Aldehydes and Ketones | 醛和酮考点精讲

📚 Aldehydes and Ketones | 醛和酮考点精讲

Aldehydes and ketones are carbonyl compounds containing the C=O functional group. In WJEC A-Level Chemistry, understanding their structure, reactivity, and identification is vital. This comprehensive revision guide covers the core concepts, reactions, and analytical tests you need to master.

醛和酮是含有C=O官能团的羰基化合物。在WJEC A-Level化学中，理解它们的结构、反应活性和鉴别方法至关重要。这份精讲涵盖了必须掌握的核心概念、反应和分析测试。

1. Introduction to the Carbonyl Group | 羰基简介

The carbonyl group (C=O) consists of a carbon atom doubly bonded to an oxygen atom. The bond is polar due to the higher electronegativity of oxygen, making the carbon atom electron-deficient and susceptible to attack by nucleophiles. In aldehydes, the carbonyl carbon is bonded to at least one hydrogen atom, while in ketones it is bonded to two carbon atoms.

羰基（C=O）由一个碳原子与一个氧原子双键连接而成。由于氧的电负性更高，该键具有极性，使碳原子缺电子，容易受到亲核试剂的进攻。在醛中，羰基碳至少与一个氢原子相连；而在酮中，羰基碳与两个碳原子相连。

2. Nomenclature and Structure | 命名与结构

For aldehydes, the suffix is -al. The carbonyl carbon is always numbered as carbon 1. Examples: methanal (HCHO), ethanal (CH₃CHO), propanal (CH₃CH₂CHO). Aromatic aldehydes include benzaldehyde (C₆H₅CHO). For ketones, the suffix is -one, with the location of the carbonyl indicated by a number. Examples: propanone (CH₃COCH₃), butan-2-one (CH₃COCH₂CH₃). The general formulas are RCHO and RCOR’.

醛的后缀是-al。羰基碳总是编号为1。例如：甲醛（HCHO）、乙醛（CH₃CHO）、丙醛（CH₃CH₂CHO）。芳香醛包括苯甲醛（C₆H₅CHO）。酮的后缀是-one，并用数字标明羰基的位置。例如：丙酮（CH₃COCH₃）、丁-2-酮（CH₃COCH₂CH₃）。通式分别为RCHO和RCOR’。

3. Physical Properties | 物理性质

Shorter aldehydes and ketones are soluble in water due to hydrogen bonding between the carbonyl oxygen and water molecules. As the hydrocarbon chain length increases, solubility decreases. They have higher boiling points than alkanes of similar molecular mass because of permanent dipole-dipole attractions, but lower boiling points than corresponding alcohols due to the absence of intermolecular hydrogen bonding.

短链醛和酮可溶于水，因为羰基氧能与水分子形成氢键。随着碳氢链增长，溶解度下降。它们的沸点高于相对分子质量相近的烷烃，这是因为存在永久偶极-偶极作用；但沸点低于相应的醇，因为缺少分子间氢键。

4. Preparation of Aldehydes and Ketones | 醛和酮的制备

Aldehydes can be prepared by the controlled oxidation of primary alcohols using acidified potassium dichromate(VI), with the aldehyde being distilled off immediately to prevent further oxidation to the carboxylic acid. Ketones are obtained by oxidising secondary alcohols; since ketones resist further oxidation under these conditions, a simple reflux setup can be used. Example equations:

醛可通过用酸性重铬酸钾(VI)有控制地氧化伯醇来制备，并需立即将醛蒸馏出来，以防进一步氧化为羧酸。酮则通过氧化仲醇获得；由于酮在此条件下不易继续氧化，可使用简单的回流装置。反应式示例：

CH₃CH₂OH + [O] → CH₃CHO + H₂O

伯醇氧化为乙醛

CH₃CH(OH)CH₃ + [O] → CH₃COCH₃ + H₂O

仲醇氧化为丙酮

5. 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, converting the C=O double bond into a single C–O⁻ bond. After protonation by an acid (or water), the final product contains a C–O–H and a C–Nu bond. The reactivity trend is: aldehydes > ketones, due to steric hindrance and the electron-donating inductive effect of alkyl groups in ketones.

