermediate alkoxide must be shown with a negative charge on oxygen.

画机理时，切勿忘记 C=O 上的偶极和弯箭号。亲核试剂的箭头必须指向 δ+ 碳，而非氧。π 键断裂的箭头必须从键中央起始，直接指向氧原子。中间体烷氧负离子必须标明氧上的负电荷。

Oxidation of aldehydes is often tested. You may need to write balanced equations for the reaction with acidified dichromate, noting the colour change from orange to green (Cr₂O₇²⁻ reduced to Cr³⁺). For example: 3CH₃CHO + Cr₂O₇²⁻ + 8H⁺ → 3CH₃COOH + 2Cr³⁺ + 4H₂O.

醛的氧化是常考点。可能需要书写酸化重铬酸盐氧化的配平方程式，并注意颜色从橙色变为绿色（Cr₂O₇²⁻ 被还原为 Cr³⁺）。例如：3CH₃CHO + Cr₂O₇²⁻ + 8H⁺ → 3CH₃COOH + 2Cr³⁺ + 4H₂O。

A common confusion is between Tollens’ reagent and Fehling’s solution. Recall: Tollens’ contains Ag⁺, gives a silver mirror; Fehling’s contains Cu²⁺, gives brick-red Cu₂O. Both are used to test for aldehydes, but Fehling’s fails for aromatic aldehydes.

常见混淆点在于托伦斯试剂和斐林溶液。牢记：托伦斯含 Ag⁺，形成银镜；斐林含 Cu²⁺，生成砖红色 Cu₂O。两者都可用于醛的测试，但斐林对芳香醛不起作用。

In naming, remember that the aldehyde carbon is always position 1, so do not insert ‘-1-al’. Ketones require a locant unless the carbonyl can only be at one position (e.g., propanone). Also, be aware of the spelling: methanal, not methanaldehyde; ethanal, not ethaldehyde.

命名时记住醛基碳永远是 1 号位，所以不要插入 ‘-1-al’。酮需要标出位次，除非羰基只有一个可能位置（如丙酮）。还要注意拼写：是 methanal，而非 methanaldehyde；是 ethanal，而非 ethaldehyde。

Finally, understand the iodoform test’s structural requirements: a methyl group directly attached to the carbonyl carbon (CH₃CO–) in ketones, or the aldehyde with that same group (i.e., ethanal). Many students incorrectly predict a positive test for propanal or for ketones like pentan-3-one.

最后，理解碘仿测试的结构要求：酮中甲基直接连在羰基碳上（CH₃CO–），或具有同样结构的醛（即乙醛）。很多学生错误地预测丙醛或 3-戊酮等为阳性。

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Aldehydes and Ketones: Key Points for A-Level OCR Chemistry | 醛和酮 考点精讲