Aldehydes and Ketones | 醛和酮

📚 Aldehydes and Ketones | 醛和酮

Aldehydes and ketones are two families of organic compounds that both contain the carbonyl group, C=O, yet they differ in the position of this group within the carbon chain. In IGCSE Edexcel Chemistry, you need to master their structures, naming, preparation from alcohols, characteristic reactions, and the tests that distinguish them from each other and from other functional groups. This revision guide covers all key specification points clearly and concisely.

醛和酮是两类含羰基（C=O）的有机化合物，只是羰基在碳链中的位置不同。在 IGCSE Edexcel 化学中，你需要掌握它们的结构、命名、由醇制备的方法、特征反应，以及区分彼此和其他官能团的检验方法。本篇考点精讲将清晰、概括地覆盖所有核心大纲要求。

1. What Are Aldehydes and Ketones? | 什么是醛和酮？

Aldehydes and ketones are carbonyl compounds, meaning they contain a carbon atom double-bonded to an oxygen atom. In an aldehyde, the carbonyl carbon is at the end of a carbon chain and is bonded to at least one hydrogen atom, giving the functional group –CHO. In a ketone, the carbonyl carbon sits between two other carbon atoms and is not bonded to any hydrogen; its functional group is –CO–.

醛和酮都是羰基化合物，即含有一个碳原子与氧原子以双键相连。在醛中，羰基碳位于碳链末端，且至少与一个氢原子相连，官能团写作 –CHO。在酮中，羰基碳位于两个碳原子之间，不与氢直接相连，官能团为 –CO–。

The general molecular formula for both families is C_nH_2nO, provided the carbonyl is the only functional group and the chain is saturated. This means aldehydes and ketones sharing the same number of carbon atoms are functional group isomers of each other – for example, propanal (CH₃CH₂CHO) and propanone (CH₃COCH₃) both have the formula C₃H₆O.

这两类化合物的通式均为 C_nH_2nO（前提是羰基为唯一官能团且碳链饱和）。这意味着碳原子数相同的醛和酮互为官能团异构体——例如丙醛（CH₃CH₂CHO）和丙酮（CH₃COCH₃）的分子式都是 C₃H₆O。

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2. The Carbonyl Functional Group | 羰基官能团

The carbonyl group consists of a carbon atom double-bonded to an oxygen atom: >C=O. The double bond is polar because oxygen is more electronegative than carbon. This polarity makes the carbonyl carbon electron-deficient and susceptible to attack by nucleophiles, as well as influencing the physical properties such as boiling points.

羰基由一个碳原子与氧原子以双键连接而成：>C=O。由于氧的电负性大于碳，该双键具有极性。这种极性使羰基碳缺电子，容易受到亲核试剂的进攻，同时也影响着沸点等物理性质。

In aldehydes, the carbonyl group is always at the end of the chain, written as –CHO (do not confuse with an alcohol –OH). In ketones, the carbonyl group is never at the end; it is written as –CO– between two carbon-containing groups. The simplest aldehyde is methanal (HCHO), and the simplest ketone is propanone (CH₃COCH₃).

在醛中，羰基总是处于碳链末端，写作 –CHO（不要与醇的 –OH 混淆）。在酮中，羰基绝不在链端，而是写为夹在两个含碳基团之间的 –CO–。最简单的醛是甲醛（HCHO），最简单的酮是丙酮（CH₃COCH₃）。

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

For IGCSE Edexcel, you must be able to name and draw straight-chain aldehydes and ketones up to four carbon atoms. Aldehydes are named by replacing the final ‘-e’ of the corresponding alkane with ‘-al’. The aldehyde carbon is always carbon number 1, so you do not need to number its position. Examples: methanal (HCHO), ethanal (CH₃CHO), propanal (CH₃CH₂CHO), butanal (CH₃CH₂CH₂CHO).

