📚 Aldehydes and Ketones | 醛与酮
Aldehydes and ketones are two of the most important families of carbonyl compounds in A-Level chemistry. They share the functional group C=O, yet their structural differences lead to distinct chemical behaviours, particularly in oxidation and nucleophilic addition reactions.
醛和酮是 A-Level 化学中最重要的两类羰基化合物。它们共有一个 C=O 官能团，但结构上的差异导致了它们在化学行为上的显著区别，特别是在氧化反应和亲核加成反应中。
1. Structure and Polarity of the Carbonyl Group | 羰基的结构与极性
The carbonyl group consists of a carbon atom double-bonded to an oxygen atom. In an aldehyde, this group is bonded to at least one hydrogen atom (RCHO); in a ketone, it is bonded to two carbon-containing groups (RCOR'). The electronegativity difference between carbon (2.5) and oxygen (3.5) makes the C=O bond strongly polar, with a permanent dipole: the carbon carries a partial positive charge (δ+) and the oxygen carries a partial negative charge (δ−).
羰基由一个碳原子与一个氧原子以双键相连而成。在醛中，羰基碳至少连接一个氢原子（RCHO）；在酮中，羰基碳连接两个含碳基团（RCOR'）。碳（电负性 2.5）和氧（电负性 3.5）的电负性差异使得 C=O 键具有强极性，产生永久偶极：碳带有部分正电荷（δ+），氧带有部分负电荷（δ−）。
2. Nomenclature | 命名规则
Under AQA nomenclature, aldehydes are named by replacing the final '-e' of the parent alkane with '-al'. The carbonyl carbon is numbered as position 1. For example, CH₃CH₂CHO is propanal, and CH₃CH(CH₃)CHO is 2-methylpropanal. Ketones use the suffix '-one', with the carbonyl carbon given the lowest possible number. For example, CH₃COCH₃ is propanone, and CH₃COCH₂CH₃ is butanone.
根据 AQA 命名体系，醛的命名是将母体烷烃词尾的“-e”改为“-al”，羰基碳编号为 1。例如，CH₃CH₂CHO 称为丙醛，CH₃CH(CH₃)CHO 称为 2-甲基丙醛。酮使用后缀“-one”，羰基碳取最小编号。例如，CH₃COCH₃ 称为丙酮，CH₃COCH₂CH₃ 称为丁酮。
3. Physical Properties | 物理性质
Because the carbonyl group cannot form intermolecular hydrogen bonds (no O–H bond), aldehydes and ketones have lower boiling points than alcohols of similar molar mass. However, the permanent dipole leads to stronger permanent dipole–dipole interactions than alkanes of similar size. The smaller aldehydes and ketones, such as methanal and propanone, are soluble in water because the C=O group can act as a hydrogen-bond acceptor with water molecules.
由于羰基无法形成分子间氢键（没有 O–H 键），醛和酮的沸点低于相对分子质量相近的醇。然而，永久偶极导致的偶极–偶极相互作用比同类烷烃更强。较小的醛和酮，如甲醛和丙酮，可溶于水，因为 C=O 基团可以作为氢键受体与水分子形成氢键。
4. Preparation of Aldehydes and Ketones | 醛和酮的制备
Primary alcohols can be oxidised to aldehydes using acidified potassium dichromate(VI) (K₂Cr₂O₇/H₂SO₄) under controlled conditions — typically by gentle heating and immediate distillation to remove the aldehyde before further oxidation to the carboxylic acid occurs. Secondary alcohols oxidise to ketones under the same conditions; the ketone is resistant to further oxidation. The orange dichromate solution turns green (Cr³⁺) during both oxidations, providing a useful visual indicator.
伯醇可在酸性重铬酸钾（K₂Cr₂O₇/H₂SO₄）作用下氧化为醛，条件需严格控制——通常采用温和加热并立即蒸馏，将醛及时移出体系，避免其进一步被氧化为羧酸。仲醇在相同条件下氧化为酮；酮对进一步氧化不敏感。两种氧化过程中，橙色的重铬酸盐溶液变为绿色（Cr³⁺），为反应提供了直观的指示。
5. Nucleophilic Addition: The Key Mechanism | 亲核加成：核心机理
The δ+ carbonyl carbon makes aldehydes and ketones susceptible to attack by nucleophiles. In AQA, the general mechanism is known as nucleophilic addition. The nucleophile attacks the electrophilic carbon, the C=O π bond breaks, and the oxygen acquires a negative charge. This intermediate is then protonated by an acid (or water) to form the final product. Common nucleophiles include CN⁻, H⁻ (from NaBH₄), and NH₃ derivatives such as NH₂OH.
