📚 Aldehydes and Ketones for CCEA A-Level Chemistry | A-Level CCEA 化学：醛和酮 考点精讲
Aldehydes and ketones are two of the most important functional groups in organic chemistry, both containing the carbonyl group C=O. In CCEA A-Level Chemistry, a deep understanding of their structure, preparation, characteristic reactions, and distinguishing tests is essential. This article systematically covers all the key knowledge points, mechanisms, and practical tests you need to master for the exam.
醛和酮是有机化学中最重要的两类官能团，都含有羰基 C=O。在 CCEA A-Level 化学考试中，深入理解它们的结构、制备方法、特征反应以及鉴别测试至关重要。本文系统梳理了你需要掌握的所有核心知识点、反应机理和实验测试，帮助你高效备考。
1. Introduction to Carbonyl Compounds | 羰基化合物简介
A carbonyl group is a carbon atom double-bonded to an oxygen atom. In aldehydes, the carbonyl carbon is bonded to at least one hydrogen atom and one alkyl or aryl group, with the general formula RCHO (except methanal, HCHO). In ketones, the carbonyl carbon is bonded to two alkyl or aryl groups, with the general formula RCOR’.
羰基是由一个碳原子与一个氧原子双键连接而成的基团。在醛中，羰基碳至少与一个氢原子以及一个烷基或芳基相连，通式为 RCHO（甲醛 HCHO 除外）。在酮中，羰基碳与两个烷基或芳基相连，通式为 RCOR’。
2. Naming Aldehydes and Ketones | 醛和酮的命名
For aldehydes, the suffix is ‘-al’. The carbonyl carbon is always carbon number 1, so it does not need a number in the name. For example, CH₃CH₂CHO is propanal. For ketones, the suffix is ‘-one’, and the position of the carbonyl group must be indicated by a number if the chain contains five or more carbons. For example, CH₃COCH₂CH₃ is butanone (no number needed), while CH₃COCH₂CH₂CH₃ is pentan-2-one.
醛的命名后缀为“-al”。羰基碳永远是 1 号碳，因此在名称中无需编号。例如，CH₃CH₂CHO 为丙醛。酮的命名后缀为“-one”，若碳链含有五个或以上碳原子，必须用数字标明羰基的位置。例如，CH₃COCH₂CH₃ 为丁酮（无需编号），而 CH₃COCH₂CH₂CH₃ 为 2-戊酮。
3. Bonding and Polarity of the C=O Group | 羰基的化学键与极性
The carbon-oxygen double bond consists of a strong sigma bond and a pi bond. Oxygen is significantly more electronegative than carbon, so the bond is highly polar, with a partial negative charge on oxygen (δ⁻) and a partial positive charge on carbon (δ⁺). This polarity makes the carbonyl carbon susceptible to nucleophilic attack.
碳氧双键由一个强的 σ 键和一个 π 键组成。氧的电负性远大于碳，因此该键极性很强，氧带部分负电荷（δ⁻），碳带部分正电荷（δ⁺）。这种极性使得羰基碳容易受到亲核试剂的进攻。
4. Preparation of Aldehydes and Ketones | 醛和酮的制备
Aldehydes can be prepared by the oxidation of primary alcohols using acidified potassium dichromate(VI), distilling off the aldehyde as it forms to prevent further oxidation to a carboxylic acid. Ketones are prepared by the oxidation of secondary alcohols; since ketones resist further oxidation, reflux can be used. Both can also be made by the dry distillation of calcium salts of carboxylic acids.
醛可以通过用酸化重铬酸钾氧化伯醇制备，需要在生成醛时立即蒸馏出来，以防止进一步氧化成羧酸。酮可通过氧化仲醇制得；由于酮难以被继续氧化，可以采用回流加热。两类化合物也可通过羧酸钙盐的干馏法制备。
5. Nucleophilic Addition Mechanism | 亲核加成机理
The most characteristic reaction of aldehydes and ketones is nucleophilic addition. A nucleophile, such as cyanide ion (:CN⁻) or hydride ion (:H⁻ from LiAlH₄), attacks the electron-deficient carbonyl carbon. The pi bond breaks, and both electrons move to oxygen, forming a tetrahedral alkoxide intermediate. This intermediate is then protonated (e.g., by water or acid) to give the final alcohol product.
醛和酮最典型的反应是亲核加成。亲核试剂，如氰根离子 (:CN⁻) 或氢负离子 (来自 LiAlH₄ 的 :H⁻)，进攻缺电子的羰基碳。π 键断裂，两个电子转移到氧上，形成一个四面体的醇盐中间体。随后该中间体被质子化（例如被水或酸），得到最终的醇产物。
C=O + Nu⁻ → C(O⁻)-Nu
C(O⁻)-Nu + H⁺ → C(OH)-Nu
6. Reaction with Hydrogen Cyanide | 与氰化氢的反应
Aldehydes and ketones react with hydrogen cyanide, HCN, in the presence of a base (cyanide ion) to form hydroxynitriles (cyanohydrins). This is an important nucleophilic addition that extends the carbon chain by one carbon atom. The reaction is reversible, and the cyanohydrin can be hydrolysed to a hydroxycarboxylic acid or reduced to an amine. Safety note: HCN is extremely toxic — the reaction is usually carried out in situ by mixing NaCN and H₂SO₄.