醛和酮最具特征的反应是亲核加成。亲核试剂Nu⁻进攻缺电子的羰基碳，使C=O双键变为C–O⁻单键。经酸（或水）质子化后，最终产物含有C–O–H键和C–Nu键。反应活性顺序为：醛 > 酮，原因是酮中烷基的空间位阻和给电子诱导效应。

6. Addition of Hydrogen Cyanide | 与氰化氢的加成

Hydrogen cyanide (HCN) adds across the C=O bond to form cyanohydrins (hydroxynitriles). This reaction is important because it extends the carbon chain by one carbon atom. The cyanohydrin can be hydrolysed to a hydroxycarboxylic acid or reduced to an amino alcohol. The mechanism involves the cyanide ion (CN⁻) acting as the nucleophile. As HCN is a weak acid and hazardous, the reaction is often carried out using NaCN and dilute H₂SO₄, which generates HCN in situ.

氰化氢（HCN）通过C=O键的加成反应生成氰醇（羟基腈）。该反应很重要，因为能将碳链延长一个碳原子。氰醇可水解为羟基羧酸，或还原为氨基醇。反应机理中，氰根离子（CN⁻）充当亲核试剂。因为HCN是弱酸且有毒，反应常使用氰化钠和稀硫酸来原位产生HCN。

CH₃CHO + HCN → CH₃CH(OH)CN

乙醛与HCN生成2-羟基丙腈

7. Addition–Elimination Reactions with Ammonia Derivatives | 与氨衍生物的加成-消除反应

Aldehydes and ketones react with ammonia derivatives such as 2,4-dinitrophenylhydrazine (2,4-DNP) through an addition–elimination mechanism (condensation). Water is eliminated and a C=N double bond is formed, producing a hydrazone derivative. These products are often brightly coloured yellow/orange/red precipitates. The reaction with 2,4-DNP is used as a general test for the carbonyl group; a positive result is the formation of a yellow/orange precipitate. The melting point of the purified 2,4-DNP derivative can be used to identify the original carbonyl compound.

醛和酮可通过加成-消除机理（缩合反应）与氨衍生物（如2,4-二硝基苯肼，2,4-DNP）反应。反应中消去一分子水，形成C=N双键，生成腙类衍生物。这些产物通常呈亮黄色、橙色或红色沉淀。与2,4-DNP的反应是检测羰基的通用方法；阳性结果是生成黄色或橙色沉淀。纯化后的2,4-DNP衍生物的熔点可用于鉴定原羰基化合物。

8. Oxidation of Aldehydes – Distinguishing Tests | 醛的氧化——鉴别测试

Aldehydes are readily oxidised to carboxylic acids, whereas ketones resist oxidation by mild oxidising agents. This difference is exploited in three key tests:

醛很容易被氧化为羧酸，而酮则不易被温和氧化剂氧化。这一区别被用于三个关键的测试：

Tollens’ reagent: Ammoniacal silver nitrate (Ag(NH₃)₂⁺). Aldehydes reduce Ag⁺ to metallic silver, forming a silver mirror on the test tube wall. Ketones give no reaction.
托伦斯试剂：氨性硝酸银（Ag(NH₃)₂⁺）。醛将Ag⁺还原为金属银，在试管壁形成银镜。酮无反应。
Fehling’s reagent: A deep blue Cu²⁺ complex. Aldehydes reduce Cu²⁺ to Cu₂O, a brick-red precipitate. Aromatic aldehydes (e.g. benzaldehyde) do not react with Fehling’s reagent.
斐林试剂：深蓝色的Cu²⁺络合物。醛将Cu²⁺还原为砖红色的Cu₂O沉淀。芳香醛（如苯甲醛）不与斐林试剂反应。
Acidified potassium dichromate(VI): Orange to green colour change on reduction of Cr₂O₇²⁻ to Cr³⁺ for aldehydes; ketones show no change.
酸化重铬酸钾(VI)：醛能使橙色的Cr₂O₇²⁻还原为绿色的Cr³⁺，溶液变色；酮则无变化。

9. Reduction of Aldehydes and Ketones | 醛和酮的还原

Both aldehydes and ketones can be reduced to alcohols. Aldehydes give primary alcohols, and ketones give secondary alcohols. The reducing agent most commonly used in the laboratory is sodium borohydride (NaBH₄) in aqueous ethanol, because it is a safer source of hydride ions (H⁻). For stronger reduction, lithium tetrahydridoaluminate (LiAlH₄) in dry ether can be used. The overall equation for ethanal reduction is:

醛和酮都能被还原为醇。醛生成伯醇，酮生成仲醇。实验室中最常用的还原剂是硼氢化钠（NaBH₄）的水/乙醇溶液，因为它是更安全的氢负离子（H⁻）来源。如需更强还原条件，可使用干醚中的四氢铝锂（LiAlH₄）。乙醛还原的总反应式为：

CH₃CHO + 2[H] → CH₃CH₂OH

乙醛被还原为乙醇

10. The Iodoform (Triiodomethane) Test | 碘仿试验

The iodoform test is specific for methyl carbonyl groups (CH₃CO–) and alcohols with a CH₃CH(OH)– group. When a methyl ketone (e.g. CH₃COCH₃) or ethanol is treated with iodine in alkaline solution, a yellow precipitate of triiodomethane (iodoform, CHI₃) with a characteristic antiseptic smell is formed. The reaction involves successive halogenation and cleavage of a triiodomethyl ketone. This test can distinguish propanone (CH₃COCH₃) from butanone (CH₃CH₂COCH₃) because butanone has a CH₃CO– group while butan-2-one does, actually butanone is CH₃COCH₂CH₃, which contains a CH₃CO– group so it gives a positive test; however, it serves to identify compounds with a methyl group directly attached to the carbonyl.

碘仿试验专属于甲基羰基（CH₃CO–）和含有CH₃CH(OH)–结构的醇。当甲基酮（如CH₃COCH₃）或乙醇在碱性溶液中与碘反应时，会生成具有特征消毒水气味的黄色三碘甲烷（碘仿，CHI₃）沉淀。该反应涉及连续卤代和三碘甲基酮的断裂。这个测试可用于区分丙酮（CH₃COCH₃）和丁酮（CH₃CH₂COCH₃），因为丁酮也含有CH₃CO–基团，会给出阳性结果；但它主要用于识别直接连在羰基上的甲基。

11. Spectroscopic Identification | 光谱鉴定

Infrared spectroscopy shows a strong, sharp absorption for the C=O stretch around 1700 cm⁻¹ (exact value depends on the environment – aldehydes typically 1720–1740 cm⁻¹, ketones around 1705–1725 cm⁻¹). Aldehydes also exhibit two weak C–H stretches near 2720 cm⁻¹ and 2820 cm⁻¹ associated with the aldehyde hydrogen. In mass spectrometry, the molecular ion peak gives the molecular mass, and characteristic fragmentation patterns include the loss of the α-hydrogen for aldehydes or cleavage adjacent to the carbonyl (McLafferty rearrangement) for ketones.

红外光谱在约1700 cm⁻¹处显示C=O伸缩振动的强而尖锐的吸收峰（确切波数取决于化学环境——醛通常在1720–1740 cm⁻¹，酮在1705–1725 cm⁻¹左右）。醛还在2720 cm⁻¹和2820 cm⁻¹附近出现两个与醛氢相关的较弱C–H伸缩峰。在质谱中，分子离子峰给出相对分子质量，特征碎裂模式包括醛失去α-氢，或酮在羰基相邻位置发生麦氏重排断裂。

12. Summary of Key Distinctions | 关键区别总结

Test / Feature	Aldehydes	Ketones
General formula	RCHO	RCOR’
Oxidation	Easily oxidised to RCOOH	Resistant to mild oxidation
Tollens’ reagent	Silver mirror formed	No reaction
Fehling’s reagent	Brick-red precipitate (except aromatic aldehydes)	No reaction
2,4-DNP test	Yellow/orange precipitate	Yellow/orange precipitate
Iodoform test	Only ethanal (CH₃CHO) gives positive	Methyl ketones give positive

Remember that while the 2,4-DNP test confirms the presence of a carbonyl group, the oxidation tests differentiate between aldehydes and ketones. By combining these tests with spectroscopic data, you can confidently identify unknown carbonyl compounds in your WJEC examination.

请记住，2,4-DNP测试确认羰基的存在，而氧化测试则可区分醛与酮。将这些测试与光谱数据相结合，你就能在WJEC考试中自信地鉴定未知羰基化合物。

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Aldehydes and Ketones | 醛和酮 考点精讲