Aldehydes and Ketones: Key Concepts for IB and OCR Chemistry | IB OCR 化学：醛和酮考点精讲

📚 Aldehydes and Ketones: Key Concepts for IB and OCR Chemistry | IB OCR 化学：醛和酮考点精讲

Aldehydes and ketones are carbonyl compounds central to organic chemistry in both IB and OCR A Level specifications. Understanding their structure, reactivity, and the key tests that differentiate them is essential for success. This article breaks down every major topic, from nomenclature and physical properties to nucleophilic addition, oxidation, and condensation reactions, providing a clear revision pathway.

醛和酮是含有羰基的有机化合物，在IB和OCR A Level化学大纲中占据核心地位。理解它们的结构、反应性以及区分它们的关键鉴别实验，是取得好成绩的关键。本文详细拆解了从命名、物理性质到亲核加成、氧化及缩合反应等每一个重要考点，为你的复习提供清晰的路线图。

1. Introduction to the Carbonyl Group | 羰基简介

The carbonyl group, C=O, consists of a carbon atom double-bonded to oxygen. In aldehydes, the carbonyl carbon is bonded to at least one hydrogen atom, giving the group the formula -CHO located at the end of a carbon chain. In ketones, the carbonyl carbon is bonded to two other carbon atoms, placing the C=O group within the chain.

羰基 C=O 由一个碳原子与氧原子以双键连接而成。在醛分子中，羰基碳至少与一个氢原子相连，官能团写作 -CHO，位于碳链末端。而在酮分子中，羰基碳与另外两个碳原子相连，C=O 基团位于碳链内部。

The carbonyl group is highly polar because oxygen is much more electronegative than carbon. This polarity creates a partial positive charge (δ⁺) on the carbon and a partial negative charge (δ⁻) on the oxygen, making the carbonyl carbon an electrophilic centre susceptible to attack by nucleophiles.

由于氧的电负性远强于碳，羰基具有强极性，使得碳上带部分正电荷 (δ⁺)，氧上带部分负电荷 (δ⁻)。这使羰基碳成为一个亲电中心，容易受到亲核试剂的进攻。

2. Nomenclature and Structure | 命名与结构

For aldehydes, the suffix is -al. The longest carbon chain containing the -CHO group is numbered such that the aldehyde carbon is always carbon-1. For example, CH₃CH₂CHO is propanal. When the -CHO group is attached to a ring, the suffix -carbaldehyde is used, e.g. cyclohexanecarbaldehyde.

醛的命名后缀为 -al。选择包含 -CHO 的最长碳链作为主链，醛基碳始终编号为 1。例如 CH₃CH₂CHO 为丙醛。当 -CHO 连接在环上时，使用后缀 -carbaldehyde，如环己基甲醛。

For ketones, the suffix is -one, and the chain is numbered to give the carbonyl carbon the lowest possible number. For instance, CH₃COCH₂CH₃ is butan-2-one. The simplest ketone, propanone (acetone), does not require a number because there is only one possible position for the carbonyl group.

酮的命名后缀为 -one，主链编号应使羰基碳的数字尽可能小。例如 CH₃COCH₂CH₃ 为丁-2-酮。最简单酮类丙酮 (propanone) 无需编号，因为羰基只有一个可能的位置。

The carbonyl carbon in both aldehydes and ketones is sp² hybridised, resulting in a trigonal planar geometry with bond angles close to 120°. The C=O double bond consists of a σ bond and a π bond; the latter is less effective in overlap due to the size difference of 2p orbitals, making it relatively weak and reactive.

醛和酮中的羰基碳均为 sp² 杂化，呈平面三角形，键角接近 120°。C=O 双键由一个 σ 键和一个 π 键组成；由于 2p 轨道大小差异，π 键重叠效率较低，相对较弱、易于发生反应。

3. Physical Properties | 物理性质

Both aldehydes and ketones are polar molecules, so they have higher boiling points than non-polar compounds of comparable molecular mass. However, they cannot form hydrogen bonds with themselves (no O–H bond), so their boiling points are lower than the corresponding alcohols. For example, propanal (49 °C) boils lower than propan-1-ol (97 °C).

