📚 Carbonyl Compounds: Aldehydes and Ketones | 羰基化合物：醛和酮
mistry, containing the C=O functional group. This article focuses on aldehydes and ketones – the simplest carbonyl compounds – ty, particularly nucleophilic addition, is fundamental to organic synthesis and analytical chemistry.
羰基化合物是A-Level化学中一个至关重要的家族，都含有 C=O 官能团。本文聚焦最简单的羰基化合物——醛和酮，涵盖它们的结构、命名、制备以及 Edexcel 考试必须掌握的关键反应。理解它们的反应性，特别是亲核加成，是有机合成和分析化学的基础。
1. Introduction to Carbonyl Compounds | 羰基化合物简介
A carbonyl group consists of a carbon atom double‑ ehydes, the carbonyl carbon is bonded to at least one hydrogen atom, giving the general formula RCHO. In ketones, the carbonyl carbon is bonded to two alkyl or aryl groups, with the general formula RCOR'.
羰基由一个碳原子与一个氧原子双键连接而成 (C=O)。在醛中，羰基碳至少与一个氢原子相连，通式为 RCHO。在酮中，羰基碳与两个烷基或芳基相连，通式为 RCOR'。
The carbonyl group is polar because oxygen is more electronegative than carbon. This polarity dominates the physical properties and chemical reactivity of aldehydes and ketones, making the carbonyl carbon electron‑deficient and susceptible to attack by nucleophiles.
由于氧的电负性大于碳，羰基具有极性。这种极性决定了醛和酮的物理性质与化学活性，使羰基碳缺电子，容易受到亲核试剂的进攻。
2. Structure and Bonding of the Carbonyl Group | 羰基的结构与成键
ed, forming three σ bonds in a trigonal planar arrangement with bond angles of approximately 120°. The remaining p orbital on carbon overlaps with a p orbital on oxygen to form a π bond. This π bond is polarised towards the oxygen atom, creating a significant dipole moment.
C=O 中的碳原子为 sp² 杂化，形成三个 σ 键，呈平面三角形排布，键角约 120°。碳原子上剩余的 p 轨道与氧原子的 p 轨道重叠形成 π 键。该 π 键电子云偏向氧原子，产生显著的偶极矩。
The electron density distribution can be represented by resonance structures, showing the carbonyl carbon carrying a partial positive charge (δ⁺) and the oxygen a partial negative charge (δ⁻). This charge separation explains why the carbonyl group readily undergoes nucleophilic addition rather than electrophilic attack.
电子密度分布可用共振结构表示，显示羰基碳带部分正电荷 (δ⁺)，氧带部分负电荷 (δ⁻)。这种电荷分离解释了为何羰基易于发生亲核加成而非亲电进攻。
3. Naming Aldehydes and Ketones | 醛和酮的命名
According to IUPAC rules, aldehydes are named by replacing the final ‘‑e’ of the parent alkane with ‘‑al’. The aldehyde carbon is always carbon‑1, so its position is not indicated in the name. For example, HCHO is methanal, CH₃CHO is ethanal, and CH₃CH₂CHO is propanal.
根据 IUPAC 规则，醛的命名是将母体烷烃词尾的“‑e”替换为“‑al”。醛基碳总是1位碳，因此名称中不标出其位置。例如，HCHO 为 methanal，CH₃CHO 为 ethanal，CH₃CH₂CHO 为 propanal。
Ketones are named by replacing the ‘‑e’ of the parent alkane with ‘‑one’, and the position of the carbonyl group is indicated by a number. For example, CH₃COCH₃ is propan‑2‑one (commonly acetone), and CH₃COCH₂CH₃ is butan‑2‑one. The chain is numbered from the end nearest the carbonyl group.
酮的命名是将母体烷烃的“‑e”替换为“‑one”，并标明羰基的位置编号。例如，CH₃COCH₃ 为 propan‑2‑one（俗称丙酮），CH₃COCH₂CH₃ 为 butan‑2‑one。编号从靠近羰基的一端开始。
4. Physical Properties | 物理性质
The polarity of the carbonyl group gives aldehydes and ketones higher boiling points than non‑polar alkanes of similar molar mass. However, because they cannot form intermolecular hydrogen bonds with themselves (they lack O–H groups), their boiling points are lower than those of corresponding alcohols.
羰基的极性使醛和酮的沸点高于摩尔质量相近的非极性烷烃。但由于它们自身无法形成分子间氢键（缺少 O–H 基团），其沸点低于相应的醇。
Short‑chain aldehydes and ketones are soluble in water because the carbonyl oxygen can form hydrogen bonds with water molecules. Solubility decreases as the length of the non‑polar hydrocarbon chain increases, typically becoming insoluble beyond about four carbon atoms in the chain.
短链醛和酮可溶于水，因为羰基氧可与水分子形成氢键。随着非极性烃链增长，溶解度下降，碳链超过约四个碳原子后通常难溶。
Solubility trend: H‑bonding with H₂O dominates in small molecules; London forces dominate in larger ones.