羰基碳上的 δ+ 使得醛和酮易受亲核试剂进攻。在 AQA 课程中，这一过程称为亲核加成机理。亲核试剂进攻缺电子的碳，C=O 的 π 键断裂，氧获得负电荷，形成中间体，随后中间体被酸（或水）质子化生成最终产物。常见的亲核试剂包括 CN⁻、H⁻（来自 NaBH₄）以及 NH₂OH 等氨的衍生物。
δ+ C=O δ− + Nu⁻ → ⁻O–C–Nu → 质子化 → HO–C–Nu
6. Oxidation Reactions of Aldehydes | 醛的氧化反应
Aldehydes are easily oxidised to carboxylic acids (RCOOH) by mild oxidising agents, because the aldehydic hydrogen attached to the carbonyl carbon can be removed. Ketones lack this hydrogen and cannot be oxidised under mild conditions. Two common distinguishing reagents are Tollens' reagent (ammoniacal silver nitrate) and Fehling's solution. Tollens' reagent contains [Ag(NH₃)₂]⁺; on warming with an aldehyde, a silver mirror forms as Ag⁺ is reduced to Ag. Fehling's solution contains Cu²⁺ complexed in alkaline tartrate; an aldehyde reduces Cu²⁺ to brick-red Cu₂O precipitate. Ketones give no reaction with either reagent.
醛极易被温和氧化剂氧化为羧酸（RCOOH），因为羰基碳上连接的醛氢可被移除。酮缺少该氢原子，因此在温和条件下不能被氧化。两种常用的区分试剂是 Tollens 试剂（氨性硝酸银）和 Fehling 溶液。Tollens 试剂含有 [Ag(NH₃)₂]⁺；与醛共热时，Ag⁺ 被还原为 Ag，形成银镜。Fehling 溶液含有碱性酒石酸络合的 Cu²⁺；醛将 Cu²⁺ 还原为砖红色的 Cu₂O 沉淀。酮与这两种试剂均不发生反应。
7. Reduction to Alcohols | 还原为醇
Aldehydes and ketones are reduced by sodium borohydride (NaBH₄) in aqueous or alcoholic solution. Aldehydes yield primary alcohols, while ketones yield secondary alcohols. The hydride ion (H⁻) acts as the nucleophile in a nucleophilic addition process, followed by protonation. For example, propanal is reduced to propan-1-ol, whereas propanone is reduced to propan-2-ol. NaBH₄ is preferred over LiAlH₄ at A-Level because it is safer and can be used in water.
醛和酮可被硼氢化钠（NaBH₄）在水溶液或醇溶液中还原。醛生成伯醇，酮生成仲醇。氢负离子（H⁻）作为亲核试剂参与亲核加成过程，随后发生质子化。例如，丙醛被还原为丙-1-醇，而丙酮被还原为丙-2-醇。在 A-Level 中，NaBH₄ 比 LiAlH₄ 更受青睐，因为它更安全且可在水中使用。
8. Reaction with HCN: Formation of Hydroxynitriles | 与HCN的反应：羟腈的生成
Hydrogen cyanide (HCN) adds across the carbonyl double bond to form hydroxynitriles (also called cyanohydrins). This reaction extends the carbon chain by one carbon atom, making it a valuable synthetic route. Because HCN is a weak acid and the CN⁻ concentration is low, the reaction is catalysed by a small amount of KCN, which provides a ready supply of CN⁻. The mechanism is nucleophilic addition: CN⁻ attacks the δ+ carbon, followed by protonation of the alkoxide intermediate. For example, propanone reacts with HCN to form 2-hydroxy-2-methylpropanenitrile.
氰化氢（HCN）可与羰基双键发生加成，生成羟腈（也称氰醇）。该反应使碳链延长一个碳原子，因此是一条重要的合成路线。由于 HCN 是弱酸，CN⁻ 浓度很低，反应需加入少量 KCN 催化，以提供稳定的 CN⁻ 来源。其机理为亲核加成：CN⁻ 进攻 δ+ 碳，随后烷氧基中间体被质子化。例如，丙酮与 HCN 反应生成 2-羟基-2-甲基丙腈。
9. Reaction with 2,4-DNPH: Testing for Carbonyls | 与2,4-DNPH的反应：羰基的检验
2,4-Dinitrophenylhydrazine (2,4-DNPH, also written as Brady's reagent) is the standard chemical test for the presence of a carbonyl group. When an aldehyde or ketone is warmed with a solution of 2,4-DNPH in acid, an orange or yellow precipitate of the corresponding 2,4-dinitrophenylhydrazone forms. This is a condensation reaction, in which water is eliminated. The melting point of the purified precipitate can be measured and compared with a data table to identify the specific aldehyde or ketone.
