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

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

Welcome to this comprehensive revision guide for Aldehydes and Ketones in the IGCSE AQA Chemistry syllabus. This article will break down the key concepts, structures, naming conventions, preparation methods, characteristic reactions, distinguishing tests, and real-world applications you need to master. Whether you are preparing for your end-of-topic test or the final exam, this bilingual guide is designed to reinforce your understanding and boost your confidence.

欢迎阅读这份针对 IGCSE AQA 化学课程中醛和酮的全面复习指南。本文将从关键概念、结构、命名规则、制备方法、特征反应、鉴别检测以及实际应用等方面，为你梳理必考要点。不论你是在准备单元测验还是大考，这份中英双语指南都能帮助你巩固知识、提升信心。

1. The Carbonyl Group – The Functional Heart | 羰基——官能团的核心

Both aldehydes and ketones are organic compounds containing the carbonyl group, a carbon atom double-bonded to an oxygen atom (C=O). The chemical and physical properties of these compounds arise from the polarity of this group. Oxygen is more electronegative than carbon, so the electrons in the double bond are pulled towards the oxygen, making the carbon slightly positive (δ+) and the oxygen slightly negative (δ-). This polarisation is key to understanding their reactivity with nucleophiles.

醛和酮都是含有羰基（碳原子与氧原子双键结合，C=O）的有机化合物。这类化合物的化学与物理性质都源于该基团的极性。氧的电负性比碳强，因此双键中的电子被拉向氧原子，使碳略带正电（δ+），氧略带负电（δ-）。这种极化是理解它们与亲核试剂发生反应的关键。

Furthermore, the carbonyl group is planar, with bond angles of approximately 120° around the carbon atom, resulting from sp² hybridisation. This geometry influences how molecules pack together and their boiling points relative to alkanes of similar molecular mass.

此外，羰基呈平面结构，碳原子周围的键角约为 120°，这是由 sp² 杂化所决定的。这种几何构型影响了分子的堆积方式，也使其沸点相较于相对分子质量相近的烷烃更高。

2. Aldehydes: Structure and General Formula | 醛：结构与通式

An aldehyde is identified by the carbonyl group bonded to at least one hydrogen atom. The aldehyde functional group is written as –CHO, where the carbon is attached to an alkyl or aryl group (R) on one side and a hydrogen atom on the other. The general formula for a saturated aliphatic aldehyde is CₙH₂ₙO, with the structural requirement of having the –CHO terminal group. For example, methanal (formaldehyde) is HCHO, and ethanal (acetaldehyde) is CH₃CHO.

醛的特征在于其羰基至少与一个氢原子相连。醛的官能团写作 –CHO，其中碳原子一侧连有烷基或芳基（R），另一侧连有氢原子。饱和脂肪醛的通式为 CₙH₂ₙO，并且结构上要求具有末端的 –CHO 基团。例如，甲醛（methanal）为 HCHO，乙醛（ethanal）为 CH₃CHO。

Because the carbonyl group is at the end of the carbon chain, aldehydes are easily oxidised to carboxylic acids. This terminal position is what gives aldehydes their distinctive reducing properties and makes them behave differently from ketones in many chemical tests.

由于羰基位于碳链的末端，醛很容易被氧化成羧酸。这种末端位置赋予了醛独特的还原性，也是它们在许多化学检测中表现与酮不同的根本原因。

3. Ketones: Structure and General Formula | 酮：结构与通式

A ketone contains a carbonyl group bonded to two alkyl or aryl groups. The general structure is R–CO–R’, where both R groups are carbon-containing substituents. The smallest ketone is propanone (acetone), CH₃COCH₃. For saturated aliphatic ketones, the general formula is also CₙH₂ₙO, but the position of the carbonyl group differs — it is never at the end of the chain.