溶解度趋势：小分子中与水的氢键占主导；较大分子中伦敦力占主导。
5. Preparation of Aldehydes and Ketones | 醛和酮的制备
Aldehydes can be prepared by the oxidation of primary alcohols using a mild oxidising agent such as acidified potassium dichromate(VI), with immediate distillation to prevent further oxidation to the carboxylic acid. For example, ethanol can be oxidised to ethanal.
醛可由伯醇的氧化制备，使用酸性重铬酸钾(VI)等温和氧化剂，并立即蒸馏以防止进一步氧化成羧酸。例如，乙醇可被氧化成乙醛。
Ketones are prepared by the oxidation of secondary alcohols, where further oxidation is not possible. For instance, propan‑2‑ol is oxidised to propan‑2‑one under reflux with acidified K₂Cr₂O₇.
酮可由仲醇氧化制备，此时无法继续氧化。例如，丙‑2‑醇与酸性 K₂Cr₂O₇ 回流反应可氧化为丙‑2‑酮。
Aldehydes and ketones can also be produced by the dry distillation of calcium salts of carboxylic acids or by the hydration of alkynes, but the alcohol oxidation routes are the most relevant for A‑Level.
醛和酮也可通过羧酸钙盐的干馏或炔烃的水合制得，但与A‑Level最相关的是醇氧化路线。
6. Nucleophilic Addition Reactions | 亲核加成反应
The most characteristic reaction of aldehydes and ketones is nucleophilic addition. A nucleophile (Nu⁻) attacks the electron‑deficient carbonyl carbon, breaking the π bond and forming a tetrahedral intermediate. After protonation by water or dilute acid, the final product is an alcohol derivative.
醛和酮最具特征性的反应是亲核加成。亲核试剂 (Nu⁻) 进攻缺电子的羰基碳，π键断裂，形成四面体中间体。经水或稀酸质子化后，最终产物为醇的衍生物。
The general mechanism involves two steps: first, the nucleophile donates its electron pair to the δ⁺ carbon, and the π electrons move entirely onto oxygen to give an alkoxide ion; second, the alkoxide ion picks up a proton (H⁺) to form the –OH group. This mechanism must be drawn with curly arrows in examinations.
通用机理分两步：首先，亲核试剂将电子对给予 δ⁺ 碳，π 电子完全转移至氧上形成烷氧负离子；其次，烷氧负离子结合一个质子 (H⁺) 生成 –OH 基团。考试中必须用弯箭头画出该机理。
7. Reaction with Hydrogen Cyanide | 与氰化氢的反应
e C=O bond to form hydroxynitriles (cyanohydrins). The nucleophile is the cyanide ion (⁻:C≡ acid. This reaction is very useful because it extends the carbon chain by one carbon atom.
氰化氢 (HCN) 加成到 C=O 键上生成羟基腈（氰醇）。亲核试剂是氰离子 (⁻:C≡N)，通常由氰化钾和稀硫酸现场生成。该反应非常有用，因为它使碳链增长一个碳原子。
CH₃CHO + HCN → CH₃CH(OH)CN (2‑hydroxypropanenitrile)
CH₃CHO + HCN → CH₃CH(OH)CN（2‑羟基丙腈）
The reaction is carried out under alkaline conditions to generate a steady supply of CN⁻ ions. With unsymmetrical carbonyl compounds, a racemic mixture of enantiomers is often produced because the planar carbonyl group can be attacked from either face with equal probability.
反应在碱性条件下进行，以稳定供给 CN⁻ 离子。对于不对称羰基化合物，常得到外消旋混合物，因为平面的羰基可被从两侧以同等概率进攻。
8. Reduction Reactions | 还原反应
Aldehydes and ketones are readily reduced to primary and secondary alcohols respectively. The most common reducing agent is sodium borohydride (NaBH₄) in water or ethanol. NaBH₄ acts as a source of hydride ions (H⁻), which attack the carbonyl carbon.
醛和酮分别可被轻易还原为伯醇和仲醇。最常用的还原剂是硼氢化钠 (NaBH₄) 的水或乙醇溶液。NaBH₄ 提供氢负离子 (H⁻)，进攻羰基碳。
CH₃COCH₃ + 2[H] → CH₃CH(OH)CH₃ (propan‑2‑ol)
CH₃COCH₃ + 2[H] → CH₃CH(OH)CH₃（丙‑2‑醇）
a nickel catalyst also works, but NaBH₄ is specific to carbonyl groups and will not reduce C=C bonds under typical conditions.
在考试答案中，还原剂常简写为 [H]，但你必须能写出 NaBH₄。使用 H₂ 和镍催化剂的催化加氢也可行，但 NaBH₄ 对羰基具有选择性，在典型条件下不会还原 C=C 双键。
9. Detection and Differentiation: 2,4‑DNP and Tollens' Reagent | 检测与鉴别：2,4‑DNP 与 Tollens 试剂
2,4‑dinitrophenylhydrazine (2,4‑DNP) reacts with both aldehydes and ketones to form a yellow or orange precipitate. This is a classic test for the presence of a carbonyl group. The melting point of the purified derivative can be used to identify the original carbonyl compound.
