Aldehydes and Ketones | IGCSE WJEC 化学：醛和酮考点精讲

📚 Aldehydes and Ketones | IGCSE WJEC 化学：醛和酮考点精讲

Aldehydes and ketones are two important families of organic compounds that both contain the carbonyl group (C=O). Understanding their structures, naming rules, and characteristic reactions – especially the difference in their behaviour towards oxidising agents – is a core requirement for the IGCSE WJEC Chemistry specification. This article will break down every essential concept, from recognising the functional group to applying tests like Fehling’s and Tollens’ reagents, helping you secure top marks in your exam.

醛和酮是两类重要的有机化合物，都含有羰基 (C=O)。理解它们的结构、命名规则和特征反应——尤其是它们对氧化剂的不同表现——是 IGCSE WJEC 化学大纲的核心要求。本文将从官能团的识别开始，逐步讲解斐林试剂和托伦斯试剂等测试方法，帮助你牢固掌握每一个考点，在考试中稳拿高分。

1. The Carbonyl Group | 羰基官能团

Both aldehydes and ketones contain a carbonyl functional group, which consists of a carbon atom double‑bonded to an oxygen atom (C=O). The carbonyl carbon is sp² hybridised, making the group planar with bond angles of approximately 120°. The C=O bond is polar because oxygen is more electronegative than carbon, giving the carbon atom a partial positive charge (δ⁺) and the oxygen a partial negative charge (δ⁻). This polarity is responsible for many of the chemical properties observed.

醛和酮都含有羰基官能团，由一个碳原子与一个氧原子以双键连接 (C=O)。羰基碳为 sp² 杂化，使基团呈平面结构，键角约为 120°。由于氧的电负性强于碳，C=O 键具有极性，碳原子带部分正电荷 (δ⁺)，氧原子带部分负电荷 (δ⁻)。这种极性决定了它们许多化学性质。

2. General Formulae and Functional Group Position | 通式与官能团位置

In an aldehyde, the carbonyl group is located at the end of the carbon chain. The carbon of the carbonyl is always bonded to at least one hydrogen atom. The general formula for a saturated aldehyde is CₙH₂ₙO. In a ketone, the carbonyl group is positioned between two carbon atoms; the carbonyl carbon is never at the end of the chain, and it is bonded to two other carbon atoms. Ketones also share the same molecular formula CₙH₂ₙO as aldehydes for the same number of carbon atoms, making them functional group isomers.

在醛中，羰基位于碳链的末端，羰基碳上至少连接一个氢原子。饱和醛的通式为 CₙH₂ₙO。在酮中，羰基位于两个碳原子之间，羰基碳绝对不出现在碳链末端，它总是与另外两个碳原子成键。对于碳原子数相同的饱和醛和酮，它们具有相同的分子式 CₙH₂ₙO，因此互为官能团异构体。

3. Naming Aldehydes | 醛的命名

To name an aldehyde, identify the longest continuous carbon chain that contains the –CHO group. Replace the final ‘e’ of the corresponding alkane name with ‘al’. The carbon of the aldehyde group is always carbon number 1, so you do not need to include a position number. For example, HCHO is methanal, CH₃CHO is ethanal, and CH₃CH₂CHO is propanal. If there are substituents, number the chain starting from the aldehyde carbon and list substituents as prefixes in alphabetical order.

命名醛时，找出含有 –CHO 基团的最长碳链，将相应烷烃名称末尾的“e”换成“al”。醛基的碳原子总是编号为 1，因此无需注明位置编号。例如 HCHO 是甲醛 (methanal)，CH₃CHO 是乙醛 (ethanal)，CH₃CH₂CHO 是丙醛 (propanal)。若存在取代基，则从醛基碳开始编号碳链，并按字母顺序列出取代基前缀。

4. Naming Ketones | 酮的命名

For ketones, select the longest chain containing the carbonyl group and replace the ‘e’ of the alkane with ‘one’. You must give the carbonyl carbon the lowest possible number and include this number before the name, except for propanone and butanone where there is only one possible position. For instance, CH₃COCH₃ is propanone, CH₃COCH₂CH₃ is butanone. For longer chains: CH₃COCH₂CH₂CH₃ is pentan‑2‑one, and CH₃CH₂COCH₂CH₃ is pentan‑3‑one.

