📚 A-Level Chemistry | Homologous Series of Aldehydes and Ketones | A-Level化学：醛与酮的同系物规律总结
In A-Level Chemistry, aldehydes and ketones are two closely related families within the same functional group class: the carbonyl compounds. Understanding their homologous series, naming conventions, physical properties, and chemical trends is essential for exam success in CIE papers.
在 A-Level 化学中，醛和酮是同一官能团类别——羰基化合物——中两个关系密切的家族。理解它们的同系物规律、命名规则、物理性质和化学趋势，是应对 CIE 考试的关键。
1. Definition and Functional Group | 定义与官能团
Aldehydes contain the carbonyl group (C=O) bonded to at least one hydrogen atom, giving the functional group -CHO. The carbonyl carbon is therefore always at the end of the carbon chain.
醛含有羰基（C=O），且羰基碳至少与一个氢原子相连，因此官能团为 -CHO。醛的羰基碳总是位于碳链末端。
Ketones contain the carbonyl group bonded to two carbon atoms, giving the functional group -CO-. The carbonyl carbon is therefore located inside the carbon chain, never at a terminal position.
酮的羰基碳与两个碳原子相连，官能团为 -CO-。因此酮的羰基碳位于碳链内部，绝不会出现在链端。
R-CHO (aldehyde) | R-CO-R’ (ketone)
The simplest aldehyde is methanal (HCHO), and the simplest ketone is propanone (CH₃COCH₃). Note that methanal is the only aldehyde with no alkyl group attached to the carbonyl carbon, and propanone is the simplest ketone because a ketone requires at least three carbon atoms in total.
最简单的醛是甲醛（HCHO），最简单的酮是丙酮（CH₃COCH₃）。注意甲醛是唯一没有烷基与羰基碳相连的醛；而丙酮是最简单的酮，因为一个酮至少需要总共三个碳原子。
2. Homologous Series: General Formula | 同系物：通式
Both aldehydes and saturated aliphatic ketones share the same general molecular formula: CₙH₂ₙO. This is because both contain one carbonyl group and no other oxygen or unsaturation.
醛和饱和脂肪酮具有相同的分子通式：CₙH₂ₙO。这是因为两者都只含一个羰基，没有其他氧原子或不饱和键。
For a straight-chain aldehyde with n carbon atoms, the molecular formula is CₙH₂ₙO. For example, ethanal is C₂H₄O, propanal is C₃H₆O, and butanal is C₄H₈O.
对于含 n 个碳原子的直链醛，分子式均为 CₙH₂ₙO。例如乙醛是 C₂H₄O，丙醛是 C₃H₆O，丁醛是 C₄H₈O。
The same formula also applies to ketones. Propanone is C₃H₆O, butanone is C₄H₈O, and pentan-2-one is C₅H₁₀O. Consequently, aldehydes and ketones with the same carbon number are structural isomers of each other.
酮同样适用此通式。丙酮是 C₃H₆O，丁酮是 C₄H₈O，戊-2-酮是 C₅H₁₀O。因此碳数相同的醛和酮互为构造异构体。
| Carbon atoms | Aldehyde name | Formula | Ketone name | Formula |
| 1 | Methanal | CH₂O | — | — |
| 2 | Ethanal | C₂H₄O | — | — |
| 3 | Propanal | C₃H₆O | Propanone | C₃H₆O |
| 4 | Butanal | C₄H₈O | Butanone | C₄H₈O |
| 5 | Pentanal | C₅H₁₀O | Pentan-2-one | C₅H₁₀O |
3. Nomenclature | 命名规则
For aldehydes, the IUPAC name uses the suffix ‘-al’. The longest chain containing the -CHO group is numbered from the carbonyl carbon, which is always carbon 1. Therefore, no locant is needed for simple aldehydes.
醛的 IUPAC 命名以后缀 ‘-al’ 结尾。包含 -CHO 的最长链从羰基碳开始编号，羰基碳始终为 1 号碳。因此简单醛无需标注位置号。
- HCHO: methanal (not methanol)
- CH₃CHO: ethanal
- CH₃CH₂CHO: propanal
- CH₃CH₂CH₂CHO: butanal
For ketones, the suffix ‘-one’ is used, and the position of the carbonyl group must be indicated by a locant when necessary. The chain is numbered to give the carbonyl carbon the lowest possible number.
