📚 IB Chemistry: Aldehydes and Ketones – Key Exam Points | IB 化学：醛和酮 考点精讲
Aldehydes and ketones are essential functional group compounds in IB Chemistry, featuring the polar carbonyl group C=O. Understanding their nomenclature, physical properties, and characteristic reactions is crucial for exam success. This revision guide covers oxidation, reduction, nucleophilic addition, and key distinguishing tests like Tollens’, Fehling’s, 2,4-DNPH, and the iodoform reaction.
醛和酮是 IB 化学中重要的官能团化合物，其核心是极性的羰基 C=O。掌握命名、物理性质以及特征反应是考试的关键。本文重点讲解氧化、还原、亲核加成以及托伦斯试剂、斐林试剂、2,4-DNPH、碘仿反应等重要鉴别测试。
1. The Carbonyl Group and Nomenclature | 羰基与命名
The carbonyl group consists of a carbon atom double-bonded to oxygen (C=O). In aldehydes, the carbonyl carbon is bonded to at least one hydrogen atom and an organic group (RCHO). In ketones, the carbonyl carbon is bonded to two organic groups (RCOR’). The general formula for aldehydes is CnH2nO, and for ketones is also CnH2nO (n≥3).
羰基由碳氧双键 (C=O) 组成。在醛中，羰基碳至少连接一个氢原子和一个有机基团 (RCHO)；在酮中，羰基碳连接两个有机基团 (RCOR’)。醛的通式为 CnH2nO，酮的通式也是 CnH2nO (n≥3)。
IUPAC naming: Aldehydes are named by replacing the terminal ‘-e’ of the parent alkane with ‘-al’. The carbonyl carbon is always carbon-1, so number is not required for simple aldehydes. Ketones are named by replacing ‘-e’ with ‘-one’, and the position of the carbonyl is specified by a number from the end nearest the C=O group.
IUPAC 命名：醛将对应烷烃词尾 ‘-e’ 改为 ‘-al’，羰基碳总是 1 号位，简单醛无需编号。酮则将 ‘-e’ 改为 ‘-one’，并从最靠近羰基的一端用数字标出羰基位置。
2. Physical Properties | 物理性质
The polar C=O bond results in dipole-dipole interactions as the predominant intermolecular forces in aldehydes and ketones (no hydrogen bonding between like molecules). Thus, their boiling points are higher than those of comparable alkanes but lower than those of corresponding alcohols, which can form hydrogen bonds.
极性的 C=O 键使醛和酮分子间最主要的力为偶极-偶极相互作用（同类分子间无氢键）。因此，它们的沸点高于相对分子质量相近的烷烃，但低于能形成氢键的相应醇类。
Short-chain aldehydes and ketones (e.g., methanal, ethanal, propanone) are soluble in water due to hydrogen bonding between the carbonyl oxygen and water molecules. As the hydrocarbon chain length increases, solubility decreases.
短链醛酮（如甲醛、乙醛、丙酮）可溶于水，因为羰基氧能与水分子形成氢键。随着碳链增长，溶解度降低。
3. Preparation: Oxidation of Alcohols | 制备：醇的氧化
Primary alcohols can be oxidized to aldehydes using acidified potassium dichromate (K₂Cr₂O₇/H⁺) under distillation to prevent further oxidation to carboxylic acid. Secondary alcohols are oxidized to ketones under similar conditions; reflux is often used as the ketone is not easily oxidized further.
伯醇可使用酸化重铬酸钾 (K₂Cr₂O₇/H⁺) 在蒸馏条件下氧化为醛，以防止进一步氧化成羧酸。仲醇在类似条件下氧化为酮，常采用回流，因为酮不易被继续氧化。
The colour change from orange Cr₂O₇²⁻ to green Cr³⁺ confirms oxidation. Tertiary alcohols resist oxidation because they lack a hydrogen atom on the carbon bearing the –OH group.
颜色从橙色 Cr₂O₇²⁻ 变为绿色 Cr³⁺ 可确认氧化发生。叔醇由于连有 –OH 的碳上没有氢，难以被氧化。
4. Oxidation Reactions of Aldehydes | 醛的氧化反应
Aldehydes are easily oxidized to carboxylic acids (or carboxylate salts in alkaline conditions). Common oxidizing agents: acidified dichromate, Tollens’ reagent, Fehling’s solution. Ketones do not react under these mild conditions, making oxidation a key distinguishing test.