酮中的羰基与两个烷基或芳基相连。通式为 R–CO–R’，其中两个 R 基团都是含碳取代基。最小的酮是丙酮（propanone），结构为 CH₃COCH₃。对于饱和脂肪酮，通式同样是 CₙH₂ₙO，但羰基的位置不同——它绝不出现在碳链末端。

Ketones are not easily oxidised under normal laboratory conditions because breaking a carbon–carbon bond would be required. This resistance to mild oxidising agents is a key distinction between ketones and aldehydes and forms the basis for chemical tests such as Fehling’s and Tollens’ tests.

酮在常规实验室条件下不易被氧化，因为氧化需要断裂碳-碳键。酮对温和氧化剂的这种抵抗性是区分酮与醛的关键，也是斐林试剂和托伦斯试剂等化学检测的基础。

4. Naming Aldehydes and Ketones – IUPAC Rules | 醛和酮的命名——IUPAC 规则

For aldehydes, select the longest continuous carbon chain that contains the –CHO group. Replace the final -e of the corresponding alkane with -al. The carbonyl carbon is always assigned number 1, so a positional number is not needed. For example, a four-carbon aldehyde is butanal. If substituents are present, number the chain starting from the aldehyde carbon. For instance, 2-methylpropanal has a methyl group on the second carbon.

对于醛，选择含有 –CHO 基团的最长连续碳链，将相应烷烃名称词尾的 -e 替换为 -al。羰基碳始终编号为 1，因此不需要位次编号。例如，四个碳的醛称为丁醛（butanal）。若存在取代基，则从醛基碳开始给碳链编号。例如，2-甲基丙醛（2-methylpropanal）在第二个碳上有一个甲基。

For ketones, find the longest chain containing the carbonyl group and replace the -e ending of the alkane with -one. The chain is numbered to give the carbonyl carbon the lowest possible number. For example, CH₃COCH₂CH₂CH₃ is pentan-2-one, not pentan-4-one. When naming cyclic ketones, the ring is numbered starting from the carbonyl carbon, and the suffix -one is used, e.g., cyclohexanone.

对于酮，找出含有羰基的最长碳链，将烷烃词尾 -e 替换为 -one。碳链编号应使羰基碳的位次尽可能小。例如，CH₃COCH₂CH₂CH₃ 应为戊-2-酮（pentan-2-one），而非戊-4-酮。命名环状酮时，从羰基碳开始对环进行编号，并使用词尾 -one，例如环己酮（cyclohexanone）。

5. Physical Properties and Trends | 物理性质及其变化趋势

Short-chain aldehydes and ketones are soluble in water due to hydrogen bonding between the lone pair on the carbonyl oxygen and water molecules. However, solubility decreases as the hydrocarbon chain length increases because the non-polar alkyl part dominates. Methanal, ethanal, and propanone are completely miscible with water, while larger molecules become increasingly insoluble.

短链醛和酮可溶于水，因为羰基氧上的孤对电子能与水分子形成氢键。但是，随着碳氢链的增长，溶解度会下降，因为非极性的烷基部分占据主导。甲醛、乙醛和丙酮与水完全互溶，而更大的分子则越来越难溶。

Boiling points of aldehydes and ketones are higher than those of alkanes of comparable molar mass due to the polarity of the carbonyl group, which causes permanent dipole–dipole attractions between molecules. However, they are lower than those of corresponding alcohols, which can form intermolecular hydrogen bonds (O–H···O). For example, propanal (boiling point 49 °C) boils lower than propan-1-ol (boiling point 97 °C) but higher than butane (boiling point -0.5 °C).

醛和酮的沸点比相对分子质量相近的烷烃高，这是因为羰基的极性使分子间产生永久的偶极-偶极吸引力。然而，它们的沸点低于相应的醇，因为醇分子间能形成氢键（O–H···O）。例如，丙醛（沸点 49 °C）的沸点低于正丙醇（沸点 97 °C），但高于丁烷（沸点 -0.5 °C）。

6. Preparation of Aldehydes and Ketones | 醛和酮的制备方法

In the IGCSE AQA course, the primary method to prepare aldehydes is the oxidation of primary alcohols. A primary alcohol such as ethanol can be oxidised to ethanal using an oxidising agent like acidified potassium dichromate(VI) (K₂Cr₂O₇/H₂SO₄). To obtain the aldehyde rather than the carboxylic acid, distillation apparatus must be used so that the aldehyde, which has a lower boiling point, is removed from the reaction mixture as soon as it is formed, preventing further oxidation.