根据 IGCSE Edexcel 要求，你需要会命名和绘制含最多四个碳原子的直链醛和酮。醛的命名是将对应烷烃末尾的“-e”换成“-al”。醛基碳总是编号为第1位，因此无需标明位置。例如：甲醛（methanal，HCHO）、乙醛（ethanal，CH₃CHO）、丙醛（propanal，CH₃CH₂CHO）、丁醛（butanal，CH₃CH₂CH₂CHO）。

Ketones are named by replacing the ‘-e’ of the alkane with ‘-one’, and the position of the carbonyl carbon must be indicated by a number, chosen to be as low as possible. The simplest ketone, propanone (CH₃COCH₃), does not need a number because the carbonyl can only be on carbon 2. For butanone, the carbonyl is on carbon 2: CH₃COCH₂CH₃ (butan-2-one).

酮的命名是将烷烃词尾的“-e”换成“-one”，并需用数字标明羰基碳的位置，编号应尽可能小。最简单的酮——丙酮（propanone，CH₃COCH₃）无需标数字，因为羰基只能在第2位。丁酮则写作 butan-2-one，结构为 CH₃COCH₂CH₃。

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4. Physical Properties | 物理性质

Aldehydes and ketones cannot form hydrogen bonds with identical molecules because they lack an –OH group. However, the polar carbonyl group allows dipole-dipole interactions, resulting in boiling points higher than those of alkanes of similar molecular mass but lower than those of corresponding alcohols. Short-chain aldehydes and ketones such as methanal, ethanal and propanone are highly soluble in water because the carbonyl oxygen can form hydrogen bonds with water molecules.

醛和酮分子间无法形成氢键，因为它们没有 –OH 基团。但极性的羰基能产生偶极-偶极作用，因此它们的沸点高于相对分子质量相近的烷烃，但低于相应的醇。短链醛酮如甲醛、乙醛和丙酮都易溶于水，因为羰基氧能与水分子形成氢键。

As the hydrocarbon chain lengthens, solubility in water decreases because the non-polar alkyl portion disrupts hydrogen bonding with water. In the IGCSE exam, you may be asked to compare boiling points or solubility trends, always linking your explanation to intermolecular forces.

随着碳链增长，水溶性降低，因为非极性的烷基部分阻碍了与水的氢键形成。在 IGCSE 考试中，你可能需要比较沸点或溶解性变化趋势，回答时一定要联系分子间作用力来解释。

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5. Preparation by Oxidation of Alcohols | 由醇氧化制备

Aldehydes are produced by the partial oxidation of primary alcohols. When a primary alcohol like ethanol is heated with an oxidising agent such as acidified potassium dichromate(VI) (K₂Cr₂O₇/H₂SO₄), it first forms an aldehyde: CH₃CH₂OH + [O] → CH₃CHO + H₂O. To obtain the aldehyde without further oxidation, the aldehyde must be distilled out of the reaction mixture as it forms, because it has a lower boiling point than the alcohol and the carboxylic acid.

醛由伯醇部分氧化制得。将乙醇等伯醇与酸化重铬酸钾（K₂Cr₂O₇/H₂SO₄）一同加热，首先生成醛：CH₃CH₂OH + [O] → CH₃CHO + H₂O。欲得到醛而不让其继续氧化，必须在其生成时立即从反应混合物中蒸馏出来，因为醛的沸点低于相应的醇和羧酸。

Ketones are prepared by the oxidation of secondary alcohols. For example, propan-2-ol (CH₃CHOHCH₃) is oxidised to propanone (CH₃COCH₃) by heating with acidified potassium dichromate(VI). Ketones are not easily oxidised further, so distillation is not essential to prevent over-oxidation; simple reflux conditions can be used. The colour change observed in these reactions is the familiar orange to green (Cr₂O₇²⁻ to Cr³⁺).

酮由仲醇氧化制备。例如，2-丙醇（CH₃CHOHCH₃）与酸化重铬酸钾加热时被氧化成丙酮（CH₃COCH₃）。酮不易被继续氧化，因此不必像制醛那样必须蒸馏；可直接采用回流条件。该反应中可观察到熟悉的颜色变化：橙色变为绿色（Cr₂O₇²⁻ 变为 Cr³⁺）。

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6. Distinguishing Aldehydes from Ketones: Selective Oxidation | 区分醛和酮：选择性氧化

A crucial difference between the two families is that aldehydes can be oxidised to carboxylic acids, whereas ketones resist oxidation under mild conditions. With acidified potassium dichromate(VI) and heating under reflux, an aldehyde is fully oxidised to a carboxylic acid: CH₃CHO + [O] → CH₃COOH. A ketone such as propanone shows no reaction with the same oxidising agent – the solution remains orange.