醛和酮都是极性分子，因此它们的沸点高于分子量相近的非极性化合物。但它们自身无法形成分子间氢键（没有 O–H 键），所以沸点低于相应的醇。例如丙醛的沸点 (49 °C) 低于正丙醇 (97 °C)。

Short-chain aldehydes and ketones, such as methanal and propanone, are miscible with water because the carbonyl oxygen can form hydrogen bonds with water molecules. As the hydrocarbon chain lengthens, solubility in water decreases rapidly due to the increasing hydrophobic character.

短链醛酮（如甲醛和丙酮）可与水混溶，因为羰基氧能与水分子形成氢键。随着碳链增长，疏水性增强，在水中的溶解度迅速下降。

4. Preparation Methods | 制备方法

Aldehydes are typically prepared by the oxidation of primary alcohols using acidified potassium dichromate(VI), with careful distillation to remove the aldehyde as it forms, preventing further oxidation to the carboxylic acid. Pyridinium chlorochromate (PCC) in anhydrous conditions can also selectively oxidise primary alcohols to aldehydes without overoxidation.

醛通常通过伯醇的氧化制备，使用酸化重铬酸钾(VI)，并采用蒸馏法及时移出生成的醛，以防止其进一步氧化成羧酸。在无水条件下使用氯铬酸吡啶盐 (PCC) 也可将伯醇选择性氧化为醛，避免过氧化。

Ketones are produced by the oxidation of secondary alcohols under the same conditions; since a ketone has no hydrogen atom on the carbonyl carbon, it resists further oxidation under these conditions. The reaction can be carried out under reflux and does not require distillation to trap the product.

酮可由仲醇在相同氧化条件下制得；由于酮的羰基碳上没有氢原子，在这些条件下可抵抗进一步氧化。该反应可在回流下进行，无需通过蒸馏截留产物。

Aldehydes and ketones can also be prepared by other routes, such as the hydration of alkynes (using HgSO₄/H₂SO₄) or the Friedel-Crafts acylation of aromatic rings, but direct oxidation of alcohols remains the most common method in the examination context.

醛和酮也可通过其他途径制备，如炔烃的水合（HgSO₄/H₂SO₄ 催化）或芳环的傅-克酰基化反应，但在考试中醇的直接氧化仍是最常见的方法。

5. Nucleophilic Addition Mechanism | 亲核加成反应机理

The most characteristic reaction of aldehydes and ketones is nucleophilic addition. A nucleophile (Nu⁻) attacks the electrophilic carbonyl carbon. The π bond breaks, and a pair of electrons moves entirely onto the oxygen, forming an alkoxide ion intermediate. This tetrahedral intermediate then picks up a proton (H⁺), usually from water or acid, to give the final addition product.

醛和酮最具特征的反应是亲核加成。亲核试剂 (Nu⁻) 进攻亲电的羰基碳，π 键断裂，一对电子完全转移至氧上，形成一个四面体烷氧负离子中间体。该中间体随后获取一个质子 (H⁺)，通常来自水或酸，生成最终的加成产物。

C=O + Nu⁻ → C(O⁻)–Nu → C(OH)–Nu (after H⁺)

C=O + Nu⁻ → C(O⁻)–Nu → C(OH)–Nu (加 H⁺ 后)

This mechanism explains many reactions, including the addition of HCN, NaHSO₃, and hydride ions (from NaBH₄ or LiAlH₄). The rate of nucleophilic addition is influenced by the size of the alkyl groups attached: aldehydes are generally more reactive than ketones due to less steric hindrance and weaker electronic stabilisation of the partial positive charge.