命名酮时，选出包含羰基的最长碳链，将烷烃词尾“e”换成“one”。必须给羰基碳尽可能小的编号，并在名称前注明该编号（丙烷酮和丁烷酮因只有一种可能位置常常省略）。例如 CH₃COCH₃ 是丙酮 (propanone)，CH₃COCH₂CH₃ 是丁酮 (butanone)。碳链更长时：CH₃COCH₂CH₂CH₃ 是戊‑2‑酮 (pentan‑2‑one)，CH₃CH₂COCH₂CH₃ 是戊‑3‑酮 (pentan‑3‑one)。

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

Short‑chain aldehydes and ketones (e.g. methanal, ethanal, propanone) are soluble in water because the polar carbonyl group can form hydrogen bonds with water molecules. As the hydrocarbon chain length increases, solubility decreases and the compounds become more soluble in organic solvents. Boiling points of aldehydes and ketones are higher than those of alkanes of similar relative molecular mass due to the presence of permanent dipole‑dipole forces between polar carbonyl groups. However, they cannot form strong intermolecular hydrogen bonds with themselves (unlike alcohols), so their boiling points are lower than the corresponding alcohols.

短链醛和酮（如甲醛、乙醛、丙酮）可溶于水，因为极性的羰基能与水分子形成氢键。随着碳链增长，水溶性下降，更易溶于有机溶剂。醛和酮的沸点高于相对分子质量相近的烷烃，这是因为极性的羰基之间存在永久偶极‑偶极作用力。但它们自身分子间不能形成强氢键（与醇不同），因此沸点低于对应的醇。

6. Distinguishing Aldehydes from Ketones by Oxidation | 通过氧化反应区分醛和酮

A key difference in the IGCSE WJEC specification is that aldehydes can be easily oxidised to carboxylic acids, while ketones cannot be oxidised under the same conditions. This is because aldehydes have a hydrogen atom attached to the carbonyl carbon, which can be removed during oxidation. Ketones lack this hydrogen, making them resistant to mild oxidising agents. This difference forms the basis of the two most common test‑tube reactions.

在 IGCSE WJEC 大纲中，一个关键区别是醛容易被氧化成羧酸，而酮在相同条件下不能被氧化。这是因为醛的羰基碳上连有一个氢原子，该氢可在氧化过程中脱去。酮缺少这个氢，因此对温和的氧化剂表现稳定。这一区别构成了两种最常见试管反应的基础。

7. Fehling’s Test | 斐林试剂检测

Fehling’s solution contains copper(II) ions (Cu²⁺) in an alkaline medium, giving it a blue colour. When warmed with an aldehyde, the aldehyde is oxidised to a carboxylic acid and the blue Cu²⁺ ions are reduced to a brick‑red precipitate of copper(I) oxide (Cu₂O). Ketones give no reaction; the solution remains blue. The chemical change can be summarised as: R–CHO + 2Cu²⁺ + 5OH⁻ → R–COO⁻ + Cu₂O + 3H₂O.

斐林试剂含有碱性介质中的铜(II)离子 (Cu²⁺)，呈深蓝色。与醛一起温热时，醛被氧化为羧酸，蓝色的 Cu²⁺ 被还原为砖红色的氧化亚铜 (Cu₂O) 沉淀。酮不发生反应，溶液保持蓝色。相关化学变化可概括为：R–CHO + 2Cu²⁺ + 5OH⁻ → R–COO⁻ + Cu₂O + 3H₂O。

8. Tollens’ Reagent (Silver Mirror Test) | 托伦斯试剂（银镜反应）

Tollens’ reagent is prepared by adding sodium hydroxide to silver nitrate solution, then adding dilute ammonia until the precipitate just dissolves. It contains the complex ion [Ag(NH₃)₂]⁺. When warmed with an aldehyde, the aldehyde is oxidised to a carboxylic acid, and silver ions are reduced to metallic silver. If the test tube is clean, the silver deposits as a shiny mirror on the glass. Ketones give no visible change. The ionic half‑equation for reduction is: [Ag(NH₃)₂]⁺ + e⁻ → Ag + 2NH₃.