在 IGCSE AQA 课程中，制备醛的主要方法是氧化伯醇（一级醇）。例如，乙醇等伯醇可以用酸化重铬酸钾(VI)（K₂Cr₂O₇/H₂SO₄）氧化成乙醛。为了得到醛而不是羧酸，必须使用蒸馏装置，使沸点较低的醛在生成时立即离开反应混合物，以防进一步氧化。

CH₃CH₂OH + [O] → CH₃CHO + H₂O

Ketones are prepared by the oxidation of secondary alcohols. For instance, propan-2-ol is oxidised to propanone. Acidified potassium dichromate(VI) can again be used, but this time reflux is employed because the ketone is resistant to further oxidation, and heating ensures complete reaction. The orange dichromate(VI) ions are reduced to green chromium(III) ions, signalling the reaction.

酮通过氧化仲醇（二级醇）来制备。例如，丙-2-醇（异丙醇）被氧化成丙酮。同样可以使用酸化重铬酸钾(VI)，但此时采用回流加热，因为酮不易被进一步氧化，加热可以确保反应完全。橙色的重铬酸根(VI)离子被还原为绿色的铬(III)离子，指示反应的发生。

CH₃CH(OH)CH₃ + [O] → CH₃COCH₃ + H₂O

7. Nucleophilic Addition Reactions – The Core Mechanism | 亲核加成反应——核心机理

The polarised C=O bond makes aldehydes and ketones susceptible to attack by nucleophiles. A nucleophile, such as the cyanide ion (CN⁻), is attracted to the slightly positive carbon atom. The nucleophile donates its electron pair to form a new bond with the carbon, while the pi bond of the C=O breaks, and the electrons move onto the oxygen, generating an alkoxide ion intermediate. Subsequent protonation (e.g., by water or dilute acid) yields the final alcohol product. This is called nucleophilic addition.

极化的 C=O 键使醛和酮容易受到亲核试剂的进攻。例如氰根离子（CN⁻）这样的亲核试剂，会被略带正电的碳原子吸引。亲核试剂提供电子对与碳形成新键，同时 C=O 的 π 键断裂，电子转移到氧上，产生一个醇盐离子中间体。随后质子化（例如通过水或稀酸）得到最终的醇产物。这就是亲核加成反应。

A classic example is the addition of hydrogen cyanide (HCN) to aldehydes and ketones. When ethanal reacts with HCN, 2-hydroxypropanenitrile is formed:

一个经典例子是醛和酮与氰化氢（HCN）的加成反应。当乙醛与 HCN 反应时，生成 2-羟基丙腈：

CH₃CHO + HCN → CH₃CH(OH)CN

This reaction is important because it introduces a new carbon–carbon bond, lengthening the carbon chain by one atom, and creates a hydroxynitrile that can be further hydrolysed to carboxylic acids or reduced to amines.

这个反应很重要，因为它引入了一个新的碳-碳键，将碳链增加一个碳原子，并生成了羟基腈，后者可以进一步水解成羧酸或被还原成胺。

8. Oxidation Reactions – Distinguishing Aldehydes from Ketones | 氧化反应——区分醛与酮

Only aldehydes can be easily oxidised to carboxylic acids because the carbonyl carbon bears a hydrogen atom that can be replaced by an –OH group. Mild oxidising agents like Tollens’ reagent and Fehling’s solution are used to test for aldehydes.