这一机理可解释许多反应，包括与 HCN、NaHSO₃ 及负氢离子（来自 NaBH₄ 或 LiAlH₄）的加成。亲核加成的速率受连接烷基大小的影响：醛通常比酮更具反应活性，因为醛基位阻较小，且对δ⁺的电子稳定作用较弱。

6. Addition of Hydrogen Cyanide | 与氰化氢加成

Aldehydes and ketones react with hydrogen cyanide, HCN, to form hydroxynitriles (cyanohydrins). The reaction proceeds by nucleophilic addition of the cyanide ion, CN⁻. Since HCN is a weak acid and a toxic gas, the reaction is normally carried out by generating CN⁻ in situ using sodium cyanide or potassium cyanide and dilute sulfuric acid under alkaline conditions.

醛和酮与氰化氢 HCN 反应生成羟基腈（氰醇）。反应通过氰根离子 CN⁻ 的亲核加成进行。由于 HCN 是弱酸且为有毒气体，通常在使用时通过氰化钠或氰化钾与稀硫酸在碱性条件下原位生成 CN⁻。

R₂C=O + HCN → R₂C(OH)CN

This reaction is synthetically important because it increases the carbon chain length by one carbon atom. The nitrile group (–CN) can later be hydrolysed to a carboxylic acid or reduced to an amine, making hydroxynitriles versatile intermediates. The reaction is a key example of nucleophilic addition in both IB and OCR specifications.

该反应在合成上至关重要，因为它使碳链增加一个碳原子。腈基 (–CN) 随后可水解为羧酸或还原为胺，使羟基腈成为用途广泛的中间体。此反应是 IB 和 OCR 考纲中亲核加成的核心示例。

7. Reaction with 2,4-Dinitrophenylhydrazine (2,4-DNP) | 与2,4-二硝基苯肼反应

Both aldehydes and ketones react with 2,4-dinitrophenylhydrazine (Brady’s reagent) to form orange or yellow precipitates of 2,4-dinitrophenylhydrazones. This is a condensation-elimination reaction that proceeds via nucleophilic addition of the –NH₂ group of 2,4-DNP to the carbonyl, followed by elimination of water.

醛和酮均可与2,4-二硝基苯肼（Brady试剂的成分）反应，生成橙色或黄色2,4-二硝基苯腙沉淀。该反应为缩合-消除反应，通过2,4-DNP的 –NH₂ 基团对羰基的亲核加成，随后消除一分子水完成。

The solid derivative has a sharp, characteristic melting point, which can be used to identify the original carbonyl compound by comparing with known data. The test is often used as a general test for the presence of a carbonyl group but does not distinguish between aldehydes and ketones.

生成的固体衍生物具有敏锐的特征熔点，可通过与已知数据比对来鉴定原始羰基化合物。该实验通常用作检测羰基存在的通用方法，但不能区分醛和酮。

8. Oxidation Reactions: Distinguishing Aldehydes | 氧化反应：鉴别醛类

Only aldehydes can be oxidised to carboxylic acids by mild oxidising agents, owing to the hydrogen atom attached to the carbonyl carbon. Ketones lack this hydrogen and resist mild oxidation. This difference provides the basis for several classical tests.

只有醛能被温和氧化剂氧化为羧酸，这是因为醛基碳上连有氢原子。酮缺少这个氢，可抵抗温和氧化。这一区别构成了几个经典鉴别试验的基础。

Tollens’ reagent (ammoniacal silver nitrate) oxidises aldehydes to carboxylate ions while reducing Ag⁺ to metallic silver, which deposits as a silver mirror on the clean glass surface. Equation: RCHO + 2[Ag(NH₃)₂]⁺ + 2OH⁻ → RCOO⁻ + 2Ag + 4NH₃ + H₂O. No reaction occurs with ketones.

Tollens 试剂（氨性硝酸银溶液）将醛氧化为羧酸根离子，同时 Ag⁺ 被还原为金属银，在洁净玻璃表面形成银镜。方程式：RCHO + 2[Ag(NH₃)₂]⁺ + 2OH⁻ → RCOO⁻ + 2Ag + 4NH₃ + H₂O。酮不发生反应。

Fehling’s solution (alkaline Cu²⁺ complexed with tartrate) and Benedict’s solution (alkaline Cu²⁺ complexed with citrate) oxidise aliphatic aldehydes to carboxylic acids, while the Cu²⁺ is reduced to a brick-red precipitate of Cu₂O. Aromatic aldehydes such as benzaldehyde do not give a positive test. Ketones give no reaction.