酮的命名以后缀 ‘-one’ 结尾，必要时需要用位置号标明羰基位置。碳链编号时使羰基碳尽可能获得最小号码。
- CH₃COCH₃: propanone (no locant needed because the carbonyl must be at position 2)
- CH₃COCH₂CH₃: butanone (also written as butan-2-one)
- CH₃CH₂COCH₂CH₃: pentan-3-one
- CH₃COCH₂CH₂CH₃: pentan-2-one
When both an aldehyde and other functional groups are present, the suffix ‘-al’ takes priority over ‘-one’ in naming. This is a common source of exam errors.
当分子中同时存在醛基和其他官能团时，’-al’ 的优先级高于 ‘-one’。这是考试中常见的易错点。
4. Structural Isomerism within the Series | 同分异构现象
Because aldehydes and ketones share the formula CₙH₂ₙO, functional group isomerism arises between them. Additionally, chain isomerism occurs within each family as the carbon skeleton branches.
由于醛和酮共用分子式 CₙH₂ₙO，它们之间存在官能团异构。此外，每一类内部还会因碳链分支而产生链异构。
For C₃H₆O, there are only two isomers: propanal (an aldehyde) and propanone (a ketone).
对于 C₃H₆O，只有两个异构体：丙醛（醛）和丙酮（酮）。
For C₄H₈O, the isomers include butanal, 2-methylpropanal, butanone, and no other simple ketone because a four-carbon ketone can only place the carbonyl at position 2 or 3, but position 3 is equivalent to position 2 by chain reversal.
对于 C₄H₈O，异构体包括丁醛、2-甲基丙醛和丁酮。四碳酮只能在 2 位或 3 位放置羰基，但 3 位与 2 位通过倒置链是等同的。
Exam tip: When asked to draw all isomers of C₅H₁₀O that are aldehydes, remember to consider both straight-chain and branched aldehydes: pentanal, 2-methylbutanal, 3-methylbutanal, and 2,2-dimethylpropanal.
考试提示：当要求画出 C₅H₁₀O 的所有醛类异构体时，记得考虑直链和支链醛：戊醛、2-甲基丁醛、3-甲基丁醛和 2,2-二甲基丙醛。
5. Physical Properties: Boiling Points | 物理性质：沸点
Aldehydes and ketones are polar molecules because the carbonyl group has a permanent dipole due to the electronegativity difference between carbon and oxygen. However, they cannot form hydrogen bonds with themselves because there is no hydrogen atom attached directly to the oxygen.
醛和酮是极性分子，因为碳与氧的电负性差异使羰基具有永久偶极。然而，由于氧原子上没有直接连氢原子，醛酮分子之间不能形成氢键。
Compared to alkanes of similar molar mass, aldehydes and ketones have higher boiling points because of dipole-dipole interactions. Compared to alcohols of similar molar mass, they have lower boiling points because alcohols can form intermolecular hydrogen bonds.
与摩尔质量相近的烷烃相比，醛酮的沸点较高，原因是偶极-偶极作用；而与摩尔质量相近的醇相比，醛酮的沸点较低，因为醇能形成分子间氢键。
As the carbon chain lengthens, boiling points increase smoothly due to greater van der Waals forces. For isomeric aldehydes and ketones, the straight-chain isomer generally has a higher boiling point than a branched isomer.
随着碳链增长，范德华力增大，沸点平稳升高。对于互为异构的醛和酮，直链异构体的沸点通常高于支链异构体。
Boiling point order: alkane < aldehyde/ketone < alcohol (for similar Mr)
6. Physical Properties: Solubility | 物理性质：溶解性
The lower members of the aldehyde and ketone series, such as methanal, ethanal, and propanone, are soluble in water. This is because the carbonyl oxygen can act as a hydrogen bond acceptor with water molecules.
同系列中较低级的成员，如甲醛、乙醛和丙酮，可溶于水。这是因为羰基氧能与水分子形成氢键，作为氢键受体。
As the alkyl chain length increases, the non-polar hydrocarbon portion dominates, and solubility in water decreases sharply. For example, pentanal is only slightly soluble, and larger aldehydes and ketones are essentially insoluble.
随着烷基链增长，非极性烃基部分占据主导，水溶性急剧下降。例如戊醛仅微溶于水，更大的醛和酮基本不溶于水。
This trend is analogous to that observed in alcohols, but aldehydes and ketones are generally less soluble than alcohols of similar molar mass because they cannot donate hydrogen bonds to water.
