📚 A-Level Edexcel Chemistry: Aldehydes and Ketones Key Points | A-Level Edexcel 化学：醛和酮 考点精讲
Aldehydes and ketones are carbonyl compounds containing the functional group C=O. They are important in organic synthesis and are tested frequently in Edexcel A-Level Chemistry. This article covers the essential knowledge you need for the exam, including structure, nomenclature, preparation, reactions, and identification tests.
醛和酮是含有羰基 C=O 的官能团化合物，在有机合成中扮演重要角色，是 Edexcel A-Level 化学考试中的高频考点。本文梳理了考试必备的核心知识，包括结构、命名、制备方法、典型反应及鉴别检验等。
1. Structure and Nomenclature | 结构与命名
Aldehydes have the general formula RCHO, with the carbonyl group at the end of the carbon chain. The functional group is –CHO. Ketones have the general formula RCOR’, where the carbonyl group is bonded to two alkyl or aryl groups, located within the chain. In IUPAC naming, aldehydes use the suffix ‘-al’ and ketones use ‘-one’. The carbonyl carbon is always assigned the lowest possible number for ketones, while for aldehydes it is always position 1, so the number is often omitted. Examples: methanal (HCHO), ethanal (CH₃CHO); propanone (CH₃COCH₃), butan-2-one (CH₃COCH₂CH₃).
醛的通式为 RCHO，羰基位于碳链末端，官能团为 –CHO。酮的通式为 RCOR’，羰基连接两个烷基或芳基，位于碳链内部。IUPAC 命名中，醛以“-al”结尾，酮以“-one”结尾。酮的羰基碳要给出编号且尽可能最小；醛的羰基始终在 1 位，编号常省略。例如：甲醛 (HCHO)、乙醛 (CH₃CHO)；丙酮 (CH₃COCH₃)、丁-2-酮 (CH₃COCH₂CH₃)。
2. Physical Properties | 物理性质
The carbonyl group is polar (C⁺=O⁻), giving aldehydes and ketones permanent dipole–dipole interactions. They have higher boiling points than alkanes of similar molar mass, but lower than corresponding alcohols because they cannot form intermolecular hydrogen bonds. Short-chain aldehydes and ketones are soluble in water due to hydrogen bonding between water and the carbonyl oxygen. Solubility decreases as the hydrocarbon chain length increases. Methanal is a gas at room temperature; other simple aldehydes and ketones are colourless liquids with characteristic smells.
羰基呈极性 (C⁺=O⁻)，使醛酮分子间存在永久偶极–偶极作用。其沸点高于相对分子质量相近的烷烃，但低于相应的醇，因为它们不能形成分子间的氢键。低碳数的醛酮可溶于水，这是因为水分子可与羰基氧形成氢键。随着碳氢链长增长，溶解性下降。甲醛在室温下为气体，其他简单醛酮多为具有特殊气味的无色液体。
3. Preparation Methods | 制备方法
Aldehydes can be prepared by the partial oxidation of primary alcohols using acidified potassium dichromate(VI). The aldehyde is distilled out of the reaction mixture as it forms to prevent further oxidation to carboxylic acid. Ketones are prepared by the oxidation of secondary alcohols under similar conditions, with no risk of over-oxidation. Both aldehydes and ketones can be synthesised via ozonolysis of alkenes, followed by reductive work-up. Industrially, methanal is produced by the oxidation of methanol using a silver or iron(III) oxide catalyst.
醛可通过伯醇在酸性重铬酸钾(VI)作用下的部分氧化制得。反应中需及时将生成的醛蒸馏出来，避免进一步氧化成羧酸。酮则可由仲醇在类似条件下氧化获得，不存在过氧化问题。醛和酮还可通过烯烃的臭氧化–还原分解得到。工业上，甲醛常用甲醇在银或氧化铁(III)催化剂存在下氧化生产。
4. Nucleophilic Addition Mechanism | 亲核加成机理
The most characteristic reaction of aldehydes and ketones is nucleophilic addition (A_N). The polarised C=O bond makes the carbonyl carbon electron-deficient and open to attack by nucleophiles. In the first step, the nucleophile donates a pair of electrons to the δ+ carbon, breaking the π bond and forming a tetrahedral alkoxide intermediate. In the second step, the negatively charged oxygen is protonated to yield the final alcohol product. Aldehydes are generally more reactive than ketones due to less steric hindrance and fewer electron-donating alkyl groups stabilising the positive charge.