醛极易被氧化成羧酸（或在碱性条件下生成羧酸盐）。常用氧化剂：酸化重铬酸盐、托伦斯试剂、斐林溶液。酮在这些温和条件下不发生反应，因此氧化反应是区分醛酮的重要依据。
Tollens’ test: Aldehyde + [Ag(NH₃)₂]⁺ + OH⁻ → Ag mirror. e.g., CH₃CHO + 2[Ag(NH₃)₂]⁺ + 2OH⁻ → CH₃COO⁻ + 2Ag(s) + 4NH₃ + 2H₂O. Fehling’s test: Aldehyde reduces blue Cu²⁺ (complex) to red Cu₂O precipitate. Ketones give no reaction.
托伦斯试验：醛 + [Ag(NH₃)₂]⁺ + OH⁻ → 银镜。例如 CH₃CHO + 2[Ag(NH₃)₂]⁺ + 2OH⁻ → CH₃COO⁻ + 2Ag(s) + 4NH₃ + 2H₂O。斐林试验：醛使蓝色 Cu²⁺（酒石酸配合物）还原为红色 Cu₂O 沉淀。酮无反应。
5. Reduction of Carbonyl Compounds | 羰基化合物的还原反应
Both aldehydes and ketones can be reduced to alcohols using reducing agents such as sodium borohydride (NaBH₄) in aqueous or alcoholic solution, or lithium aluminium hydride (LiAlH₄) in dry ether. NaBH₄ is preferred in the lab due to safety.
醛和酮均可被还原为醇，常用的还原剂有硼氢化钠 (NaBH₄)（在水或醇溶液中使用）或氢化铝锂 (LiAlH₄)（在无水乙醚中）。实验室中更常用 NaBH₄，因为更安全。
Aldehydes are reduced to primary alcohols: RCHO + 2[H] → RCH₂OH. Ketones are reduced to secondary alcohols: RCOR’ + 2[H] → RCH(OH)R’. The reduction involves nucleophilic addition of hydride (H⁻) to the carbonyl carbon.
醛被还原为伯醇：RCHO + 2[H] → RCH₂OH。酮被还原为仲醇：RCOR’ + 2[H] → RCH(OH)R’。该还原反应实质是负氢离子 (H⁻) 对羰基碳的亲核加成。
6. Nucleophilic Addition with Hydrogen Cyanide | 与氰化氢的亲核加成
Hydrogen cyanide (HCN) adds across the C=O bond of aldehydes and ketones to form hydroxynitriles (cyanohydrins). This reaction is catalyzed by base (CN⁻) and increases the carbon chain by one carbon atom. It is a classic nucleophilic addition.
氰化氢 (HCN) 与醛酮的 C=O 键加成，生成羟腈（氰醇）。反应由碱 (CN⁻) 催化，使碳链增加一个碳原子。这是一个典型的亲核加成反应。
Mechanism: The nucleophile CN⁻ attacks the electrophilic carbonyl carbon, breaking the π bond. The negatively charged oxygen then protonates (from HCN or H₂O) to give –OH. For example: CH₃COCH₃ + HCN → CH₃C(OH)(CN)CH₃.
机理：亲核试剂 CN⁻ 进攻亲电的羰基碳，π 键断裂；带负电荷的氧随后质子化（来自 HCN 或 H₂O），生成 –OH。例如：CH₃COCH₃ + HCN → CH₃C(OH)(CN)CH₃。
7. The 2,4-DNPH Test for Carbonyl Group | 2,4-二硝基苯肼检测羰基
2,4-Dinitrophenylhydrazine (Brady’s reagent) reacts with aldehydes and ketones to form orange or yellow precipitates of 2,4-dinitrophenylhydrazones. This confirms the presence of a carbonyl group (C=O), but does not distinguish between aldehyde and ketone.
2,4-二硝基苯肼（布雷迪试剂）与醛和酮反应，生成橙黄色 2,4-二硝基苯腙沉淀。此反应可确认羰基 (C=O) 的存在，但不能区分醛和酮。
The precipitate can be purified, and its melting point measured to identify the original carbonyl compound by comparison with known derivatives.
该沉淀经纯化后可通过测定熔点并与已知衍生物对照，鉴定原羰基化合物。
8. Iodoform (Haloform) Reaction | 碘仿反应
The iodoform test gives a pale yellow precipitate of triiodomethane (CHI₃) with compounds containing the CH₃CO– group, such as ethanal (acetaldehyde) and all methyl ketones (RCOCH₃). The reaction is carried out with iodine
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