📚 The Homologous Series of Aldehydes and Ketones | 醛和酮的同系物
parate homologous series characterised by a regular gradation in physical properties and common chemical behaviour. This article explores their structures, nomenclature, preparation, characteristic reactions and the tests that distinguish them, tailored to the Cambridge A‑Level Chemistry syllabus.
醛和酮是两类最重要的有机化合物，都含有活泼的羰基（C=O）。它们各自构成独立的同系物，其物理性质呈现规律性递变，并且拥有一致的化学行为。本文按照剑桥A‑Level化学大纲的要求，将深入探讨醛和酮的结构、命名、制备、特征反应以及区分二者的鉴别试验。
1. Understanding Homologous Series | 理解同系物的概念
unctional group and similar chemical properties, where each successive member differs by a –CH₂– group. In aldehydes, the functional group is – s. Both series exhibit a gradual increase in boiling points and a decrease in water solubility as the carbon chain lengthens.
同系物是一类具有相同官能团和相似化学性质的有机化合物，相邻成员之间相差一个 –CH₂– 基团。在醛中，官能团是始终位于碳链末端的 –CHO；而在酮中，官能团是连接在两个烷基上的羰基（C=O）。随着碳链增长，两个同系列的沸点都会逐渐升高，水溶性则逐渐下降。
It is the presence of the polar carbonyl group that dominates the chemistry of both aldehydes and ketones. The carbon atom of the carbonyl is electron‑deficient and thus susceptible to attack by nucleophiles, a key theme in their reactions. The homologous series concept allows chemists to predict properties and reactivity patterns for unfamiliar members simply by knowing their position in the series.
正是极性羰基的存在决定了醛和酮的化学性质。羰基碳原子缺电子，容易受到亲核试剂的进攻——这是它们反应中的一个核心主题。同系物的概念使化学家只要知道某一化合物在系列中的位置，就能预测陌生同系物的性质和反应规律。
2. General Formula and Functional Group | 通式与官能团
for aldehydes (the simplest being methanal, HCHO) and n ≥ 3 for ketones (the simplest being propanone, CH₃COCH₃). The difference lies in the arrangement: an aldehyde has at least one hydrogen atom attached to the carbonyl carbon, while a ketone has two carbon atoms attached.
当碳原子数相同时，醛和酮具有相同的分子式：CₙH₂ₙO。但该通式对醛要求 n ≥ 1（最简单的醛是甲醛，HCHO），对酮则要求 n ≥ 3（最简单的酮是丙酮，CH₃COCH₃）。区别在于结构：醛的羰基碳上至少连接一个氢原子，而酮的羰基碳上连接着两个碳原子。
The functional group of aldehydes is often written as – o carbon atoms. This structural distinction leads to important differences in reactivity, especially towards oxidation.
醛的官能团常写作 –CHO，羰基碳是碳链的一部分。酮的官能团则是位于两个碳原子之间的羰基（>C=O）。这种结构上的区别导致了反应活性的重要差异，特别是在氧化反应中表现得尤为明显。
3. IUPAC Nomenclature | IUPAC 命名法
For aldehydes, the suffix ‘‑al’ is used. The parent chain is the longest continuous chain containing the –CHO group, and the carbonyl carbon is always assigned number 1. For example, CH₃CH₂CHO is propanal, and CH₃CH₂CH₂CHO is butanal. When substituents are present, numbering starts so that the aldehyde carbon remains C‑1, e.g. 2‑methylbutanal.
醛的命名使用后缀“‑al”。选取包含 –CHO 的最长连续碳链作为主链，羰基碳始终编号为 1。例如，CH₃CH₂CHO 是丙醛，CH₃CH₂CH₂CHO 是丁醛。如果有取代基，编号仍以使醛基碳为 1 的方式进行，如 2‑甲基丁醛。
Ketones are named using the suffix ‘‑one’. The parent chain is numbered so that the carbonyl carbon receives the lowest possible number. Thus CH₃COCH₂CH₃ is butan‑2‑one and CH₃COCH₃ is propanone (commonly known as acetone). The position of the carbonyl group is indicated by a number before the suffix, e.g. pentan‑3‑one.
酮的命名使用后缀“‑one”。主链编号要使羰基碳获得尽可能小的数字。因此 CH₃COCH₂CH₃ 是丁‑2‑酮，CH₃COCH₃ 是丙酮（俗称 acetone）。羰基的位置用后缀前的数字表示，如戊‑3‑酮。
| Formula | Aldehyde name | Ketone name |
|---|---|---|
| HCHO | Methanal | — |
| CH₃CHO | Ethanal | — |
| C₃H₆O | Propanal | Propanone |
| C₄H₈O | Butanal | Butan‑2‑one |
4. Physical Properties | 物理性质
The boiling points of aldehydes and ketones are higher than those of comparable alkanes because the polar carbonyl group gives rise to permanent dipole–dipole interactions between molecules. However, since neither aldehydes nor ketones can donate hydrogen bonds to themselves, their boiling points are lower than those of the corresponding alcohols, which can form hydrogen bonds.