2,4‑二硝基苯肼 (2,4‑DNP) 与醛和酮均反应，生成黄色或橙色沉淀。这是检验羰基存在的经典方法。纯化衍生物的熔点可用于鉴别原始羰基化合物。
Tollens' reagent ([Ag(NH₃)₂]⁺) is a mild oxidising agent that distinguishes aldehydes from ketones. Aldehydes are oxidised to carboxylate ions, reducing silver ions to metallic silver, which forms a characteristic silver mirror on the test tube. Ketones give no reaction.
Tollens 试剂 ([Ag(NH₃)₂]⁺) 是一种温和氧化剂，可区分醛和酮。醛被氧化为羧酸根离子，银离子被还原为金属银，在试管壁上形成特征性的银镜。酮不发生反应。
CH₃CHO + 2[Ag(NH₃)₂]⁺ + 2OH⁻ → CH₃COO⁻ + 2Ag + 4NH₃ + H₂O
CH₃CHO + 2[Ag(NH₃)₂]⁺ + 2OH⁻ → CH₃COO⁻ + 2Ag + 4NH₃ + H₂O
10. The Iodoform (Triiodomethane) Reaction | 碘仿（三碘甲烷）反应
Methyl ketones (CH₃CO–R) and compounds with the CH₃CH(OH)– group give a positive iodoform test. The reaction involves treatment with iodine and sodium hydroxide, producing a pale yellow precipitate of triiodomethane (CHI₃) with a characteristic antiseptic smell.
甲基酮 (CH₃CO–R) 以及含有 CH₃CH(OH)– 基团的化合物会呈现碘仿反应阳性。该反应用碘和氢氧化钠处理，生成淡黄色的三碘甲烷 (CHI₃) 沉淀，具有特征性的消毒剂气味。
For example, ethanal (CH₃CHO) and propan‑2‑one (CH₃COCH₃) give positive results, whereas propanal (CH₃CH₂CHO) and butan‑2‑one (CH₃COCH₂CH₃) do not. The reaction first involves halogenation of the methyl group adjacent to the carbonyl, followed by cleavage to form CHI₃.
例如，乙醛 (CH₃CHO) 和丙‑2‑酮 (CH₃COCH₃) 呈阳性，而丙醛 (CH₃CH₂CHO) 和丁‑2‑酮 (CH₃COCH₂CH₃) 则不会。反应首先是羰基邻位甲基的卤代，继而断裂生成 CHI₃。
11. Oxidation of Aldehydes | 醛的氧化
Aldehydes can be oxidised to carboxylic acids by a variety of oxidising agents, including acidified potassium dichromate(VI) and Tollens' reagent. This reaction is used to differentiate aldehydes from ketones, as ketones resist oxidation under normal conditions.
醛可被多种氧化剂氧化成羧酸，包括酸性重铬酸钾(VI) 和 Tollens 试剂。此反应用于区别醛与酮，因为酮在常规条件下难以被氧化。
With acidified K₂Cr₂O₇, the colour changes from orange to green, indicating the reduction of Cr(VI) to Cr(III). This test is easy to perform in the laboratory and is frequently assessed in practical examinations. Ketones do not produce any colour change.
使用酸性 K₂Cr₂O₇ 时，颜色由橙变绿，表明 Cr(VI) 被还原为 Cr(III)。该测试在实验室中简便易行，实验考试中经常考查。酮不会导致任何颜色变化。
12. Summary of Key Reactions | 关键反应总结
The table below summarises the major reactions of aldehydes and ketones that are essential for the Edexcel A‑Level specification. Being able to recall these conditions, reagents, and products is critical for both structured and multiple‑choice questions.
下表总结了 Edexcel A‑Level 考试大纲中醛和酮的主要反应。牢记这些条件、试剂和产物对于简答题和选择题都至关重要。
| Reaction 反应 | Reagents 试剂 | Product 产物 | Notes 注释 |
|---|---|---|---|
| Nucleophilic addition of HCN | KCN + dil. H₂SO₄ | Hydroxynitrile | Increases chain length |
| Reduction | NaBH₄ in water/ethanol | 1° or 2° alcohol | H⁻ acts as nucleophile |
| 2,4‑DNP test | 2,4‑DNP in methanol / H₂SO₄ | Yellow/orange ppt | Detects C=O group |
| Tollens' test | [Ag(NH₃)₂]⁺, warm | Silver mirror | Positive for aldehydes only |
| Iodoform test | I₂ + NaOH, warm | Yellow ppt of CHI₃ | Positive for methyl ketones |
| Oxidation of aldehydes | K₂Cr₂O₇ / H⁺, reflux | Carboxylic acid | Orange to green |
Understanding these reactions not only helps you answer direct recall questions but also allows you to tackle synthesis and analysis problems with confidence. Always practise drawing mechanisms, especially for nucleophilic addition, to secure full marks.
掌握这些反应不仅能帮助你回答直接的记忆题，还能让你自信地应对合成与分析问题。务必练习画机理，尤其是亲核加成，以确保拿到满分。
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