醛和酮最具特征的反应是亲核加成。极化的 C=O 键使羰基碳带 δ+ 电荷，易受亲核试剂进攻。第一步中，亲核试剂向该碳提供一对电子，π 键断裂，形成四面体的烷氧负离子中间体。第二步，带负电的氧被质子化，生成最终的醇类产物。醛的反应活性通常高于酮，因为空间位阻较小，且给电子的烷基较少，不足以稳定部分正电荷。
5. Addition of Hydrogen Cyanide (HCN) | 与氰化氢的加成
Hydrogen cyanide adds across the carbonyl group to form a hydroxynitrile (cyanohydrin). This reaction increases the carbon chain length by one and is particularly useful in synthesis. Because HCN is a toxic gas, it is usually generated in situ from sodium cyanide and dilute sulfuric acid. The mechanism involves nucleophilic attack by the cyanide ion (CN⁻), followed by protonation. The product hydroxynitrile contains both a hydroxyl (–OH) and a nitrile (–CN) group, which can be hydrolysed to a carboxylic acid, providing access to amino acids and other derivatives. The reaction is a key example of nucleophilic addition in the Edexcel specification.
氰化氢可与羰基发生加成反应，生成羟腈（氰醇）。该反应使碳链增加一个碳原子，在合成中十分有用。由于 HCN 为有毒气体，通常在体系中用氰化钠和稀硫酸原位产生。机理为氰根离子 (CN⁻) 亲核进攻，随后质子化。产物羟腈同时含有羟基 (–OH) 和腈基 (–CN)，后者可水解为羧酸，进而合成氨基酸等衍生物。该反应是 Edexcel 考纲中亲核加成的重要实例。
6. Reduction with Sodium Borohydride | 硼氢化钠还原
Aldehydes and ketones are reduced to primary and secondary alcohols respectively using sodium borohydride (NaBH₄) in aqueous or alcoholic solution. The hydride ion (H⁻) from NaBH₄ acts as a nucleophile, attacking the carbonyl carbon. The resulting alkoxide ion is then protonated by water or alcohol solvent to give the alcohol. The overall process is summarised as: RCHO + 2[H] → RCH₂OH (primary alcohol); RCOR’ + 2[H] → RCH(OH)R’ (secondary alcohol). In equations, [H] represents the reducing agent. Lithium aluminium hydride (LiAlH₄) is a stronger reductant but is not required for simple aldehyde/ketone reduction in the A-level lab context.
醛和酮可被硼氢化钠 (NaBH₄) 在水或醇溶液中还原，分别得到伯醇和仲醇。NaBH₄ 提供的负氢离子 (H⁻) 作为亲核试剂进攻羰基碳，生成的烷氧负离子随后被溶剂质子化得到醇。总反应可表示为：RCHO + 2[H] → RCH₂OH（伯醇）；RCOR’ + 2[H] → RCH(OH)R’（仲醇）。方程式中 [H] 代表还原剂。氢化铝锂 (LiAlH₄) 还原性更强，但在 A-level 实验中简单醛酮的还原通常使用 NaBH₄ 即可。
7. Oxidation Reactions | 氧化反应
A key difference between aldehydes and ketones is their susceptibility to oxidation. Aldehydes are readily oxidised to carboxylic acids by mild oxidising agents such as acidified potassium dichromate(VI) or Tollens’ reagent. Ketones lack a hydrogen atom on the carbonyl carbon, so they resist oxidation under similar conditions. This forms the basis of chemical tests to distinguish between them. Vigorous oxidation of ketones can break carbon–carbon bonds, but this is beyond the scope of Edexcel A-level simple tests. The oxidation of aldehydes is often written as: RCHO + [O] → RCOOH, where [O] is the oxidising agent.
醛与酮的一个关键区别在于它们对氧化的敏感性。醛可被酸性重铬酸钾(VI)或 Tollens 试剂等温和氧化剂氧化为羧酸。酮的羰基碳上没有氢原子，因此在类似条件下不易被氧化。这一差异构成了化学鉴别检验的基础。酮在剧烈条件下可发生碳碳键断裂氧化，但这超出了 Edexcel A-level 简单检验的范围。醛的氧化通常记作：RCHO + [O] → RCOOH，[O] 表示氧化剂。
8. Identification Tests: 2,4-DNP and Tollens/Fehling | 鉴别检验：2,4-DNP 与 Tollens/Fehling 试剂
2,4-dinitrophenylhydrazine (2,4-DNP) is used to detect the presence of a carbonyl group (both aldehydes and ketones). A positive test gives an orange or yellow precipitate of the corresponding 2,4-dinitrophenylhydrazone. The melting point of the derivative can be used to identify the specific carbonyl compound. Tollens’ reagent (ammoniacal silver nitrate) contains [Ag(NH₃)₂]⁺ ions. Aldehydes reduce it to metallic silver, forming a ‘silver mirror’ on the test tube walls, while ketones give no reaction. Fehling’s solution (alkaline Cu²⁺ complex) is reduced by aldehydes to give a brick-red precipitate of Cu₂O; ketones do not react. Thus, Tollens and Fehling tests distinguish aldehydes from ketones.
