📚 The Homologous Series of Aldehydes and Ketones | 醛酮同系列
Welcome to this A-Level Chemistry revision guide on the homologous series of aldehydes and ketones. These carbonyl compounds are central to organic chemistry, and Cambridge questions regularly test their structure, naming, physical trends, preparation and key reactions.
欢迎阅读本篇 A-Level 化学复习指南，主题为醛和酮的同系列。这类羰基化合物是有机化学的核心内容，剑桥考试常考查它们的结构、命名、物理性质递变、制备方法和重要反应。
1. The Carbonyl Functional Group | 羰基官能团
Both aldehydes and ketones contain the carbonyl group, C=O, in which an oxygen atom is double-bonded to a carbon atom. In an aldehyde, the carbonyl carbon is bonded to at least one hydrogen atom and is therefore located at the end of a carbon chain. In a ketone, the carbonyl carbon is bonded to two other carbon atoms, so the C=O group lies within the chain.
醛和酮都含有羰基 C=O，即氧原子与碳原子以双键连接。在醛中，羰基碳至少与一个氢原子相连，因此位于碳链末端；在酮中，羰基碳与两个其他碳原子相连，因此 C=O 官能团位于碳链内部。
For saturated aliphatic aldehydes and ketones, both series share the general molecular formula CₙH₂ₙO. Methanal, HCHO, is the simplest aldehyde, while propanone, CH₃COCH₃, is the simplest ketone.
对于饱和脂肪族醛和酮，两个系列具有相同的通式 CₙH₂ₙO。最简单的醛是甲醛 HCHO，最简单的酮是丙酮 CH₃COCH₃。
2. Nomenclature and Homologous Series | 命名与同系列
IUPAC names for aldehydes end in ‘-al’. The aldehyde carbon is always carbon-1, so no locant is needed when naming a straight-chain aldehyde. For example, HCHO is methanal, CH₃CHO is ethanal, and CH₃CH₂CHO is propanal.
醛的 IUPAC 名称以 “-al” 结尾。醛基碳始终是 1 号碳，因此直链醛命名时无需标出位次。例如 HCHO 为 methanal（甲醛），CH₃CHO 为 ethanal（乙醛），CH₃CH₂CHO 为 propanal（丙醛）。
Ketones are named with the suffix ‘-one’, and a locant is normally needed to show the position of the carbonyl group in chains with four or more carbon atoms. For example, CH₃COCH₃ is propanone, CH₃COCH₂CH₃ is butanone, and CH₃CH₂COCH₂CH₃ is pentan-3-one.
酮的命名以 “-one” 结尾，当碳链含有四个或更多碳原子时，通常需要标出羰基的位置。例如 CH₃COCH₃ 为 propanone（丙酮），CH₃COCH₂CH₃ 为 butanone（丁酮），CH₃CH₂COCH₂CH₃ 为 pentan-3-one（3-戊酮）。
As a homologous series, successive members differ by a CH₂ unit, show a gradual change in physical properties, and share the same functional group and similar chemical behaviour.
作为同系列，相邻成员相差一个 CH₂ 单元，物理性质呈现递变规律，并且具有相同的官能团和相似的化学性质。
3. Physical Properties | 物理性质
The polar carbonyl group gives aldehydes and ketones permanent dipole-dipole attractions. Their boiling points are therefore higher than those of alkanes of similar relative molecular mass, but lower than the corresponding alcohols, because they cannot form hydrogen bonds with their own molecules.
极性羰基使醛和酮分子间存在永久偶极-偶极引力。因此它们的沸点高于相对分子质量相近的烷烃，但低于相应的醇，因为它们自身分子之间不能形成氢键。
Shorter-chain aldehydes and ketones are soluble in water because the carbonyl oxygen can accept hydrogen bonds from water molecules. As the non-polar hydrocarbon chain lengthens, solubility in water decreases rapidly.
短链醛和酮可溶于水，因为羰基氧可以接受水分子的氢键。随着非极性烃链增长，它们在水中的溶解度迅速下降。
4. Preparation from Alcohols | 由醇制备
Aldehydes are formed by the partial oxidation of primary alcohols. In the laboratory, the aldehyde is removed by distillation as soon as it forms to prevent further oxidation to the carboxylic acid. A typical oxidising agent is acidified potassium dichromate(VI), K₂Cr₂O₇/H₂SO₄.
