Aldehydes and Ketones Key Points Review | IB WJEC 化学：醛和酮考点精讲

📚 Aldehydes and Ketones Key Points Review | IB WJEC 化学：醛和酮考点精讲

Aldehydes and ketones are fundamental carbonyl compounds that appear frequently in both IB and WJEC chemistry specifications. Understanding their structure, nomenclature, physical properties, preparation, and characteristic reactions — especially nucleophilic addition, oxidation, and reduction — is essential. This article systematically breaks down every key topic you need to master for your exams, with clear comparisons and exam tips.

醛和酮是 IB 和 WJEC 化学大纲中频繁出现的基本羰基化合物。掌握它们的结构、命名、物理性质、制备方法以及特征反应——特别是亲核加成、氧化和还原——至关重要。本文系统梳理了你必须掌握的每一个关键主题，并附有清晰的对比和考试技巧。

1. Functional Group and Structure | 官能团与结构

Both aldehydes and ketones contain the carbonyl group (C=O). In aldehydes, the carbonyl carbon is bonded to at least one hydrogen atom and to an R group (which can be alkyl or aryl) or another hydrogen. In ketones, the carbonyl carbon is bonded to two R groups, which may be the same or different, neither of which can be hydrogen.

醛和酮都含有羰基 (C=O)。在醛中，羰基碳至少与一个氢原子以及一个 R 基团（可以是烷基或芳基）或另一个氢原子相连。在酮中，羰基碳与两个 R 基团相连，这两个基团可以相同或不同，但都不能是氢。

The general formula for aldehydes (excluding the simplest methanal) is RCHO; for ketones it is RCOR’. The carbonyl carbon is sp² hybridised with a trigonal planar geometry and bond angles of approximately 120°.

醛（除最简单的甲醛外）的通式为 RCHO；酮的通式为 RCOR’。羰基碳为 sp² 杂化，具有平面三角形构型，键角约为 120°。

The C=O double bond is polarised because oxygen is more electronegative than carbon, making the carbon electron‑deficient and susceptible to attack by nucleophiles.

C=O 双键因氧的电负性大于碳而具有极性，这使得碳原子缺电子，容易受到亲核试剂的进攻。

2. Nomenclature of Aldehydes and Ketones | 醛和酮的命名

According to IUPAC rules, aldehydes are named by replacing the final ‘-e’ of the parent alkane with ‘-al’. The carbonyl carbon is always carbon number 1 in the chain, so its position does not need to be indicated. Methanal (formaldehyde), ethanal (acetaldehyde), and propanal are common examples.

根据 IUPAC 规则，醛的命名是将母体烷烃末尾的“-e”改为“-al”。羰基碳始终是链上的 1 号碳，因此无需标明其位置。甲醛、乙醛和丙醛是常见的例子。

Ketones are named by replacing the ‘-e’ of the alkane with ‘-one’, and the position of the carbonyl group is indicated by a number unless the chain is short enough that there is no ambiguity (e.g., propanone). The ketone functional group takes precedence over alkyl and halogen substituents in numbering.

酮的命名是将烷烃的“-e”改为“-one”，并用数字标明羰基的位置，除非碳链很短且没有歧义（例如丙酮）。在编号时，酮官能团优先于烷基和卤素取代基。

In WJEC and IB exams, you must be able to draw structures from systematic names and vice versa, including handling substituents, branched chains, and cyclic ketones. Common names such as acetone (propanone) and benzaldehyde are also frequently tested.

在 WJEC 和 IB 考试中，你必须要能根据系统命名画出结构，反之亦然，包括处理取代基、支链和环酮。诸如丙酮（propanone）和苯甲醛等常用名也时常考查。

3. Physical Properties | 物理性质

The polarity of the carbonyl group gives aldehydes and ketones higher boiling points than alkanes of comparable molar mass, but lower boiling points than the corresponding alcohols because they cannot form intermolecular hydrogen bonds among themselves (they lack an –OH group).

