Carbonyl Compounds: Aldehydes and Ketones - Structure, Reactions and Tests | 羰基化合物：醛与酮 —— 结构、反应与检测

📚 Carbonyl Compounds: Aldehydes and Ketones - Structure, Reactions and Tests | 羰基化合物：醛与酮 —— 结构、反应与检测

The carbonyl group is one of the most important functional groups in organic chemistry, and it sits at the heart of the A-Level Chemistry syllabus. Aldehydes and ketones both contain the carbonyl group, but they differ in the atoms bonded to the carbonyl carbon. In aldehydes, the carbonyl carbon is bonded to at least one hydrogen atom, while in ketones it is bonded to two carbon-containing groups. This seemingly small difference leads to significant variations in reactivity, especially towards oxidation, and it is exactly this difference that examiners love to test. Mastering carbonyl chemistry is essential for Cambridge A-Level students, because these compounds appear in synthesis routes, analytical tests, and reaction mechanisms.

羰基是有机化学中最重要的官能团之一，也是 A-Level 化学考纲的核心内容。醛和酮都含有羰基，但羰基碳所连接的原子不同：醛的羰基碳上至少连有一个氢原子，而酮的羰基碳则与两个含碳基团相连。这一细微差别导致它们的反应活性出现显著差异，尤其表现在氧化反应上，而正是这种差异成为考试的高频命题点。掌握羰基化合物化学对剑桥 A-Level 考生至关重要，因为它们广泛存在于合成路线、分析测试和反应机理中。

1. Introduction to Carbonyl Compounds | 羰基化合物简介

² hybridised, giving a trigonal planar arrangement with bond angles close to 120°. The double bond is polar because oxygen is more electronegative than carbon, so the carbonyl carbon carries a partial positive charge (δ⁺) and the oxygen carries a partial negative charge (δ⁻). This polarity makes the carbonyl carbon electron-deficient and highly susceptible to nucleophilic attack, which is the single most important idea in the whole topic.

羰基由一个碳原子与一个氧原子以双键相连构成，写作 C=O。碳原子为 sp² 杂化，呈平面三角形构型，键角接近 120°。由于氧的电负性大于碳，该双键具有极性，羰基碳带部分正电荷（δ⁺），氧带部分负电荷（δ⁻）。这种极性使羰基碳缺电子，极易受到亲核试剂进攻——这是整个专题中最重要的核心思想。

• Aldehyde general formula: RCHO, where R can be H or an alkyl/aryl group. | 醛的通式：RCHO，R 可以是 H 或烷基/芳基。
• Ketone general formula: RCOR', where R and R' are alkyl or aryl groups. | 酮的通式：RCOR'，R 与 R' 均为烷基或芳基。
• Simplest aldehyde: methanal (formaldehyde), HCHO. | 最简单的醛：甲醛 methanal，HCHO。
• Simplest ketone: propanone (acetone), CH₃COCH₃. | 最简单的酮：丙酮 propanone，CH₃COCH₃。

Carbonyl compounds are everywhere: methanal is used to make urea-formaldehyde resins and adhesives, ethanal is an intermediate in industrial chemistry, and propanone is a common laboratory and industrial solvent. In living organisms, aldehydes and ketones appear as sugars (glucose contains an aldehyde group in its open-chain form), as ketone bodies in metabolism, and as intermediates in enzyme-catalysed pathways.

羰基化合物无处不在：甲醛用于制造脲醛树脂和黏合剂，乙醛是重要的工业中间体，丙酮则是实验室和工业常用溶剂。在生物体内，醛和酮以糖类（葡萄糖开链形式含醛基）、代谢中的酮体以及酶催化途径的中间体等形式出现。

A useful way to remember the distinction is this pair of general structures:

R-CH=O (aldehyde)  vs  R-CO-R' (ketone)

2. Structure and Bonding of the Carbonyl Group | 羰基的结构与成键

The carbonyl carbon forms three sigma bonds and one pi bond, so the geometry around it is trigonal planar. The three atoms attached to the carbon lie in the same plane, and the oxygen sits in that plane too. Because the pi bond is formed from a p orbital on carbon overlapping with a p orbital on oxygen, the electron cloud is concentrated above and below the molecular plane. This flat arrangement allows a nucleophile to approach the carbonyl carbon from either face, which explains why the products of nucleophilic addition are often formed as a racemic mixture when a new chiral centre is created.

羰基碳形成三个 σ 键和一个 π 键，因此其几何构型为平面三角形。与碳相连的三个原子共平面，氧原子也位于同一平面内。由于 π 键由碳的 p 轨道与氧的 p 轨道侧面重叠形成，电子云分布在分子平面的上下两侧。这种平面结构允许亲核试剂从任一方向进攻羰基碳，从而解释了当反应产生新的手性中心时，产物通常以外消旋混合物形式存在。

consequences: the carbon becomes electrophilic, and the oxygen becomes basic enough to accept a proton in the second step of many mechanisms. The C=O bond is also stronger than a C=C bond, with a bond enthalpy of roughly 743 kJ mol⁻¹ compared with about 612 kJ mol⁻¹ for C=C. This is why carbonyls prefer addition-elimination or simple addition, rather than the electrophilic addition typical of alkenes.