托伦斯试剂通过向硝酸银溶液加入氢氧化钠，再滴加稀氨水至沉淀恰好溶解而制得。它含有配合离子 [Ag(NH₃)₂]⁺。与醛一起温热时，醛被氧化为羧酸，银离子被还原为金属银。如果试管洁净，银会附着在管壁上形成光亮的银镜。酮则无明显变化。还原反应的离子半方程式为：[Ag(NH₃)₂]⁺ + e⁻ → Ag + 2NH₃。

9. Oxidation with Acidified Potassium Dichromate(VI) | 用酸性重铬酸钾(VI)氧化

Acidified potassium dichromate(VI) solution (K₂Cr₂O₇ / H₂SO₄) is orange due to Cr₂O₇²⁻ ions. Aldehydes are oxidised to carboxylic acids, causing a colour change from orange to green as Cr³⁺ ions are formed. Ketones, again, show no colour change. This reaction is less often used as a specific test for aldehydes because primary alcohols, which also give a positive result, may be present, but it is a standard method to demonstrate the oxidation of aldehydes.

酸性重铬酸钾(VI) 溶液 (K₂Cr₂O₇ / H₂SO₄) 因含有 Cr₂O₇²⁻ 离子而呈橙色。醛被氧化为羧酸，溶液由橙色变为绿色，因为生成了 Cr³⁺ 离子。酮同样不发生颜色变化。由于一级醇也能产生阳性结果，此反应较少用作醛的专属检测，但它是演示醛可被氧化的标准方法。

10. Nucleophilic Addition Reactions | 亲核加成反应

Because the carbonyl carbon carries a partial positive charge, aldehydes and ketones both undergo nucleophilic addition. The simplest example in the IGCSE WJEC specification is the addition of hydrogen cyanide (HCN) across the C=O bond. The cyanide ion (CN⁻) acts as a nucleophile and attacks the δ⁺ carbon, forming a hydroxynitrile (cyanohydrin). For ethanal: CH₃CHO + HCN → CH₃CH(OH)CN. The product contains both a –OH group and a –CN group. This reaction extends the carbon chain by one carbon atom, making it useful in organic synthesis.

由于羰基碳带有部分正电荷，醛和酮都可以发生亲核加成反应。IGCSE WJEC 大纲中最简单的例子是氰化氢 (HCN) 对 C=O 键的加成。氰根离子 (CN⁻) 作为亲核试剂进攻 δ⁺ 碳，生成羟基腈（氰醇）。例如乙醛的反应：CH₃CHO + HCN → CH₃CH(OH)CN。产物中同时含有 –OH 和 –CN 基团。该反应使碳链延长一个碳原子，在有机合成中很有用。

11. Safety Considerations with HCN | 氰化氢的使用安全

Hydrogen cyanide is extremely toxic. In the laboratory, it is generated in situ by adding sulfuric acid to a mixture of sodium cyanide or potassium cyanide. The reaction must be carried out in a fume cupboard, and appropriate personal protective equipment must be worn. The toxicity of cyanide is an important point that examiners expect you to mention when describing this addition reaction.

氰化氢有剧毒。在实验室中，通常通过向氰化钠或氰化钾的混合物中加入硫酸来当场产生。反应必须在通风橱中进行，并佩戴合适的个人防护装备。氰化物的毒性是考官希望你在描述该加成反应时提到的要点，不要遗漏。

12. Summary of Aldehyde and Ketone Reactivity | 醛与酮反应性总结

The key difference to remember is that aldehydes are oxidisable, while ketones are not. This allows you to use Fehling’s, Tollens’, or acidified dichromate(VI) to discriminate between them. Both aldehydes and ketones can undergo nucleophilic addition due to the polar C=O bond. Structurally, aldehydes have the carbonyl at the end of the carbon chain with a hydrogen attached, whereas ketones have it between two alkyl groups. Naming follows the suffixes –al for aldehydes and –one (with a position number if needed) for ketones. Mastering these core facts guarantees confidence in any IGCSE WJEC Chemistry question on this topic.

最关键的区分点是醛可以被氧化，而酮不能。因此你可以用斐林试剂、托伦斯试剂或酸性重铬酸钾(VI)来鉴别它们。由于极性的 C=O 键，醛和酮都能发生亲核加成。在结构上，醛的羰基位于碳链末端并接有氢原子，酮的羰基则位于两个烷基之间。命名时醛使用后缀“‑al”，酮使用“‑one”（必要时标定位号）。牢牢掌握这些核心知识，你就能轻松应对 IGCSE WJEC 化学中有关醛酮的任何考题。

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Aldehydes and Ketones | IGCSE WJEC 化学：醛和酮 考点精讲