醛和酮的沸点高于相应烷烃，因为极性羰基使分子间产生永久的偶极‑偶极相互作用。但由于醛和酮自身不能提供氢键给体，它们的沸点低于能形成分子间氢键的对应醇。
Short‑chain aldehydes and ketones (up to about four carbons) are appreciably soluble in water. This is because the carbonyl oxygen can form hydrogen bonds with water molecules. As the non‑polar hydrocarbon chain lengthens, solubility decreases rapidly because the hydrophobic alkyl part becomes dominant. Methanal is a gas at room temperature, while ethanal and propanone are volatile liquids.
短链醛和酮（约四个碳以内）在水中溶解性较好。这是因为羰基氧能与水分子形成氢键。随着非极性的碳氢链增长，疏水的烷基部分逐渐占主导地位，溶解度迅速下降。甲醛在室温下是气体，乙醛和丙酮则是挥发性液体。
5. Preparation Methods | 制备方法
Aldehydes can be prepared by the careful oxidation of primary alcohols. The oxidising agent is typically acidified potassium dichromate(VI) solution, K₂Cr₂O₇/H₂SO₄, and the aldehyde must be distilled out of the reaction mixture as it forms to prevent further oxidation to the carboxylic acid. In the laboratory, pyridinium chlorochromate (PCC) is sometimes used to achieve a controlled oxidation.
醛可以通过伯醇的谨慎氧化来制备。常用的氧化剂是酸化重铬酸钾溶液（K₂Cr₂O₇/H₂SO₄），反应中生成的醛需要立即蒸馏出来，以防止进一步氧化成羧酸。实验室中有时会使用吡啶氯铬酸盐（PCC）来实现可控氧化。
Ketones are obtained by oxidising secondary alcohols. Since ketones contain no hydrogen on the carbonyl carbon, they are resistant to further oxidation under these conditions, so distillation is not essential. Industrially, propanone is made via the cumene process or by catalytic oxidation of propene.
酮由仲醇氧化得到。由于酮的羰基碳上没有氢原子，在此条件下不易进一步氧化，因此不需要蒸馏分离。工业上，丙酮是通过异丙苯法或丙烯催化氧化制得的。
RCH₂OH + [O] → RCHO + H₂O
RCH(OH)R' + [O] → RCOR' + H₂O
6. Nucleophilic Addition Mechanism | 亲核加成机理
. The oxygen atom, being more electronegative, withdraws electron density, making the carbonyl carbon electron‑ tacks the carbon, causing the π bond to break and the electrons to move onto the oxygen, forming an alkoxide ion. In the second step, this negatively charged oxygen is protonated, often by a solvent or acid, to give the addition product.
醛和酮最具特征的反应是按极性 C=O 双键进行的亲核加成。电负性更强的氧原子拉走电子云密度，使羰基碳缺电子，成为亲核试剂进攻的理想靶点。机理分两步进行：首先亲核试剂进攻碳，导致 π 键断裂，电子移到氧上生成烷氧负离子；第二步，带负电荷的氧被（通常来自溶剂或酸的）质子化，得到加成产物。
This mechanism explains why aldehydes are generally more reactive than ketones towards nucleophiles. Two factors are at play: electronic effects and steric hindrance. Alkyl groups in ketones donate electron density towards the carbonyl carbon, reducing its partial positive charge, and also physically obstruct the approach of the nucleophile. Aldehydes, having at least one small hydrogen atom, are less hindered and more electrophilic.
该机理解释了为什么醛对亲核试剂的反应性通常高于酮。这其中有两个因素：电子效应和空间位阻。酮中的烷基会将电子推向羰基碳，降低其正电性，同时也在空间上阻碍亲核试剂的靠近。醛至少有一个小体积的氢原子，位阻较小，亲电性更强。
7. Addition of Hydrogen Cyanide | 与氰化氢的加成
Hydrogen cyanide, HCN, adds to aldehydes and ketones to form hydroxynitriles (cyanohydrins). Because HCN is highly toxic, the reaction is usually carried out by generating it in situ from sodium cyanide and dilute sulfuric acid. The nucleophile is the cyanide ion, CN⁻.
氰化氢（HCN）与醛和酮发生加成反应，生成羟腈（氰醇）。由于 HCN 剧毒，实验通常通过氰化钠和稀硫酸现场生成。亲核试剂是氰根离子 CN⁻。
CH₃CHO + HCN → CH₃CH(OH)CN (2‑hydroxypropanenitrile)
This reaction is extremely useful in organic synthesis because it lengthens the carbon chain by one carbon atom. The nitrile group (–CN) can subsequently be hydrolysed to a carboxylic acid or reduced to an amine, opening routes to amino acids and other compounds.
该反应在有机合成中非常有用，因为它能使碳链增加一个碳原子。氰基（–CN）随后可以水解成羧酸，或还原成胺，从而为制备氨基酸和其他化合物开辟了途径。
The carbonyl compound must be one that can react fast enough; simple aldehydes and symmetrical ketones work well. Due to the lower reactivity, aromatic ketones may require more forcing conditions.