两类化合物最重要的区别在于：醛能被氧化成羧酸，酮在温和条件下则不被氧化。将醛与酸化重铬酸钾一同回流加热，醛会彻底氧化为羧酸：CH₃CHO + [O] → CH₃COOH。而像丙酮这样的酮在同样条件下不发生反应，溶液保持橙色。

This difference arises because aldehyde molecules possess a hydrogen atom attached to the carbonyl carbon, which can be removed during oxidation. Ketones lack this hydrogen, making oxidation require breaking a strong C–C bond, which does not occur with common oxidising agents at IGCSE level. This selective oxidation is the basis for chemical tests to tell aldehydes and ketones apart.

造成这一差异的原因是醛分子的羰基碳上连有一个氢原子，该氢原子在氧化过程中可被脱去。酮没有这个氢原子，氧化需要断裂较强的 C–C 键，这在 IGCSE 阶段常用的氧化剂条件下不会发生。这种选择性氧化正是化学检验区分醛酮的基础。

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7. Reduction of Aldehydes and Ketones | 醛酮的还原反应

Both aldehydes and ketones can be reduced to alcohols by the addition of hydrogen. This is an important reaction specified in the Edexcel IGCSE syllabus. The reducing agent is usually represented as [H] or H₂ with a nickel catalyst and heat. An aldehyde is reduced to a primary alcohol: CH₃CHO + 2[H] → CH₃CH₂OH. A ketone is reduced to a secondary alcohol: CH₃COCH₃ + 2[H] → CH₃CHOHCH₃.

醛和酮都可经加氢还原生成醇。这是 Edexcel IGCSE 大纲明确要求的重要反应。还原剂通常用 [H] 表示，或用 H₂ 加镍催化剂并加热。醛被还原为伯醇：CH₃CHO + 2[H] → CH₃CH₂OH。酮被还原为仲醇：CH₃COCH₃ + 2[H] → CH₃CHOHCH₃。

This transformation is the reverse of alcohol oxidation. In exam questions, you may be asked to identify the product of reduction or to choose an appropriate starting material to prepare a target alcohol. Remember: reducing an aldehyde always gives a primary alcohol; reducing a ketone always gives a secondary alcohol.

该转化是醇氧化的逆反应。考试中可能会要求你判断还原产物，或选择适当原料以制备目标醇。请记住：还原醛总是得到伯醇，还原酮总是得到仲醇。

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8. Tests for Aldehydes: Fehling’s and Tollens’ Reagents | 醛的检验：斐林试剂和托伦斯试剂

Because aldehydes are easily oxidised and ketones are not, we can use mild oxidising agents to test for the presence of an aldehyde. Two such reagents are used in IGCSE Edexcel: Fehling’s solution and Tollens’ reagent. Both give a positive result only with aldehydes, leaving ketones unchanged.

由于醛易被氧化而酮不易，我们可以用温和的氧化剂来检验醛的存在。IGCSE Edexcel 考试涉及两种这类试剂：斐林溶液和托伦斯试剂。两者仅对醛显阳性反应，酮则不发生变化。

Fehling’s solution is a deep blue complex containing Cu²⁺ ions. When heated with an aldehyde, the Cu²⁺ is reduced to Cu⁺, which forms a brick-red precipitate of copper(I) oxide (Cu₂O). The aldehyde is oxidised to a carboxylic acid. No precipitate forms with a ketone – the blue colour remains.

斐林溶液是含 Cu²⁺ 离子的深蓝色络合物。与醛共热时，Cu²⁺ 被还原为 Cu⁺，生成砖红色的氧化亚铜（Cu₂O）沉淀。醛则被氧化成羧酸。酮在此条件下不产生沉淀，蓝色不变。

Tollens’ reagent contains the complex ion [Ag(NH₃)₂]⁺. When warmed with an aldehyde, Ag⁺ is reduced to metallic silver, which deposits on the clean inner wall of the test tube as a silver mirror. The aldehyde is oxidised to a carboxylate ion under the alkaline conditions. Ketones give no silver mirror.