这一趋势与醇类似，但相同摩尔质量的醛酮通常比醇更难溶于水，因为它们不能向水提供氢键。
7. Reactivity of the Carbonyl Group | 羰基的反应活性
The carbonyl carbon is electron-deficient because the electronegative oxygen atom withdraws electron density. This makes the carbonyl carbon susceptible to attack by nucleophiles.
羰基碳因氧原子吸电子而缺电子，容易受到亲核试剂的进攻。
Aldehydes are generally more reactive than ketones toward nucleophilic addition. There are two main reasons: sterically, aldehydes have at most one alkyl group attached to the carbonyl carbon, so the nucleophile can approach more easily; electronically, alkyl groups are electron-donating, so ketones have two alkyl groups that stabilise the positive character of the carbonyl carbon and reduce its reactivity.
醛通常比酮更容易发生亲核加成。原因有两个：空间上，醛的羰基碳最多连一个烷基，亲核试剂更容易接近；电子上，烷基是给电子基团，酮的两个烷基稳定了羰基碳的正电性，降低了它的反应活性。
This difference explains why aldehydes are more easily oxidised and why they form more stable cyanohydrins and hemiacetals with higher equilibrium constants than ketones.
这一差异解释了为什么醛更容易被氧化，以及为什么醛形成氰醇和半缩醛时的平衡常数比酮更大。
8. Oxidation of Aldehydes and Ketones | 醛与酮的氧化
The most important chemical distinction between aldehydes and ketones is their behaviour toward oxidising agents. Aldehydes can be oxidised to carboxylic acids, whereas ketones resist oxidation under mild conditions.
醛与酮最重要的化学区别在于它们对氧化剂的反应。醛能被氧化成羧酸，而酮在温和条件下不被氧化。
With Tollens’ reagent (ammoniacal silver nitrate), aldehydes reduce Ag⁺ to metallic silver, producing a silver mirror. Ketones do not react. This forms the basis of a classic test-tube identification.
用托伦试剂（氨性硝酸银）时，醛将 Ag⁺ 还原为金属银，生成银镜；酮不发生反应。这是经典试管鉴别的依据。
With Fehling’s or Benedict’s solution, aldehydes reduce Cu²⁺ to Cu⁺, giving a brick-red precipitate of copper(I) oxide. Ketones do not react, except α-hydroxy ketones which can give a false positive.
用斐林试剂或本尼迪特试剂时，醛将 Cu²⁺ 还原为 Cu⁺，生成砖红色的氧化亚铜沉淀；酮不反应（α-羟基酮除外，可能产生假阳性）。
Acidified potassium dichromate(VI) oxidises aldehydes to carboxylic acids, with a colour change from orange to green. Ketones are not oxidised under these conditions.
酸化的重铬酸钾可将醛氧化为羧酸，颜色由橙色变为绿色；酮在此条件下不被氧化。
| Reagent | Observation with aldehyde | Observation with ketone |
| Tollens’ reagent | Silver mirror | No change |
| Fehling’s solution | Brick-red precipitate | No change |
| Acidified K₂Cr₂O₇ | Orange to green | No change |
9. Reduction to Alcohols | 还原为醇
Aldehydes and ketones can be reduced to primary and secondary alcohols respectively, using reducing agents such as NaBH₄ or LiAlH₄.
醛和酮可分别被还原为伯醇和仲醇，常用还原剂为 NaBH₄ 或 LiAlH₄。
When an aldehyde is reduced, the product is a primary alcohol because the -CHO group becomes -CH₂OH. For example, ethanal is reduced to ethanol.
醛被还原时产物为伯醇，因为 -CHO 变成 -CH₂OH。例如乙醛被还原为乙醇。
When a ketone is reduced, the product is a secondary alcohol. For example, propanone is reduced to propan-2-ol.
酮被还原时产物为仲醇。例如丙酮被还原为丙-2-醇。
R-CHO + 2[H] → R-CH₂OH
R-CO-R’ + 2[H] → R-CH(OH)-R’
NaBH₄ is preferred in A-Level practical work because it is safer and can be used in aqueous or alcoholic solution. LiAlH₄ is more powerful but requires anhydrous conditions.