所用的羰基化合物必须有足够的反应速率；简单的醛和对称酮均可顺利进行。由于芳香酮的反应活性较低，可能需要更强烈的反应条件。
8. Reaction with 2,4‑Dinitrophenylhydrazine (2,4‑DNPH) | 与 2,4‑二硝基苯肼的反应
2,4‑DNPH is a reagent used to detect the presence of a carbonyl group. When an aldehyde or ketone is mixed with a solution of 2,4‑DNPH in acidic methanol, a yellow or orange precipitate of the corresponding 2,4‑dinitrophenylhydrazone forms.
2,4‑二硝基苯肼（2,4‑DNPH）是用来检测羰基的试剂。将醛或酮与 2,4‑DNPH 的酸性甲醇溶液混合，会生成相应 2,4‑二硝基苯腙的黄色或橙色沉淀。
>C=O + H₂N‑NH‑C₆H₃(NO₂)₂ → >C=N‑NH‑C₆H₃(NO₂)₂ + H₂O
The identity of the original carbonyl compound can then be deduced by filtering, recrystallising and measuring the melting point of the solid derivative, which is compared with literature values. This test does not distinguish between aldehydes and ketones, as both give a positive result.
原始羰基化合物可以通过过滤、重结晶并测定该固体衍生物的熔点，再与文献值比对来鉴定。该试验不能区分醛和酮，因为两者都会产生阳性结果。
9. Oxidation of Aldehydes – Why Ketones Resist | 醛的氧化——酮为何难以被氧化
Aldehydes are readily oxidised to carboxylic acids because the carbonyl carbon bears a hydrogen atom that can be removed. Mild oxidising agents such as acidified dichromate, warm Fehling's solution or Tollens' reagent bring about this transformation. Ketones, lacking this hydrogen, are not oxidised under similar conditions. Only strong oxidising agents that break carbon–carbon bonds (such as hot, concentrated HNO₃) can oxidise ketones, giving a mixture of carboxylic acids.
醛很容易被氧化成羧酸，因为其羰基碳上带有一个可被脱去的氢原子。温和的氧化剂如酸化重铬酸盐、温热的斐林溶液或托伦斯试剂均能完成这一转化。酮缺乏这个氢原子，在类似条件下不被氧化。只有能断裂碳‑碳键的强氧化剂（如热浓硝酸）才能氧化酮，生成羧酸的混合物。
RCHO + [O] → RCOOH
This difference in behaviour is the foundation of several chemical tests used to distinguish aldehydes from ketones, which are essential in qualitative organic analysis.
这一行为差异是几种用于区分醛和酮的化学试验的基础，在有机定性分析中至关重要。
10. Distinguishing Aldehydes from Ketones: Diagnostic Tests | 醛和酮的鉴别试验
Tollens' silver mirror test: Tollens' reagent contains the diamminesilver(I) ion, [Ag(NH₃)₂]⁺. Aldehydes reduce this to metallic silver, which deposits as a shiny mirror on the inner surface of the test tube. Ketones give no reaction.
托伦斯银镜试验：托伦斯试剂含有二氨合银(I)离子 [Ag(NH₃)₂]⁺。醛能将其还原为金属银，在试管内壁形成光亮的银镜。酮不发生反应。
RCHO + 2[Ag(NH₃)₂]⁺ + 2OH⁻ → RCOO⁻ + 2Ag↓ + 4NH₃ + 2H₂O
Fehling's test: Fehling's solution is an alkaline copper(II) tartrate complex. On warming with an aldehyde, the blue Cu²⁺ ions are reduced to a brick‑red precipitate of copper(I) oxide, Cu₂O. Aromatic aldehydes do not react, and ketones remain blue.
斐林试验：斐林溶液是碱性的酒石酸铜(II)配合物。与醛共热时，蓝色的 Cu²⁺ 被还原为砖红色的氧化亚铜（Cu₂O）沉淀。芳香醛不反应，酮则保持蓝色。
RCHO + 2Cu²⁺ + 5OH⁻ → RCOO⁻ + Cu₂O↓ + 3H₂O
Iodoform (triiodomethane) test: A compound containing a methyl ketone group (CH₃CO–) or ethanol (which is oxidised in situ to ethanal) gives a positive iodoform test. Warming with iodine and sodium hydroxide produces a pale yellow precipitate of triiodomethane, CHI₃, with a characteristic antiseptic smell. Ethanal and all methyl ketones (e.g. propanone, butanone) give this test; aldehydes without a methyl group adjacent to the carbonyl do not.
碘仿（三碘甲烷）试验：含有甲基酮基团（CH₃CO–）的化合物或乙醇（可被原位氧化成乙醛）会给出阳性碘仿反应。与碘和氢氧化钠共热，产生具有特征防臭味的浅黄色三碘甲烷（CHI₃）沉淀。乙醛和所有甲基酮（如丙酮、丁酮）会得到阳性结果；羰基邻位没有甲基的醛则不反应。
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