托伦斯试剂含有 [Ag(NH₃)₂]⁺ 络离子。与醛微热时，Ag⁺ 被还原为金属银，沉积在洁净的试管内壁上形成银镜。醛在碱性条件下被氧化成羧酸根离子。酮则不会产生银镜。

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9. Using Bromine Water to Distinguish Alkenes from Carbonyl Compounds | 用溴水区分烯烃和羰基化合物

The IGCSE Edexcel specification also asks you to understand how aqueous bromine can be used to tell apart alkenes from compounds containing only the carbonyl group. Alkenes decolourise bromine water rapidly by electrophilic addition, turning it from orange to colourless. Carbonyl compounds – both aldehydes and ketones – do not react with bromine water under these test conditions, so the orange colour persists.

IGCSE Edexcel 大纲还要求你理解如何利用溴水区分烯烃与仅含羰基的化合物。烯烃通过亲电加成使溴水迅速褪色，由橙色变为无色。羰基化合物（包括醛和酮）在该测试条件下不与溴水反应，橙色保持不变。

Be careful: although aldehydes can be oxidised by bromine water slowly, in IGCSE you should consider that neither aldehydes nor ketones decolourise bromine water in the standard test. The key idea is that bromine water tests for C=C double bonds, and the carbonyl double bond does not behave in the same way.

注意：虽然醛可被溴水缓慢氧化，但在 IGCSE 层面应认为标准测试中醛和酮都不能使溴水褪色。核心思想在于溴水检测的是 C=C 双键，而羰基双键不会发生同样的反应。

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10. Key Reaction Summary and Comparison Table | 核心反应总结与对比表

The table below brings together the most important reactions that distinguish aldehydes, ketones and related compounds. You can use this as a quick revision checklist for the exam.

下表汇总了区分醛、酮及相关化合物的最重要反应，可作为考前快速复习清单。

Test / Reaction | 检验/反应	Aldehyde | 醛	Ketone | 酮	Alkene | 烯烃
Acidified K₂Cr₂O₇, heat | 酸化重铬酸钾，加热	Orange → green; aldehyde → carboxylic acid | 橙变绿；醛 → 羧酸	No reaction; stays orange | 无反应，橙色不变	Oxidation may occur slowly, not a standard test | 可能缓慢氧化，不作标准检验
Fehling’s solution, warm | 斐林溶液，温热	Blue → brick-red precipitate | 蓝色 → 砖红色沉淀	No change; remains blue | 无变化，蓝色不变	No change | 无变化
Tollens’ reagent, warm | 托伦斯试剂，温热	Silver mirror formed | 形成银镜	No silver mirror | 无银镜	No silver mirror | 无银镜
Bromine water, shake | 溴水，振荡	No rapid decolourisation | 不迅速褪色	No rapid decolourisation | 不迅速褪色	Orange → colourless | 橙色 → 无色
Reduction with H₂/Ni | H₂/Ni 还原	Forms primary alcohol | 生成伯醇	Forms secondary alcohol | 生成仲醇	Forms alkane | 生成烷烃

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11. Real-World Context and Exam Tips | 实际应用与考试技巧

Methanal (formaldehyde) is used as a preservative and in the production of plastics. Propanone (acetone) is a common solvent, for example in nail varnish remover. Knowing these examples can help you link the chemistry to everyday contexts, which often appears in application-style questions.

甲醛（methanal）用作防腐剂并用于塑料生产。丙酮（propanone）是常见溶剂，例如洗甲水中就含有它。了解这些实例可以将化学知识与日常生活联系起来，应用类题目中常会涉及。

When drawing displayed formulae, always show the double bond between C and O clearly. For aldehydes, write the hydrogen attached to the carbonyl carbon; for ketones, ensure the carbonyl is between two carbon atoms. In naming, remember that the suffix for aldehyde is always ‘-al’ and for ketone is ‘-one’. Practice writing balanced equations for oxidation and reduction as these are frequently tested.

在绘制显示式时，务必清晰画出 C 与 O 之间的双键。醛要写出与羰基碳相连的氢原子，酮则要确保羰基位于两个碳原子之间。命名时牢记醛的后缀总是“-al”，酮的后缀是“-one”。多练写氧化与还原的配平方程式，因为它们是高频考点。

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