醛可由伯醇部分氧化制得。在实验室中，醛生成后应立即蒸馏移出，以防止进一步氧化生成羧酸。常用氧化剂为酸化重铬酸钾(VI)，K₂Cr₂O₇/H₂SO₄。
Ketones are formed by the oxidation of secondary alcohols. Since a ketone cannot be oxidised further without breaking carbon-carbon bonds, it is not necessary to distil it away from the reaction mixture. A typical equation is CH₃CH(OH)CH₃ + [O] → CH₃COCH₃ + H₂O.
酮可由仲醇氧化制得。由于酮在断裂碳-碳键之前不能被继续氧化，因此无需将其从反应混合物中蒸出。典型反应式为 CH₃CH(OH)CH₃ + [O] → CH₃COCH₃ + H₂O。
5. Nucleophilic Addition Mechanism | 亲核加成机理
The carbonyl carbon is electron-deficient because oxygen is more electronegative than carbon, making the C=O bond polar. Nucleophiles attack the δ⁺ carbon atom, and the π electrons of the C=O bond move onto the oxygen to form an alkoxide intermediate. Protonation of this intermediate gives the final addition product.
由于氧的电负性大于碳，羰基碳缺电子，使 C=O 键具有极性。亲核试剂进攻 δ⁺ 碳原子，C=O 键的 π 电子转移到氧上，形成烷氧负离子中间体。该中间体质子化后得到最终加成产物。
This addition mechanism is characteristic of both aldehydes and ketones and explains many of their reactions with reagents such as NaBH₄, HCN and 2,4-DNPH.
这种加成机理是醛和酮的典型反应特征，可以解释它们与 NaBH₄、HCN 和 2,4-DNPH 等试剂发生的许多反应。
6. Reduction of Aldehydes and Ketones | 醛酮的还原
Reduction of an aldehyde gives a primary alcohol, while reduction of a ketone gives a secondary alcohol. Suitable reducing agents include sodium borohydride, NaBH₄, in aqueous ethanol, or lithium aluminium hydride, LiAlH₄, in dry ether.
醛还原生成伯醇，酮还原生成仲醇。常用还原剂包括硼氢化钠 NaBH₄（溶于乙醇水溶液）或氢化铝锂 LiAlH₄（溶于无水乙醚）。
For example, ethanal is reduced to ethanol: CH₃CHO + 2[H] → CH₃CH₂OH. Propanone is reduced to propan-2-ol: CH₃COCH₃ + 2[H] → CH₃CH(OH)CH₃.
例如，乙醛被还原为乙醇：CH₃CHO + 2[H] → CH₃CH₂OH。丙酮被还原为 2-丙醇：CH₃COCH₃ + 2[H] → CH₃CH(OH)CH₃。
7. Addition of Hydrogen Cyanide | 氰化氢加成
Hydrogen cyanide adds across the carbonyl group to form a hydroxynitrile, also called a cyanohydrin. In the laboratory, HCN is generated in situ by adding dilute sulfuric acid to potassium cyanide, KCN, because hydrogen cyanide is highly toxic.
氰化氢与羰基发生加成反应，生成羟基腈（又称氰醇）。由于氰化氢剧毒，实验室中通常用稀硫酸与氰化钾 KCN 现场生成 HCN。
The reaction increases the carbon chain length by one carbon atom, which makes it useful in organic synthesis. For example, ethanal reacts with HCN to form 2-hydroxypropanenitrile: CH₃CHO + HCN → CH₃CH(OH)CN.
该反应使碳链增加一个碳原子，因此在有机合成中非常有用。例如，乙醛与 HCN 反应生成 2-羟基丙腈：CH₃CHO + HCN → CH₃CH(OH)CN。
The mechanism follows nucleophilic addition: the cyanide ion, CN⁻, attacks the δ⁺ carbonyl carbon, and the negatively charged oxygen is then protonated by HCN or H⁺.
该机理遵循亲核加成：氰根离子 CN⁻ 进攻 δ⁺ 羰基碳，带负电的氧随后被 HCN 或 H⁺ 质子化。
8. Distinguishing Aldehydes from Ketones |
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