羰基的极性使醛和酮的沸点高于摩尔质量相近的烷烃，但低于相应的醇，因为它们自身之间无法形成分子间氢键（它们缺少 –OH 基团）。

Short-chain aldehydes and ketones are soluble in water because the oxygen atom of the carbonyl can form hydrogen bonds with water molecules. As the hydrocarbon chain grows longer, solubility in water decreases while solubility in non‑polar solvents increases.

短链醛和酮可溶于水，因为羰基氧原子能与水分子形成氢键。随着烃链增长，水溶性降低，而在非极性溶剂中的溶解性增强。

Methanal is a gas at room temperature, ethanal is a volatile liquid, and most other simple aldehydes and ketones are liquids. Their characteristic odours are often used to describe them; many have pleasant fruity smells, although some lower aldehydes have pungent odours.

甲醛在室温下为气体，乙醛为挥发性液体，其它简单的醛和酮大多为液体。它们的特征气味常被用来描述它们；许多具有令人愉快的水果香味，但一些低级醛有刺激性气味。

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

Aldehydes can be prepared by the oxidation of primary alcohols using a controlled amount of oxidising agent and immediate distillation to prevent further oxidation to carboxylic acid. Acidified potassium dichromate(VI) is the typical oxidising agent; the colour change from orange to green indicates the reaction.

醛可通过伯醇的氧化制备，使用控制量的氧化剂并立即蒸馏，以防进一步氧化成羧酸。典型的氧化剂是酸化重铬酸钾(VI)；颜色从橙色变为绿色表明反应发生。

Ketones are prepared by the oxidation of secondary alcohols under reflux, again using acidified K₂Cr₂O₇. No further oxidation occurs under these conditions because breaking a C–C bond would be required to produce a carboxylic acid.

酮可通过仲醇的回流氧化制备，同样使用酸化 K₂Cr₂O₇。在此条件下不会进一步氧化，因为要生成羧酸需要断裂 C–C 键。

Another important synthetic route is the hydration of alkynes. Symmetrical internal alkynes give ketones; terminal alkynes give aldehydes (via Markovnikov addition followed by tautomerism). This reaction uses HgSO₄/H₂SO₄ as catalyst.

另一种重要的合成途径是炔烃的水合。对称的内炔生成酮；末端炔烃通过 Markovnikov 加成和互变异构生成醛。该反应使用 HgSO₄/H₂SO₄ 作为催化剂。

Both WJEC and IB may also ask about ozonolysis of alkenes followed by reductive work‑up, which yields aldehydes or ketones depending on the substitution of the alkene.

WJEC 和 IB 也都可能考查烯烃的臭氧化分解及其后的还原处理，这根据烯烃的取代情况生成醛或酮。

5. Nucleophilic Addition Reactions | 亲核加成反应

The most characteristic reaction of aldehydes and ketones is nucleophilic addition across the polar C=O bond. A nucleophile attacks the electrophilic carbonyl carbon, breaking the π bond and forming a tetrahedral alkoxide intermediate, which is then protonated.

醛和酮最具特征的反应是跨越极性 C=O 双键的亲核加成。亲核试剂进攻缺电子的羰基碳，断裂 π 键，形成四面体状的醇盐中间体，随后质子化。

Addition of hydrogen cyanide (HCN) yields cyanohydrins (hydroxynitriles). The reaction is catalysed by base (CN⁻ is generated in situ from KCN with acid). This is an important reaction for extending the carbon chain and is stereospecific if the carbonyl carbon has two different substituents.

与氰化氢（HCN）加成生成氰醇（羟基腈）。该反应由碱催化（用 KCN 和酸原位生成 CN⁻）。这是增长碳链的重要反应，当羰基碳连有两个不同取代基时具有立体特异性。

Sodium hydrogensulfite (NaHSO₃) adds to aldehydes and methyl ketones to form crystalline bisulfite addition products. This reaction is often used for the purification of aldehydes.