由于碳（2.5）与氧（3.5）的电负性差异，C=O 双键被强烈极化。这种极性带来两个结果：碳具有亲电性，而氧的碱性足以在许多机理的第二步中接受质子。此外，C=O 键比 C=C 键更强，键能约为 743 kJ mol⁻¹，而 C=C 约为 612 kJ mol⁻¹。这正是羰基化合物倾向于发生加成-消除或简单加成，而不是烯烃典型的亲电加成的原因。

δ⁺   δ⁻    >C=O   ( sp², ~120°, planar, polar)

3. Naming Aldehydes and Ketones | 醛与酮的命名

IUPAC naming follows predictable rules. Aldehydes take the suffix -al, and the carbonyl carbon is always carbon number 1, so no number is needed. Ketones take the suffix -one, and the position of the carbonyl group must be stated using the lowest possible number, unless the chain is so short that no ambiguity exists. Common (trivial) names are still widely used in textbooks and exam questions, so both systems should be recognised.

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IUPAC 命名遵循规律性规则。醛使用后缀 -al，且羰基碳永远是 1 号碳，因此无需标号。酮使用后缀 -one，必须标出羰基的位置，并取最小的可能编号，除非碳链太短不存在歧义。习惯命名法（俗名）在教科书和考题中仍被广泛使用，因此两套体系都需要认识。

Several special cases are worth memorising. A molecule with an aldehyde group at each end is a dialdehyde, such as OHC-CHO, named ethanedial (glyoxal). A ring ketone such as cyclohexanone keeps the -one suffix with the number 1 implied. When an aldehyde group is attached to a benzene ring, the name is based on the aromatic ring, as in benzaldehyde (benzenecarbaldehyde). Compounds containing both an aldehyde and a ketone group require the -al suffix to take priority, and the carbonyl of the aldehyde is numbered 1.

几个特殊情况需要记住。两端都有醛基的分子称为二醛，例如 OHC-CHO 命名为乙二醛（glyoxal）。环酮如环己酮保留 -one 后缀，默认为 1 号位。当醛基连接在苯环上时，命名以芳环为基础，如苯甲醛（benzenecarbaldehyde）。若分子中同时含醛基和酮羰基，-al 后缀优先级更高，醛基碳编为 1 号。

4. Physical Properties: Boiling Points and Solubility | 物理性质：沸点与溶解性

nt dipole-dipole attractions between molecules. However, their boiling points are much lower than those of alcohols of similar size, because carbonyl compounds have no hydrogen atom bonded directly to oxygen, so they cannot act as hydrogen bond donors to each other. The lone pairs on the carbonyl oxygen can accept hydrogen bonds, which is important for solubility but not for boiling point.

与相对分子质量相近的烷烃相比，醛和酮的沸点更高，因为极性的 C=O 基团在分子间产生永久偶极-偶极作用。但它们的沸点明显低于相近大小的醇，因为羰基化合物没有直接连在氧上的氢原子，分子之间无法作为氢键给体。羰基氧上的孤对电子可以接受氢键，这一点对溶解性很重要，但对沸点没有贡献。

Solubility follows a simple trend. Methanal, ethanal and propanone are completely miscible with water because their carbonyl oxygen accepts hydrogen bonds from water molecules. As the hydrocarbon chain lengthens, the non-polar part of the molecule dominates and solubility falls sharply; hexanal, for example, is only slightly soluble. Smaller aldehydes also have sharp, pungent smells, which is why methanal and ethanal must be handled in a fume cupboard.

溶解性呈现简单规律。甲醛、乙醛和丙酮可与水完全互溶，因为其羰基氧能接受水分子提供的氢键。随着烃链增长，分子的非极性部分占主导，溶解度急剧下降，例如己醛仅微溶于水。较小的醛还具有强烈刺鼻的气味，因此甲醛和乙醛必须在通风橱中操作。

5. Preparing Aldehydes and Ketones | 醛与酮的制备

The most important school-level preparation is the oxidation of alcohols using acidified potassium dichromate(VI), K₂Cr₂O₇/H₂SO₄. A primary alcohol can be oxidised to an aldehyde, but the aldehyde is easily oxidised further to a carboxylic acid, so the product must be distilled off as it forms. A secondary alcohol is oxidised to a ketone, which resists further oxidation, so the mixture can be heated under reflux to drive the reaction to completion. In both cases the orange dichromate(VI) ion is reduced to the green chromium(III) ion.

中学阶段最重要的制备方法是使用酸性重铬酸钾 K₂Cr₂O₇/H₂SO₄ 氧化醇。伯醇可被氧化为醛，但醛易被进一步氧化成羧酸，因此必须边生成边蒸出产物。仲醇被氧化为酮，酮不易继续氧化，因此可以加热回流使反应进行完全。两种情况下，橙色的重铬酸根离子都被还原为绿色的铬(III)离子。

CH₃CH₂OH + [O] → CH₃CHO + H₂O

(CH₃)₂CHOH + [O] → (CH₃)₂CO + H₂O

Distillation versus reflux is a classic exam distinction. Distilling the product out of the reaction flask removes the aldehyde before it can be oxidised again, whereas refluxing returns the condensed vapour to the flask so that oxidation goes all the way to the carboxylic acid. If a question asks how to obtain ethanal from ethanol, the answer must mention warming with acidified dichromate(VI) and distilling the product.

蒸馏与回流是经典考点。把产物蒸出反应瓶可使醛在被再次氧化前离开体系；而回流则把冷凝液返回瓶中，使氧化一直进行到羧酸。若题目问如何由乙醇制取乙醛，答案必须提到用酸性重铬酸钾温热并蒸出产物。

Other routes are worth knowing as extension knowledge. Ketones

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