Fehling 溶液（酒石酸根络合的碱性 Cu²⁺）和 Benedict 溶液（柠檬酸根络合的碱性 Cu²⁺）可将脂肪醛氧化为羧酸，同时 Cu²⁺ 被还原为砖红色 Cu₂O 沉淀。芳香醛如苯甲醛不产生阳性结果。酮无反应。

Acidified potassium dichromate(VI) can also oxidise aldehydes; the orange solution turns green (Cr³⁺). This test is less specific but still useful. In summary, Tollens, Fehling’s, and dichromate tests are positive for aldehydes and negative for ketones.

酸化重铬酸钾(VI) 也可氧化醛，橙色溶液变绿（生成 Cr³⁺）。该试验特异性较低，但仍然有用。总之，Tollens、Fehling’s 和重铬酸盐试验对醛呈阳性，对酮呈阴性。

9. Reduction of Aldehydes and Ketones | 醛酮的还原

Both aldehydes and ketones can be reduced to alcohols by the addition of hydrogen. The most common reducing agents are sodium tetrahydridoborate(III) (NaBH₄) in aqueous or alcoholic solution, and lithium tetrahydridoaluminate(III) (LiAlH₄) in dry ether. NaBH₄ is milder and selective for carbonyl groups, while LiAlH₄ is more powerful and reduces a wider range of groups.

醛和酮均可通过加氢被还原为醇。最常用的还原剂是硼氢化钠 (NaBH₄，于水或醇溶液中) 和氢化铝锂 (LiAlH₄，于无水乙醚中)。NaBH₄ 较温和，对羰基具有选择性；LiAlH₄ 活性更强，能还原更多类型的基团。

In mechanistic terms, the hydride ion (H⁻) acts as a nucleophile, attacking the carbonyl carbon. This is another example of nucleophilic addition, followed by protonation of the alkoxide intermediate. Aldehydes yield primary alcohols, while ketones yield secondary alcohols.

机理上，负氢离子 (H⁻) 作为亲核试剂进攻羰基碳，这是亲核加成的又一范例，随后烷氧负离子中间体质子化。醛还原得伯醇，酮还原得仲醇。

Aldehyde → 1° alcohol; Ketone → 2° alcohol

醛 → 伯醇；酮 → 仲醇

10. The Iodoform (Triiodomethane) Test | 碘仿反应

The iodoform reaction is a specific test for the presence of a methyl ketone (CH₃CO–) group or a secondary alcohol with a methyl group adjacent to the –OH (ethanol is the only primary alcohol that gives a positive test). When a compound containing acetyl group (CH₃CO–) is treated with iodine and sodium hydroxide, a pale yellow precipitate of triiodomethane (CHI₃, iodoform) is formed, along with the carboxylate salt.

碘仿反应是检测甲基酮 (CH₃CO–) 基团或邻位带有甲基的仲醇（乙醇是唯一能产生阳性结果的伯醇）的特征实验。含有乙酰基 (CH₃CO–) 的化合物与碘和氢氧化钠反应，会生成淡黄色的三碘甲烷 (CHI₃，碘仿) 沉淀及羧酸盐。

CH₃COR + 3I₂ + 4NaOH → CHI₃↓ + RCOONa + 3NaI + 3H₂O

This test is very useful for identifying propanone and other methyl ketones. Note that ethanal and ethanol also give a positive iodoform test. Ketones without the CH₂CO– group, such as butan-2-one (CH₃COCH₂CH₃, which does contain CH₃CO– and actually gives a positive test), need careful interpretation. The key structural requirement is the presence of CH₃CO– or CH₃CH(OH)– moiety.