亚硫酸氢钠 (NaHSO₃) 与醛和甲基酮加成，生成晶体状的亚硫酸氢盐加成产物。此反应常用于提纯醛类。

Addition–elimination reactions with ammonia derivatives (e.g., 2,4‑dinitrophenylhydrazine, hydroxylamine, hydrazines) yield imines, oximes, and hydrazones, respectively. The 2,4‑DNP reaction is particularly important as a test for the carbonyl group, producing a yellow/orange precipitate.

与氨的衍生物（例如 2,4-二硝基苯肼、羟胺、肼）发生的加成-消除反应分别生成亚胺、肟和腙。2,4‑DNP 反应尤其重要，是检测羰基的鉴定试验，生成黄色/橙色沉淀。

6. Oxidation Reactions | 氧化反应

Aldehydes are readily oxidised to carboxylic acids by a variety of mild oxidising agents, including acidified dichromate, Tollens’ reagent, and Fehling’s/Benedict’s solution. Ketones resist oxidation under mild conditions because it would involve the breaking of a C–C bond, which requires more vigorous reagents.

醛容易被多种温和氧化剂氧化成羧酸，包括酸化重铬酸盐、Tollens 试剂以及 Fehling/Benedict 试剂。酮在温和条件下难以被氧化，因为这需要断裂 C–C 键，需使用更剧烈的试剂。

The fact that aldehydes are oxidised while ketones are not is the basis for distinguishing between the two using chemical tests. Strong oxidising agents such as hot concentrated HNO₃ or KMnO₄ can oxidise ketones, but this is not required for the distinguishing tests.

醛能被氧化而酮不能，这是通过化学测试区分二者的基础。热的浓 HNO₃ 或 KMnO₄ 等强氧化剂能氧化酮，但在鉴别测试中不作要求。

In IB, the oxidation of aldehydes is often linked to redox half‑equations and to the use of Fehling’s and Tollens’ tests. WJEC may also ask for equations and observations — Tollen’s gives a silver mirror, Fehling’s a brick‑red precipitate of Cu₂O.

在 IB 中，醛的氧化常与氧化还原半反应以及 Fehling 试验和 Tollens 试验的使用联系起来。WJEC 也可能要求书写方程式并描述现象——Tollens 试验产生银镜，Fehling 试验产生砖红色 Cu₂O 沉淀。

7. Reduction Reactions | 还原反应

Both aldehydes and ketones can be reduced to alcohols. Aldehydes are reduced to primary alcohols, and ketones to secondary alcohols. The typical reducing agent is sodium tetrahydridoborate(III) (NaBH₄) in aqueous or alcoholic solution, often represented by [H] in equations.

醛和酮都可以被还原为醇。醛被还原为伯醇，酮被还原为仲醇。典型的还原剂是硼氢化钠 (NaBH₄)，在水或醇溶液中使用，方程式中常用 [H] 表示。

Lithium tetrahydridoaluminate(III) (LiAlH₄) is a stronger reducing agent that can also be used, but must be handled in anhydrous conditions followed by careful hydrolysis. IB and WJEC both use NaBH₄ as the standard laboratory reductant for carbonyls.

氢化铝锂 (LiAlH₄) 是更强的还原剂，也可使用，但必须在无水条件下操作，然后小心水解。IB 和 WJEC 都以 NaBH₄ 作为羰基化合物的标准实验室还原剂。

The mechanism of reduction is again nucleophilic addition: H⁻ (hydride ion) attacks the carbonyl carbon, and the resulting alkoxide ion gains a proton from the solvent (e.g., water or alcohol).

还原反应的机理同样是亲核加成：H⁻（氢负离子）进攻羰基碳，生成的醇盐离子从溶剂（如水或醇）中获取一个质子。

The reduction of aldehydes and ketones is the reverse of the oxidation of alcohols, establishing an important redox relationship that is frequently examined.

醛和酮的还原是醇氧化的逆反应，这建立起一个重要的氧化还原关系，经常被考查。

8. Iodoform (Triiodomethane) Reaction | 碘仿反应

Aldehydes or ketones containing the CH₃CO– group (a methyl group directly attached to the carbonyl, i.e., methyl ketones) or alcohols that can be oxidised to such a structure give a positive iodoform test. The only aldehyde that gives this test is ethanal (CH₃CHO).

