📚 Aldehydes and Ketones Key Points for CIE A-Level Chemistry | A-Level CIE 化学：醛和酮考点精讲
Aldehydes and ketones are carbonyl compounds containing the C=O functional group. In aldehydes the carbonyl group is at the end of the carbon chain bonded to at least one hydrogen atom, while in ketones it is bonded to two carbon atoms. This small structural difference leads to striking variations in reactivity, especially in oxidation. For CIE A-Level Chemistry, you need to master their preparation, nucleophilic addition mechanism, distinguishing tests, and characteristic reactions like the iodoform test. Let’s explore these key areas in depth.
醛和酮是含有 C=O 官能团的羰基化合物。醛的羰基位于碳链末端，至少与一个氢原子相连；酮的羰基则与两个碳原子相连。这个微小的结构差异导致它们反应活性不同，尤其在氧化反应中。在 CIE A-Level 化学考试中，你需要掌握它们的制备方法、亲核加成机理、鉴别试验以及碘仿反应等特征反应。让我们深入梳理这些考点。
1. Structure and Bonding of the Carbonyl Group | 羰基的结构与成键
The carbonyl group consists of a carbon atom double-bonded to an oxygen atom. The carbon is sp² hybridised, giving a planar arrangement with bond angles of about 120°. The C=O bond is polar because oxygen is more electronegative than carbon; this polarity makes the carbonyl carbon electron-deficient and susceptible to nucleophilic attack.
羰基由碳原子与氧原子双键连接而成。碳为 sp² 杂化，呈平面结构，键角约为 120°。C=O 键是极性键，因为氧的电负性大于碳；这种极性使得羰基碳缺电子，容易受到亲核试剂的进攻。
In aldehydes, the partial positive charge on the carbonyl carbon is slightly more accessible than in ketones, because the alkyl groups in ketones donate electron density and also introduce steric hindrance. This explains why aldehydes are generally more reactive than ketones towards nucleophilic addition.
在醛中，羰基碳上的部分正电荷比酮中更容易被进攻，因为酮中的烷基给电子且带来空间位阻。这就解释了为什么醛通常比酮在亲核加成反应中更活泼。
2. Nomenclature of Aldehydes and Ketones | 醛和酮的命名
For aldehydes, the suffix is ‘-al’. The aldehyde carbon is always position 1, so no number is needed. For example, methanal (HCHO), ethanal (CH₃CHO), propanal (CH₃CH₂CHO). When a substituent is present, number the chain starting from the aldehyde carbon. For ketones, the suffix is ‘-one’ and the position of the carbonyl group must be indicated by a number, e.g., propanone (CH₃COCH₃), butan-2-one (CH₃COCH₂CH₃).
醛的命名后缀为“-al”。醛基碳总是位于 1 号位，所以不需要编号。例如：methanal (甲醛)、ethanal (乙醛)、propanal (丙醛)。若存在取代基，从醛基碳开始编号。酮的后缀为“-one”，必须用数字标明羰基的位置，例如：propanone (丙酮)、butan-2-one (丁‑2‑酮)。
In cyclic ketones, the carbonyl carbon is counted as part of the ring. Cyclohexanone is a common example. It is essential to practise naming compounds where both an aldehyde and a ketone group could exist; the aldehyde takes priority in nomenclature.
在环状酮中，羰基碳作为环的一部分。环己酮是一个常见例子。必须练习命名同时含有醛基和酮基的化合物；在命名中醛基优先。
3. Physical Properties and Solubility | 物理性质与溶解性
Short-chain aldehydes and ketones are soluble in water because the carbonyl oxygen can hydrogen-bond with water molecules. Methanal, ethanal, and propanone are completely miscible with water. As the hydrocarbon chain lengthens, solubility decreases rapidly. They have higher boiling points than alkanes of similar relative molecular mass due to permanent dipole-dipole interactions, but lower than corresponding alcohols because they cannot form intermolecular hydrogen bonds with themselves.
短链醛和酮可溶于水，因为羰基氧可与水分子形成氢键。甲醛、乙醛和丙酮与水完全互溶。随着碳链增长，溶解度迅速下降。由于存在永久偶极‑偶极作用，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，因为它们自身分子间不能形成氢键。
Volatility is also worth noting: many aldehydes and ketones have distinctive smells, with some ketones being used in perfumes. Propanone is a widely used solvent due to its ability to dissolve many organic substances.