只有醛容易被氧化成羧酸，因为其羰基碳上连有一个可被 –OH 取代的氢原子。托伦斯试剂和斐林试剂等温和氧化剂可用于检测醛。

Tollens’ test (silver mirror test): Tollens’ reagent contains [Ag(NH₃)₂]⁺ ions. When warmed with an aldehyde, the aldehyde is oxidised, and Ag⁺ ions are reduced to metallic silver, which deposits on the clean test tube as a silver mirror. Ketones give no reaction.
托伦斯试验（银镜反应）： 托伦斯试剂含有 [Ag(NH₃)₂]⁺ 离子。与醛温热时，醛被氧化，Ag⁺ 离子被还原成金属银，沉积在洁净的试管壁上形成银镜。酮不发生反应。

Fehling’s / Benedict’s test: These reagents contain Cu²⁺ ions complexed in alkaline solution (blue). Aldehydes reduce Cu²⁺ to Cu⁺, forming a brick-red precipitate of Cu₂O. Ketones produce no colour change.
斐林/本尼迪克特试验： 这些试剂含有碱性溶液中络合的 Cu²⁺ 离子（蓝色）。醛将 Cu²⁺ 还原成 Cu⁺，形成砖红色的 Cu₂O 沉淀。酮无颜色变化。

The balanced equation for the oxidation of ethanal by Fehling’s solution can be simplified as:

乙醛被斐林试剂氧化的配平方程式可简化表示为：

CH₃CHO + 2Cu²⁺ + 2H₂O → CH₃COOH + Cu₂O + 4H⁺

Acidified potassium dichromate(VI) also oxidises aldehydes but not ketones, showing an orange-to-green colour change.

酸化重铬酸钾(VI) 也能氧化醛而不能氧化酮，颜色从橙色变为绿色。

9. Reduction of Aldehydes and Ketones | 醛和酮的还原反应

Both aldehydes and ketones can be reduced to alcohols by adding hydrogen across the carbonyl double bond. The most common reducing agent in the lab is sodium tetrahydridoborate(III), NaBH₄ (often called sodium borohydride), in aqueous or alcoholic solution. This acts as a source of hydride ions (H⁻), which are the true reducing species.

醛和酮都可以通过向羰基双键上加氢而被还原成醇。实验室中最常用的还原剂是四氢硼酸钠（通常称为硼氢化钠，NaBH₄），在水或醇溶液中使用。它提供氢负离子（H⁻），后者是真正的还原物种。

Aldehydes are reduced to primary alcohols, and ketones are reduced to secondary alcohols. For example:

醛被还原成伯醇（一级醇），酮被还原成仲醇（二级醇）。例如：

CH₃CHO + 2[H] → CH₃CH₂OH

CH₃COCH₃ + 2[H] → CH₃CH(OH)CH₃

The [H] in the equation represents the reducing agent. This reduction is important in organic synthesis because it allows alcohols to be regenerated from carbonyl compounds.

方程式中的 [H] 代表还原剂。这一还原反应在有机合成中很重要，因为它能由羰基化合物再生为醇。

10. 2,4-DNP Test – Detecting the Carbonyl Group | 2,4-二硝基苯肼测试——检测羰基

2,4-dinitrophenylhydrazine (2,4-DNP) is a chemical reagent that reacts with both aldehydes and ketones to form a bright yellow, orange, or red precipitate. This test confirms the presence of a carbonyl group but does not distinguish between aldehydes and ketones.

2,4-二硝基苯肼（2,4-DNP）是一种能与醛和酮反应生成亮黄色、橙色或红色沉淀的化学试剂。该测试可证实羰基的存在，但不能区分醛和酮。

The product is a 2,4-dinitrophenylhydrazone. Each different aldehyde or ketone produces a crystalline derivative with a specific, sharp melting point. Once the precipitate is purified and dried, its melting point can be measured and compared to a database to identify the original carbonyl compound.