含有 CH₃CO– 基团（甲基与羰基直接相连，即甲基酮）的醛或酮，或者能被氧化成该结构的醇，会给出阳性碘仿反应。唯一能给出该反应的醛是乙醛 (CH₃CHO)。

The test involves warming the sample with iodine and sodium hydroxide. A pale yellow precipitate of triiodomethane (iodoform, CHI₃) with a characteristic antiseptic smell indicates a positive result.

该试验是将样品与碘和氢氧化钠一起温热。产生具有特征防腐剂气味的淡黄色三碘甲烷（碘仿，CHI₃）沉淀，表明结果为阳性。

The reaction proceeds via halogenation of the methyl group followed by cleavage. In exam questions, the structural requirement is often tested: CH₃CO–R or CH₃CH(OH)–R.

反应经由甲基卤代然后断裂的历程。考试题目常考查结构要求：CH₃CO–R 或 CH₃CH(OH)–R。

9. Distinguishing Tests for Aldehydes and Ketones | 醛和酮的鉴别测试

While 2,4‑DNP gives a positive test for both aldehydes and ketones (formation of a yellow/orange precipitate), the differentiation between the two relies on the fact that aldehydes can be further oxidised but ketones cannot.

尽管 2,4‑DNP 对醛和酮均呈阳性反应（生成黄色/橙色沉淀），区分二者仍需依赖醛能被进一步氧化而酮不能这一事实。

Tollens’ reagent (ammoniacal silver nitrate) oxidises aldehydes to carboxylate ions and is reduced to metallic silver, forming a characteristic silver mirror on the inner surface of a clean test tube. Ketones give no reaction.

Tollens 试剂（氨性硝酸银）将醛氧化为羧酸根离子，自身被还原为金属银，在洁净试管内壁形成特征性的银镜。酮无反应。

Fehling’s solution (or Benedict’s solution) contains Cu²⁺ ions complexed with tartrate (or citrate) in alkaline medium. Aldehydes reduce the blue Cu²⁺ to a brick‑red precipitate of Cu₂O. Aliphatic aldehydes react; aromatic aldehydes (e.g., benzaldehyde) do not. Ketones give no reaction.

Fehling 溶液（或 Benedict 溶液）含有酒石酸根（或柠檬酸根）络合的 Cu²⁺ 离子，在碱性介质中。醛将蓝色的 Cu²⁺ 还原为砖红色的 Cu₂O 沉淀。脂肪醛可以反应；芳香醛（如苯甲醛）不反应。酮无反应。

In WJEC, students should be able to write ionic equations for these redox processes and describe the colour changes. In IB, the tests are part of the organic practical work and exam questions often link observations to functional groups.

在 WJEC 中，学生应能写出这些氧化还原过程的离子方程式并描述颜色变化。在 IB 中，这些测试是有机实验操作的一部分，考题常将观察现象与官能团联系起来。

10. Addition–Elimination with Ammonia Derivatives | 与氨衍生物的加成-消除反应

Condensation (addition–elimination) reactions with 2,4‑dinitrophenylhydrazine (2,4‑DNP), hydroxylamine (NH₂OH), and hydrazines (NH₂NH₂ or phenylhydrazine) follow a common mechanism: nucleophilic attack by the nitrogen lone pair on the carbonyl carbon, loss of water, and formation of a C=N bond.

与 2,4‑二硝基苯肼 (2,4‑DNP)、羟胺 (NH₂OH) 和肼类（NH₂NH₂ 或苯肼）的缩合（加成-消除）反应遵循共同的机理：氮上的孤对电子对羰基碳亲核进攻，脱去一分子水，形成 C=N 键。

The product from 2,4‑DNP has a sharp melting point that can be used to identify the original aldehyde or ketone. This is a classic method for characterising carbonyl compounds.

与 2,4‑DNP 生成的产物具有敏锐的熔点，可用于鉴定原始醛或酮。这是表征羰基化合物的经典方法。

In exam questions, you must be able to recognise the structural features of imines, oximes, and hydrazones, and understand that these reactions are acid‑catalysed, with the optimum pH being slightly acidic.