挥发性也值得注意：许多醛和酮有特殊气味，有些酮被用于香水。丙酮因其能溶解多种有机物而广泛用作溶剂。
4. Laboratory and Industrial Preparation | 实验室和工业制备方法
Aldhydes and ketones are commonly prepared by the oxidation of alcohols. Controlled oxidation of primary alcohols using acidified potassium dichromate(VI) with distillation produces aldehydes, while further oxidation yields carboxylic acids. To stop at the aldehyde stage, the aldehyde must be distilled out as it forms. Secondary alcohols are oxidised to ketones; no further oxidation is possible under normal conditions because there is no hydrogen atom on the carbonyl carbon that can be removed.
醛和酮通常由醇的氧化制备。在酸性重铬酸钾(VI)中，一级醇经控制氧化并蒸馏可得到醛，进一步氧化则生成羧酸。为使反应停留在醛阶段，必须在醛生成时立即将其蒸馏出来。二级醇氧化生成酮；由于酮的羰基碳上没有可离去的氢，一般条件下不会继续氧化。
Ketones can also be produced by the dry distillation of calcium salts of carboxylic acids, and both aldehydes and ketones may be prepared by the hydration of alkynes using the Kucherov reaction (HgSO₄/H₂SO₄). Industrially, ethanal is made by the Wacker process from ethene. Propanone is a by-product of phenol manufacture and can also be made from propan-2-ol dehydrogenation.
酮还可通过羧酸钙盐的干馏制备。醛和酮均可通过炔烃水合（Kucherov 反应，HgSO₄/H₂SO₄）制得。工业上，乙醛由乙烯经 Wacker 法制取，而丙酮是苯酚生产的副产物，也可由异丙醇脱氢得到。
5. Nucleophilic Addition Mechanism | 亲核加成机理
The most important reaction of aldehydes and ketones is nucleophilic addition. The nucleophile attacks the electrophilic carbonyl carbon, breaking the π-bond. This results in a tetrahedral intermediate with a negative charge on oxygen. Protonation then gives the final alcohol derivative. With HCN, the nucleophile is the cyanide ion, CN⁻. The reaction is catalysed by a base to generate sufficient CN⁻, but an acid is avoided because HCN is a weak acid and needs CN⁻ in solution.
醛和酮最重要的反应是亲核加成。亲核试剂进攻缺电子的羰基碳，π键断裂，形成带负电荷的氧负离子四面体中间体。随后质子化得到最终的醇衍生物。以 HCN 为例，亲核试剂是 CN⁻ 离子。反应需碱催化以产生足够的 CN⁻，但避免强酸性条件，因为 HCN 是弱酸，需要溶液中有 CN⁻。
The general mechanism follows:
Nu⁻ + >C=O → >C(O⁻)–Nu → (H⁺) >C(OH)–Nu
. With NaHSO₃, the nucleophile is the bisulfite ion; with LiAlH₄, it is a hydride ion, H⁻. Aldehydes react faster than ketones. Steric hindrance and inductive effects of alkyl groups stabilise the ketone and make the carbonyl carbon less electrophilic.
一般机理如下：
Nu⁻ + >C=O → >C(O⁻)–Nu → (H⁺) >C(OH)–Nu
。与 NaHSO₃ 反应时亲核试剂是亚硫酸氢根；与 LiAlH₄ 反应时是 H⁻。醛反应比酮快。烷基的空间位阻和诱导效应稳定了酮，使其羰基碳亲电性减弱。
6. Addition of Hydrogen Cyanide and Hydroxynitriles | 氰化氢加成与羟基腈的形成
Hydrogen cyanide adds to aldehydes and ketones to form hydroxynitriles (cyanohydrins). The product contains one more carbon atom, making it useful for chain lengthening. The OH group can be substituted further and the CN group can be hydrolysed to a carboxylic acid. These products are important synthetic intermediates. The reaction must be carried out carefully as HCN is extremely toxic.
氰化氢与醛酮加成生成羟基腈（氰醇）。产物增加一个碳原子，可用于碳链增长。羟基可进一步取代，氰基可水解为羧酸。这些产物是重要的合成中间体。反应需谨慎操作，因为 HCN 剧毒。
With ethanal, the product is 2-hydroxypropanenitrile:
CH₃CHO + HCN → CH₃CH(OH)CN
. With propanone, it is 2-hydroxy-2-methylpropanenitrile. The nucleophilic addition of CN⁻ is often examined with the mechanism and the subsequent acid hydrolysis to a hydroxy acid.