该试验在鉴定丙酮及其他甲基酮时非常有用。注意乙醛和乙醇也会给出阳性结果。没有 CH₃CO– 结构的酮需要仔细解读——丁-2-酮 (CH₃COCH₂CH₃) 实际含有乙酰基，反应结果为阳性。结构上的关键要求是存在 CH₃CO– 或 CH₃CH(OH)– 结构片段。

11. Acidity of α-Hydrogens and Aldol Condensation | α-氢的酸性与羟醛缩合

The hydrogens on the carbon adjacent to the carbonyl group (α-hydrogens) are unusually acidic (pKa ≈ 20 for ketones, slightly lower for aldehydes). This acidity arises because the enolate ion formed after deprotonation is stabilised by resonance delocalisation of the negative charge between the α-carbon and the carbonyl oxygen.

羰基邻位碳（α-碳）上的氢具有超常的酸性（酮的 pKa ≈ 20，醛的稍低）。这种酸性是由于去质子化后形成的烯醇负离子可通过共振作用，将负电荷离域至 α-碳和羰基氧之间而得到稳定。

In base, aldehydes and ketones that possess α-hydrogens can undergo aldol (or ketol) condensation. Two molecules combine: the enolate ion formed from one molecule attacks the carbonyl carbon of another molecule. The initial product is a β-hydroxy aldehyde or β-hydroxy ketone, which can often undergo further dehydration to give an α,β-unsaturated carbonyl compound under heating.

在碱性条件下，具有 α-氢的醛或酮可发生羟醛缩合反应。两个分子结合：一个分子形成烯醇负离子，进攻另一个分子的羰基碳。初始产物为 β-羟基醛或 β-羟基酮，加热下常可进一步脱水生成 α,β-不饱和羰基化合物。

2CH₃CHO → CH₃CH(OH)CH₂CHO → CH₃CH=CHCHO + H₂O

This reaction is a crucial method of carbon–carbon bond formation in organic synthesis and is frequently examined in both IB and OCR papers. The mechanism must clearly show the enolate intermediate and proton transfer steps.

该反应是有机合成中形成碳-碳键的重要方法，在 IB 和 OCR 试卷中频繁出现。反应机理必须清晰展示烯醇负离子中间体和质子转移步骤。

12. Summary of Key Reactions and Tests | 关键反应与鉴别总结

To summarise, aldehydes and ketones share many reactions due to the common carbonyl group, but the presence of the aldehydic hydrogen allows specific oxidation reactions. Below is a comparison table of key tests and reactions:

总结而言，醛和酮由于共有羰基而有许多相似反应，但醛基氢的存在使其能发生特征的氧化反应。以下为关键测试与反应的对比表：

Reaction / Test	Aldehyde	Ketone
2,4-DNP	Orange/yellow ppt	Orange/yellow ppt
Tollens’ reagent	Silver mirror	No reaction
Fehling’s / Benedict’s	Brick-red Cu₂O (aliphatic)	No reaction
Acidified K₂Cr₂O₇	Orange → green	No reaction (unless strong conditions)
Iodoform test (NaOH + I₂)	Positive only if CH₃CHO (ethanal)	Positive if CH₃CO– group present
Reduction (NaBH₄)	1° alcohol	2° alcohol
Nucleophilic addition (HCN)	Hydroxynitrile	Hydroxynitrile
Aldol condensation	Yes, if α-H present	Yes, if α-H present

Mastering these reactions and understanding their mechanistic details will allow you to confidently tackle synthesis, mechanism, and reaction-prediction questions. In exams, always pay attention to the reagent conditions: acidic or basic, cold or heated, as they often determine the product.

掌握这些反应及其机理细节，你将能自信地应对合成、机理推演和反应预测题。在考试中，务必留意试剂条件：酸性或碱性、冷或热，因为它们常常决定产物的去向。

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Aldehydes and Ketones: Key Concepts for IB and OCR Chemistry | IB OCR 化学：醛和酮 考点精讲