在 A-Level 实验中更常用 NaBH₄，因为它更安全且可在水或醇溶液中使用；LiAlH₄ 更强，但需无水条件。
10. Nucleophilic Addition Reactions | 亲核加成反应
The carbonyl group undergoes nucleophilic addition. Two textbook reactions are particularly important: reaction with hydrogen cyanide (HCN) to form hydroxynitriles, and reaction with 2,4-dinitrophenylhydrazine (Brady’s reagent) to form orange precipitates.
羰基能发生亲核加成。课本中两个重要反应是：与氰化氢（HCN）反应生成羟腈，以及与 2,4-二硝基苯肼（布兰德试剂）反应生成橙色沉淀。
With HCN, the nucleophile is CN⁻, and the carbonyl carbon forms a new C-C bond. The product is a 2-hydroxynitrile. For ethanal, the product is 2-hydroxypropanenitrile (CH₃CH(OH)CN).
与 HCN 反应时，亲核试剂为 CN⁻，羰基碳形成新的 C-C 键，产物为 2-羟腈。例如乙醛的产物是 2-羟基丙腈（CH₃CH(OH)CN）。
This reaction is valuable because it increases the carbon chain length by one carbon, an important synthetic strategy in organic chemistry.
这个反应很有价值，因为它将碳链增加一个碳原子，是有机合成中重要的延长碳链策略。
With Brady’s reagent (2,4-DNPH), both aldehydes and ketones form orange or yellow precipitates of 2,4-dinitrophenylhydrazones. This test confirms the presence of a carbonyl group but does not distinguish between aldehydes and ketones.
与布兰德试剂（2,4-二硝基苯肼）反应时，醛和酮都会形成橙色或黄色沉淀，即 2,4-二硝基苯腙。该测试确认羰基的存在，但不能区分醛和酮。
11. Acidity of α-Hydrogens | α-氢的酸性
Hydrogen atoms attached to the carbon adjacent to the carbonyl group (α-hydrogens) are weakly acidic. This is because the conjugate base (enolate ion) is stabilised by resonance delocalisation of the negative charge onto the oxygen atom.
与羰基相邻碳上的氢原子（α-氢）具有弱酸性。这是因为其共轭碱（烯醇负离子）通过共振将负电荷离域到氧原子上而得到稳定。
This acidity, though weak (pKa ≈ 20), enables condensation reactions. In the iodoform test, the α-methyl group of ethanol or a methyl ketone is halogenated and then cleaved to give iodoform (CHI₃), a yellow solid with a distinctive antiseptic smell.
这种酸性虽然较弱（pKa ≈ 20），但能发生缩合反应。在碘仿试验中，乙醇或甲基酮的 α-甲基被卤代后裂解，生成具有特殊气味的黄色固体碘仿（CHI₃）。
The iodoform test is positive for ethanal and for all ketones of the form R-CO-CH₃. Propanone gives a positive result, while propanal gives a negative result because it lacks an α-methyl group adjacent to the carbonyl.
碘仿试验对乙醛和所有 R-CO-CH₃ 型甲基酮呈阳性。丙酮呈阳性，而丙醛呈阴性，因为丙醛的羰基旁没有 α-甲基。
12. Summary of Key Trends | 核心规律总结
In the homologous series of aldehydes and ketones, the following trends are central to the CIE specification: both families share the general formula CₙH₂ₙO; boiling points increase with molar mass; solubility decreases with chain length; aldehydes are more reactive than ketones toward nucleophiles; and aldehydes are readily oxidised while ketones are not.
在醛和酮的同系物中，以下规律是 CIE 考纲核心：两大家族共用通式 CₙH₂ₙO；沸点随摩尔质量增大而升高；溶解度随链长增加而降低；醛比酮对亲核试剂更活泼；醛易被氧化而酮不被氧化。
For identification tests, remember: Tollens’ and Fehling’s tests distinguish aldehydes from ketones, while Brady’s reagent detects any carbonyl group, and the iodoform test specifically detects methyl ketones and ethanal.
鉴别试验方面请记住：托伦和斐林试验区分醛与酮；布兰德试剂检测任何羰基；碘仿试验专门检测甲基酮和乙醛。
Mastering these patterns allows you to predict properties, draw isomers, and design simple synthetic routes involving carbonyl compounds with confidence.
掌握这些规律，你就能自信地预测性质、画出异构体，并设计涉及羰基化合物的简单合成路线。
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