醛和酮在碱（氰离子）存在下与氰化氢 HCN 反应，生成羟基腈（氰醇）。这是一个重要的亲核加成反应，可使碳链增长一个碳原子。该反应是可逆的，生成的氰醇可水解为羟基羧酸，或还原为胺。安全提示：HCN 剧毒——通常通过现场混合 NaCN 和 H₂SO₄ 来产生。
CH₃COCH₃ + HCN → CH₃C(OH)(CN)CH₃
7. Reduction Reactions | 还原反应
Aldehydes are reduced to primary alcohols, and ketones to secondary alcohols. The classic reducing agent is lithium tetrahydridoaluminate(III), LiAlH₄, in dry ether, which provides the nucleophilic hydride ion, :H⁻. Sodium tetrahydridoborate(III), NaBH₄, in water or alcohol is a milder and more selective reducing agent that also works for both. The reaction mechanism is nucleophilic addition of hydride followed by protonation.
醛被还原为伯醇，酮被还原为仲醇。经典的还原剂是四氢合铝(III)酸锂 LiAlH₄（溶于干燥乙醚），它提供亲核的氢负离子 :H⁻。四氢合硼(III)酸钠 NaBH₄ 溶于水或醇中，是一种更温和、选择性更高的还原剂，同样可以还原醛和酮。反应机理为氢负离子亲核加成，随后质子化。
8. Oxidation Reactions | 氧化反应
Aldehydes are easily oxidised to carboxylic acids by mild oxidising agents such as Tollens’ reagent, Fehling’s solution, or acidified potassium dichromate(VI). Ketones do not undergo oxidation under similar conditions; they can only be oxidised under vigorous conditions that break carbon-carbon bonds. This difference forms the basis of chemical tests to distinguish aldehydes from ketones.
醛极易被温和的氧化剂如托伦斯试剂、费林溶液或酸化的重铬酸钾氧化成羧酸。酮在类似条件下不会被氧化；只有在剧烈条件下（断裂碳-碳键）它们才能被氧化。这一差异构成了区分醛与酮的化学测试基础。
9. Distinguishing Tests: Tollens’ and Fehling’s | 鉴别测试：托伦斯试剂与费林试剂
Tollens’ reagent is [Ag(NH₃)₂]⁺. When warmed with an aldehyde, the Ag⁺ is reduced to metallic silver, forming a silver mirror on the test tube. Ketones give no reaction. Fehling’s solution contains Cu²⁺ complexed with tartrate in alkaline solution. Aldehydes reduce the blue Cu²⁺ to a brick-red precipitate of Cu₂O. Ketones show no change. Both tests rely on the aldehyde being oxidised to a carboxylate ion.
托伦斯试剂 为 [Ag(NH₃)₂]⁺。当与醛共热时，Ag⁺ 被还原为金属银，在试管内壁形成银镜。酮无此反应。费林溶液 含有酒石酸根配位的 Cu²⁺（碱性溶液）。醛将蓝色的 Cu²⁺ 还原为砖红色的 Cu₂O 沉淀。酮无变化。这两个测试都基于醛被氧化为羧酸根离子的反应。
10. Reaction with 2,4-Dinitrophenylhydrazine (2,4-DNP) | 与 2,4-二硝基苯肼的反应
Both aldehydes and ketones react with Brady’s reagent (a solution of 2,4-dinitrophenylhydrazine in methanol/sulfuric acid) to form a bright yellow or orange precipitate of the corresponding 2,4-dinitrophenylhydrazone. This confirms the presence of a carbonyl group. The melting point of the derivative can be measured and compared with literature values to identify the specific carbonyl compound.
醛和酮都可以与布雷迪试剂（2,4-二硝基苯肼的甲醇/硫酸溶液）反应，生成亮黄色或橙色的 2,4-二硝基苯腙沉淀。这个反应证实了羰基的存在。衍生物的熔点可以通过实验测定，与文献值对比，从而鉴定具体的羰基化合物。
11. Iodoform (Triiodomethane) Test | 碘仿反应
The iodoform test gives a positive result (pale yellow precipitate of CHI₃ with a characteristic antiseptic smell) for compounds containing the CH₃CO– group (methyl ketones) or CH₃CH(OH)– group (secondary alcohols with methyl attached to the carbinol carbon). Thus, ethanal and all methyl ketones (e.g., propanone, butanone) give a positive test, while other aldehydes and ketones do not. The reagent is alkaline aqueous iodine (I₂ in NaOH).
含有 CH₃CO– 结构（甲基酮）或 CH₃CH(OH)– 结构（与甲醇碳相连的甲基醇）的化合物，碘仿测试呈阳性——生成具有特殊消毒水气味的淡黄色 CHI₃ 沉淀。因此，乙醛和所有甲基酮（如丙酮、丁酮）都会产生阳性反应，而其他醛和酮则不能。试剂为碱性碘溶液（I₂ 溶于 NaOH）。
12. Summary Table of Tests | 测试总结表
The following table summarises the key test results for identifying and distinguishing aldehydes and ketones at CCEA A-Level. Knowing these is crucial for structured questions on organic analysis.
下表总结了在 CCEA A-Level 中鉴定和区分醛与酮的关键测试结果。掌握这些内容对于有机分析的结构化题目至关重要。
| Test / 测试 | Aldehyde / 醛 | Ketone / 酮 |
|---|---|---|
| 2,4-DNP (Brady’s reagent) | Orange/yellow ppt | Orange/yellow ppt |
| Tollens’ reagent (Ag⁺) | Silver mirror formed | No reaction |
| Fehling’s solution (Cu²⁺) | Blue → brick-red ppt | No reaction (remains blue) |
| Iodoform test (I₂/OH⁻) | Only ethanal gives yellow ppt | Only methyl ketones give yellow ppt |
Published by TutorHao | CCEA A-Level Chemistry Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导Cancel reply