以乙醛为例，产物为 2‑羟基丙腈：
CH₃CHO + HCN → CH₃CH(OH)CN
。以丙酮为例，产物是 2‑羟基‑2‑甲基丙腈。CN⁻ 的亲核加成常常与机理以及随后的酸性水解生成羟基酸一同考查。
7. Oxidation Reactions – Distinguishing Aldehydes from Ketones | 氧化反应——区分醛和酮
Aldhydes are easily oxidised to carboxylic acids, but ketones resist oxidation. This difference is exploited in simple chemical tests: Fehling’s solution and Tollens’ reagent. Fehling’s solution (blue, containing Cu²⁺ ions complexed with tartrate) is reduced by aldehydes to a brick-red precipitate of Cu₂O upon warming. Ketones give no reaction.
醛极易被氧化为羧酸，而酮不会被氧化。这一差异被用于简单的化学测试：斐林溶液和托伦斯试剂。斐林溶液（蓝色，含与酒石酸根配合的 Cu²⁺ 离子）在加热时被醛还原，生成砖红色的 Cu₂O 沉淀。酮则无反应。
Tollens’ reagent (the silver mirror test) contains [Ag(NH₃)₂]⁺ ions. Aldehydes reduce this to metallic silver, forming a silver mirror on the inner wall of a clean test tube. Ketones do not react. These tests are specific to aldehydes and are commonly tested in practical assessments.
托伦斯试剂（银镜试验）含有 [Ag(NH₃)₂]⁺ 离子。醛将其还原为金属银，在洁净试管内壁形成银镜。酮不发生反应。这些试验对醛具有专属性，常在实验考核中出现。
8. Reduction of Aldehydes and Ketones | 醛和酮的还原
Aldhydes are reduced to primary alcohols and ketones to secondary alcohols. The most widely used reducing agent is sodium borohydride, NaBH₄, in aqueous or alcoholic solution. Lithium aluminium hydride, LiAlH₄, is a stronger reducing agent and works in dry ether, but it can also reduce carboxylic acids and esters, so NaBH₄ is preferred for chemoselectivity. The hydride ion, H⁻, acts as the nucleophile in these reactions.
醛被还原为一级醇，酮被还原为二级醇。最常用的还原剂是硼氢化钠 NaBH₄，在水或醇溶液中使用。氢化铝锂 LiAlH₄ 是更强的还原剂，需在无水乙醚中反应，但它还能还原羧酸和酯，因此 NaBH₄ 更具化学选择性。这些反应中 H⁻ 离子充当亲核试剂。
The reduction of propanal yields propan-1-ol, and reduction of propanone yields propan-2-ol. The mechanism is essentially nucleophilic addition of hydride followed by protonation. In equations, it is conventional to represent the reducing agent as [H], e.g.,
CH₃COCH₃ + 2[H] → CH₃CH(OH)CH₃
.
丙醛还原得到正丙醇，丙酮还原得到异丙醇。机理本质上是 H⁻ 的亲核加成，随后质子化。方程式中常用 [H] 表示还原剂，例如：
CH₃COCH₃ + 2[H] → CH₃CH(OH)CH₃
。
9. The Iodoform (Triiodomethane) Test | 碘仿反应
Methyl ketones (compounds with CH₃CO– group) and secondary alcohols with the CH₃CH(OH)– group give a positive iodoform test. The test involves warming the sample with iodine in sodium hydroxide. A pale yellow precipitate of triiodomethane (CHI₃), with an antiseptic smell, indicates a positive result. Ethanol is the only primary alcohol that gives this test because its oxidation product is ethanal containing the necessary CH₃CO– group.
甲基酮（含 CH₃CO– 基团的化合物）和含有 CH₃CH(OH)– 基团的二级醇在碘仿试验中呈阳性。试验将样品与碘的氢氧化钠溶液混合加热。生成淡黄色的三碘甲烷 (CHI₃) 沉淀，伴有消毒水气味，即为阳性结果。乙醇是唯一呈阳性的伯醇，因为其氧化产物乙醛含有所需的 CH₃CO– 基团。
The reactions involve halogenation and cleavage: first the methyl group is tri-iodinated in the alpha position, then the C–C bond is broken by nucleophilic attack by hydroxide, forming CHI₃ and a carboxylate ion. It’s important to be able to write the overall equation: e.g.,
CH₃COCH₃ + 3I₂ + 4NaOH → CHI₃ + CH₃COONa + 3NaI + 3H₂O
.