在考题中，你必须能识别亚胺、肟和腙的结构特征，并理解这些反应是酸催化的，最适 pH 为弱酸性。

11. Spectroscopic Identification | 光谱鉴定

Infrared (IR) spectroscopy shows a strong, sharp absorption around 1700–1750 cm⁻¹ due to C=O stretching. In aldehydes, two characteristic weak C–H stretches around 2700–2800 cm⁻¹ are often observed, distinguishing them from ketones.

红外 (IR) 光谱显示，约在 1700–1750 cm⁻¹ 处有一个强而尖锐的吸收峰，归属于 C=O 伸缩振动。醛常在 2700–2800 cm⁻¹ 附近观察到两个特征的弱 C–H 伸缩振动峰，这可与酮区分开。

In proton NMR, the aldehyde proton is highly deshielded and appears as a singlet in the region δ 9–10 ppm. In ketones, protons on carbon atoms α to the carbonyl group appear at δ 2–2.5 ppm.

在质子核磁共振谱中，醛氢高度去屏蔽，在 δ 9–10 ppm 区域出现单峰。酮中 α 碳上的质子出现在 δ 2–2.5 ppm。

Mass spectrometry often shows a characteristic fragmentation pattern: for aldehydes, the loss of H (M – 1) or the formation of an acylium ion (RCO⁺) is common. For methyl ketones, the peak at m/z = 43 (CH₃CO⁺) is diagnostic.

质谱常呈现特征碎片模式：醛可失去 H (M – 1) 或形成酰基正离子 (RCO⁺)。甲基酮在 m/z = 43 (CH₃CO⁺) 处的峰具诊断意义。

Both IB and WJEC expect the integration of spectroscopic data to deduce the structure of unknown aldehydes and ketones.

IB 和 WJEC 都要求整合光谱数据来推断未知醛和酮的结构。

12. Summary and Exam Tips | 总结与考试技巧

Focus on the key differences: aldehydes have a hydrogen attached to the carbonyl, giving them distinctive oxidation behaviour and a characteristic NMR signal. Ketones lack this hydrogen and cannot be oxidised under mild conditions. Master the mechanisms of nucleophilic addition for HCN and NaBH₄, and the redox interconversions between alcohols, aldehydes/ketones, and carboxylic acids.

重点掌握关键区别：醛有一个氢原子连在羰基上，这赋予它们独特的氧化行为和特征 NMR 信号。酮缺少这个氢，在温和条件下不能被氧化。掌握 HCN 和 NaBH₄ 亲核加成的机理，以及醇、醛/酮和羧酸之间的氧化还原相互转化。

Always be systematic: compare boiling points and solubility using intermolecular force arguments. Draw mechanisms with curly arrows starting from the nucleophile to the carbonyl carbon, and don’t forget to show the alkoxide intermediate and subsequent protonation.

始终系统化：运用分子间作用力的论证比较沸点和溶解度。绘制机理时，弯箭头从亲核试剂出发指向羰基碳，不要忘记画出醇盐中间体和后续的质子化步骤。

For the iodoform test, memorise the structural requirement (CH₃CO– or CH₃CH(OH)–). For distinguishing tests, link the reagent to the redox principle and the observation (silver mirror, brick‑red precipitate, or no reaction).

对于碘仿反应，牢记结构要求（CH₃CO– 或 CH₃CH(OH)–）。对于鉴别测试，将试剂与氧化还原原理及实验现象（银镜、砖红色沉淀或无反应）联系起来。

Practice drawing skeletal structures quickly and accurately. Double‑check your numbering when naming ketones, and remember that in cyclic compounds the carbonyl position is assumed to be 1 unless otherwise stated.

练习快速且准确地绘制骨架结构。给酮命名时仔细核对编号，并记住在环状化合物中羰基位置默认为 1 号，除非另有说明。

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Aldehydes and Ketones Key Points Review | IB WJEC 化学：醛和酮 考点精讲