2,4-二硝基苯肼 (2,4-DNP) 用于检测羰基的存在（醛和酮均可）。阳性反应产生橙色或黄色沉淀，即相应的 2,4-二硝基苯腙。通过测定衍生物的熔点可鉴定具体的羰基化合物。Tollens 试剂（氨银溶液）含有 [Ag(NH₃)₂]⁺ 离子。醛能将其还原为金属银，在试管壁上形成“银镜”，而酮不发生反应。Fehling 溶液（碱性铜(II)配合物）可被醛还原，生成砖红色 Cu₂O 沉淀；酮不反应。因此，Tollens 和 Fehling 试剂可用于区分醛和酮。
9. The Iodoform (Triiodomethane) Reaction | 碘仿（三碘甲烷）反应
The iodoform test detects a methyl carbonyl group (CH₃CO–) or a secondary alcohol with a methyl group on the same carbon (CH₃CH(OH)–). When a carbonyl compound possessing the structure RCOCH₃ is treated with iodine and sodium hydroxide, a pale yellow precipitate of triiodomethane (iodoform, CHI₃) is formed. The reaction involves initial substitution of the methyl hydrogens by iodine, followed by cleavage of the C–C bond. For example, ethanal (CH₃CHO) and propanone (CH₃COCH₃) give positive iodoform tests. This test is a useful way to narrow down the identity of an unknown carbonyl compound.
碘仿反应可检测甲基羰基 (CH₃CO–) 或带有甲基的仲醇结构 (CH₃CH(OH)–)。当具有 RCOCH₃ 结构的羰基化合物与碘和氢氧化钠作用时，会生成淡黄色的三碘甲烷（碘仿，CHI₃）沉淀。反应先是甲基上的氢逐步被碘取代，随后 C–C 键断裂。例如，乙醛 (CH₃CHO) 和丙酮 (CH₃COCH₃) 均呈碘仿反应阳性。这一检验有助于缩小未知羰基化合物的鉴定范围。
10. Spectroscopic Analysis | 光谱分析
Spectroscopy provides valuable structural information. In IR spectroscopy, aldehydes and ketones show a strong absorption band around 1700–1750 cm⁻¹ due to C=O stretching. Aldehydes additionally exhibit a distinctive C–H stretch for the –CHO hydrogen at around 2700–2900 cm⁻¹, which is often seen as two small peaks. In mass spectrometry, aldehydes and ketones frequently undergo α-cleavage and McLafferty rearrangement. The fragmentation pattern can help distinguish between isomers. For example, propanal and propanone give different mass spectra. NMR spectroscopy is used (beyond this article) to confirm the environment of hydrogens, with the aldehyde proton appearing at δ 9–10 ppm in ¹H NMR.
光谱分析可提供丰富的结构信息。红外光谱 中，醛和酮的 C=O 伸缩振动在约 1700–1750 cm⁻¹ 处有强吸收。醛还在约 2700–2900 cm⁻¹ 出现 –CHO 氢的 C–H 伸缩特征峰，常表现为两个小峰。质谱 中，醛和酮常发生 α-裂解和 McLafferty 重排。碎片模式有助于区分同分异构体，例如丙醛和丙酮的质谱不同。核磁共振波谱 中（超出本文详述），醛氢在 ¹H NMR 中出现在 δ 9–10 ppm 区域，可进一步确证结构。
11. Summary of Key Reactions and Patterns | 核心反应与规律总结
In summary, aldehydes and ketones share many reactions due to the carbonyl group, but their differences in oxidation behaviour and the presence of an aldehyde hydrogen make them distinguishable. The main reaction types you must master for Edexcel A-Level Chemistry are: nucleophilic addition (HCN, NaBH₄), oxidation (only aldehydes to carboxylic acids), and qualitative tests (2,4-DNP, Tollens, Fehling, iodoform). Always consider steric and electronic effects when comparing reactivity. Spectroscopic data is an essential tool for confirming structures. Using a systematic approach to reaction conditions, reagents and observations will help you answer examination questions accurately.
总结而言，醛和酮因羰基的存在共享许多反应，但氧化行为的差异和醛氢的存在使它们得以区分。Edexcel A-Level 化学要求掌握的主要反应类型有：亲核加成（HCN, NaBH₄）、氧化（仅醛可被氧化成羧酸）以及定性检验（2,4-DNP, Tollens, Fehling, 碘仿）。比较反应活性时，始终要考虑空间效应和电子效应。光谱数据是确证结构的重要工具。有条理地掌握反应条件、试剂和实验现象，将有助于准确解答考试问题。
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