反应涉及卤化与断裂：首先甲基的 α 位被三碘取代，然后 C–C 键受 OH⁻ 亲核进攻而断裂，生成 CHI₃ 和羧酸根。需要能写出总方程式，例如：
CH₃COCH₃ + 3I₂ + 4NaOH → CHI₃ + CH₃COONa + 3NaI + 3H₂O
。
10. Condensation Reactions – 2,4-DNP and Derivatives | 缩合反应——2,4‑二硝基苯肼及衍生物
Both aldehydes and ketones react with 2,4-dinitrophenylhydrazine (2,4-DNP) to form yellow or orange precipitates. This is a condensation reaction that eliminates water, producing the corresponding hydrazone. The reaction is used as a general test for the carbonyl group. The melting point of the purified derivative can help identify the original aldehyde or ketone.
醛和酮均可与 2,4‑二硝基苯肼 (2,4‑DNP) 反应生成黄色或橙色沉淀。这是一个缩合反应，脱去一分子水，生成相应的腙。该反应可作为羰基的普遍检测方法。纯化衍生物的熔点可用于鉴别原来的醛或酮。
Similarly, aldehydes and ketones condense with hydroxylamine (NH₂OH) to form oximes, and with hydrazine to form hydrazones. These reactions are less prominent in CIE practical assessments but may appear in questions about identifying carbonyl compounds.
类似地，醛酮与羟胺 (NH₂OH) 缩合生成肟，与肼缩合生成腙。这些反应在 CIE 实验考核中不太突出，但可能在鉴别羰基化合物的题目中出现。
11. Key Distinctions and Summary of Tests | 重要区分与测试总结
| Test / Reagent | Aldehyde | Ketone |
|---|---|---|
| 2,4-DNP | Yellow/orange ppt | Yellow/orange ppt |
| Tollens’ reagent | Silver mirror | No reaction |
| Fehling’s solution | Brick-red ppt | No reaction |
| Acidified K₂Cr₂O₇ | Orange → green | No reaction |
| Iodoform test | Only if CH₃CO– present (e.g., ethanal) | Positive for methyl ketones |
Remember that a ketone with a methyl group next to the carbonyl, such as butan-2-one, will also give a positive iodoform test. The table above helps you systematically distinguish these compounds.
记住，羰基邻位有甲基的酮，如丁‑2‑酮，也会在碘仿试验中呈阳性。上表有助于系统地鉴别这些化合物。
12. Typical Exam Questions and Common Errors | 经典考题与常见错误
CIE exam questions often ask for the mechanism of nucleophilic addition of CN⁻ to an aldehyde. Make sure your curly arrows start from the lone pair on CN⁻ and point to the carbonyl carbon, and a second arrow from the C=O bond to oxygen. The intermediate must show a negative oxygen; then protonation from HCN or H₃O⁺. Never use H⁺ from a strong acid directly because that would protonate CN⁻.
CIE 考试常要求写出 CN⁻ 对醛的亲核加成机理。确保弯箭头从 CN⁻ 的孤对电子出发指向羰基碳，第二个箭头从 C=O 键指向氧。中间体必须显示负氧；随后是来自 HCN 或 H₃O⁺ 的质子化。切勿直接使用强酸的 H⁺，否则会使 CN⁻ 质子化。
Another pitfall is confusing oxidation products. Remember primary alcohols → aldehyde → carboxylic acid; secondary alcohols → ketone. Aldehydes are oxidised to carboxylic acids, ketones are not oxidised. In the Fehling’s test, the active species is Cu²⁺ (complexed) reduced to Cu₂O, not CuO.
另一个易错点是混淆氧化产物。记住一级醇 → 醛 → 羧酸；二级醇 → 酮。醛氧化为羧酸，酮不被氧化。在斐林试验中，活性物种是 Cu²⁺ (配合物) 被还原为 Cu₂O，而不是 CuO。
For the iodoform test, the yellow precipitate is CHI₃, not CH₂I₂ or CI₄. The reaction requires a methyl carbonyl or methyl carbinol group. Always check the structure of the given compound before concluding.
对于碘仿试验，黄色沉淀是 CHI₃，而非 CH₂I₂ 或 CI₄。该反应需要甲基羰基或甲基甲醇基团。作答前一定要检查给定化合物的结构。
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