2,4-二硝基苯肼（2,4-DNPH，也称为 Brady 试剂）是检验羰基存在的标准试剂。当醛或酮与酸化的 2,4-DNPH 溶液共热时，会生成橙黄色沉淀，即相应的 2,4-二硝基苯腙。这是一个缩合反应，反应中脱去一分子水。纯化后沉淀的熔点可被测定，并与数据表对比，从而鉴定具体的醛或酮。
10. Reaction with Ammonia Derivatives | 与氨衍生物的反应
Beyond 2,4-DNPH, aldehydes and ketones react with other ammonia derivatives via condensation. Hydroxylamine (NH₂OH) gives oximes, and hydrazine (NH₂NH₂) gives hydrazones. These reactions follow the same pattern: the nitrogen lone pair attacks the carbonyl carbon, followed by loss of water to form a C=N bond. These products are often crystalline solids with sharp melting points, useful for identification purposes.
除 2,4-DNPH 外，醛和酮还可与其他氨衍生物发生缩合反应。羟胺（NH₂OH）生成肟，肼（NH₂NH₂）生成腙。这些反应遵循相同模式：氮上的孤对电子进攻羰基碳，随后脱去一分子水形成 C=N 键。所得产物通常是具有尖锐熔点的晶体固体，可用于物质鉴定。
11. Iodoform Reaction | 碘仿反应
The iodoform test is specific to methyl ketones (RCOCH₃) and to compounds that can be oxidised to methyl ketones, such as ethanol and ethanal. When warmed with iodine in alkaline solution (I₂/NaOH), a methyl ketone is oxidised and cleaved to produce a pale yellow precipitate of iodoform (CHI₃), which has a distinctive antiseptic smell. This reaction is also positive for ethanol because it is oxidised to ethanal, which has the required CH₃CO– unit.
碘仿试验专用于甲基酮（RCOCH₃）以及可被氧化为甲基酮的化合物，如乙醇和乙醛。将样品与碘的碱性溶液（I₂/NaOH）共热时，甲基酮被氧化并断裂，生成淡黄色的碘仿沉淀（CHI₃），具有特征性的消毒水气味。乙醇也呈阳性，因为它先被氧化为乙醛，而乙醛含有所需的 CH₃CO– 结构单元。
12. Key Exam Points and Common Pitfalls | 考试要点与常见误区
When writing the nucleophilic addition mechanism, always show the curly arrow from the nucleophile to the δ+ carbon, and clearly display the intermediate alkoxide ion before protonation. For oxidation of an aldehyde, state that the product is a carboxylic acid and that Tollens' or Fehling's is used as a mild oxidising agent. Remember that ketones do not react with Tollens' or Fehling's, and that NaBH₄ reduces both but cannot reduce a C=C bond. In the iodoform test, count the hydrogens: only CH₃CO– groups give a positive result.
书写亲核加成机理时，务必从亲核试剂画出指向 δ+ 碳的弯箭头，并清楚展示质子化之前的烷氧基负离子中间体。对于醛的氧化，需写明产物为羧酸，并指出 Tollens 或 Fehling 作为温和氧化剂。牢记酮不与 Tollens 或 Fehling 反应；NaBH₄ 可还原 C=O 但不能还原 C=C 键。在碘仿试验中，注意计数氢原子：只有 CH₃CO– 基团才给出阳性结果。
One common mistake is to write the formula of Fehling's precipitate as CuO instead of Cu₂O. Another is to forget that HCN must be buffered with KCN to generate sufficient CN⁻. Finally, when naming products of nucleophilic additions, check the longest carbon chain after the new bond has been formed — the –CN group introduces an extra carbon that must be counted.
一个常见错误是将 Fehling 反应的沉淀写成 CuO 而非 Cu₂O。另一个错误是忘记 HCN 需要与 KCN 构成缓冲体系才能产生足够的 CN⁻。最后，在命名亲核加成产物时，务必检查新键形成后的最长碳链——–CN 基团引入了一个额外碳原子，必须计入主链。
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