产物是 2,4-二硝基苯腙。每种不同的醛或酮都会生成具有特定、敏锐熔点的晶体衍生物。沉淀纯化并干燥后，通过测量熔点并与数据库对比，可以鉴定原来的羰基化合物。

This technique is often used in combination with the oxidation tests: 2,4-DNP confirms a carbonyl compound, and Tollens’ or Fehling’s then decides whether it is an aldehyde or a ketone.

该技术常与氧化测试联用：2,4-DNP 确认羰基化合物，托伦斯或斐林测试再判断是醛还是酮。

11. Triiodomethane (Iodoform) Test – Methyl Ketones and Ethanal | 碘仿反应——甲基酮和乙醛的鉴别

The iodoform test specifically detects compounds containing the CH₃CO– group attached to an H or carbon, i.e., methyl ketones or ethanal (the only aldehyde with a methyl group next to the carbonyl). The reaction involves alkaline iodine solution, which oxidises the methyl group and then substitutes hydrogen atoms with iodine, eventually forming a pale yellow precipitate of triiodomethane (CHI₃, iodoform) with a characteristic antiseptic smell.

碘仿反应专一性地检测含有 CH₃CO– 基团（连接氢或碳）的化合物，即甲基酮或乙醛（唯一一种紧邻羰基有甲基的醛）。该反应使用碱性碘溶液，先将甲基氧化，继而碘原子取代氢原子，最终生成淡黄色的三碘甲烷（CHI₃，碘仿）沉淀，并带有特征的消毒水气味。

For example, propanone (CH₃COCH₃) and ethanal (CH₃CHO) give a positive iodoform test, while butanone (CH₃COCH₂CH₃) also gives a positive result because it possesses a methyl ketone group. However, pentan-3-one, which lacks a methyl group adjacent to the carbonyl, gives a negative result. This test is useful for identifying the structure of unknown carbonyl compounds.

例如，丙酮（CH₃COCH₃）和乙醛（CH₃CHO）呈阳性碘仿反应，丁酮（CH₃COCH₂CH₃）也呈阳性，因为它含有甲基酮基团。然而，戊-3-酮因邻接羰基的基团中没有甲基，则呈阴性反应。该测试可用于推断未知羰基化合物的结构。

12. Summary and Exam Tips | 总结与考场技巧

In the IGCSE AQA Chemistry exam, you may be asked to draw and name aldehydes and ketones, describe laboratory preparations, predict products of oxidation and reduction, or outline test-tube reactions to distinguish between given organic compounds. Common pitfalls include confusing aldehyde oxidation with ketone reactions, forgetting that Tollens’ and Fehling’s tests work only for aldehydes, and misnaming ketones by not assigning the lowest number to the carbonyl group.

在 IGCSE AQA 化学考试中，你可能会遇到要求画图和命名醛酮、描述实验室制备方法、预测氧化与还原产物、或设计试管反应以区分给定有机物的题目。常见易错点包括混淆醛的氧化与酮的反应，忘记托伦斯和斐林测试仅对醛有效，以及酮命名时没有给羰基以最小位次。

Test / 测试	Aldehyde / 醛	Ketone / 酮
2,4-DNP	Orange/yellow precipitate	Orange/yellow precipitate
Tollens’ reagent	Silver mirror	No reaction
Fehling’s / Benedict’s	Brick-red precipitate	No reaction (stays blue)
Acidified K₂Cr₂O₇	Orange to green	No reaction
Iodoform test	Positive only for ethanal	Positive for methyl ketones

Remember that mechanisms are not required in depth at IGCSE level, but you should be able to recognise nucleophilic addition as the key reaction type. Always use precise terminology: ‘reduction’ means addition of hydrogen; ‘oxidation’ for aldehydes yields carboxylic acid. And practice writing balanced equations using [O] and [H] to represent oxidising and reducing agents.

请记住，IGCSE 阶段不要求深入的机理，但你应能识别亲核加成为关键反应类型。始终使用准确术语：“还原”指加氢；“氧化”对醛而言生成羧酸。多练习用 [O] 和 [H] 表示氧化剂和还原剂的配平方程式。

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