📚 Aldehydes and Ketones: IB & AQA Chemistry Key Points | IB AQA 化学：醛和酮 考点精讲
Aldehydes and ketones are essential carbonyl compounds that feature prominently in IB and AQA Chemistry curricula. Understanding their structure, reactivity, and characteristic reactions is vital for success in organic chemistry assessments. This guide consolidates the core content, from nomenclature and nucleophilic addition mechanisms to distinguishing tests, presented in a dual-language format to support your revision.
醛和酮是重要的羰基化合物，在 IB 和 AQA 化学课程中占有突出地位。理解它们的结构、反应性及特征反应对于有机化学考试成功至关重要。本文汇总了核心内容，从命名和亲核加成机理到鉴别检验，以双语形式呈现，助力你的复习。
1. Structure and Functional Group | 结构与官能团
f a carbon atom sp² hybridised and doubly bonded to oxygen. In aldehydes, the carbonyl carbon is bonded to at least one hydrogen atom, giving the general formula RCHO, where R can be H, alkyl, or aryl. The –CHO group is called the formyl group.
羰基 (>C=O) 由 sp² 杂化的碳原子与氧双键连接而成。在醛中，羰基碳至少与一个氢原子相连，通式为 RCHO，其中 R 可以是 H、烷基或芳基。–CHO 基团称为醛基或甲酰基。
Ketones have the carbonyl carbon bonded to two carbon-containing groups; their general formula is RCOR'. The simplest ketone is propanone (acetone), CH₃COCH₃. The carbonyl group is never at the end of the carbon chain in ketones.
酮的羰基碳与两个含碳基团相连，通式为 RCOR'。最简单的酮是丙酮 (CH₃COCH₃)。在酮中，羰基从不出现在碳链末端。
2. IUPAC Nomenclature | IUPAC 命名法
For aldehydes, select the longest continuous carbon chain containing the –CHO group. Replace the final 'e' of the parent alkane with 'al'. The carbonyl carbon is always position 1, so no locant is needed for the aldehyde group. Substituents are numbered accordingly. Examples: methanal (HCHO), ethanal (CH₃CHO), 2-methylpropanal.
醛的命名：选取包含 –CHO 的最长碳链，将母体烷烃末尾的 'e' 替换为 'al'。羰基碳总是 1 号位，因此无需标明醛基的位置。取代基据此编号。例如：甲 醛 (HCHO)、乙 醛 (CH₃CHO)、2-甲基丙醛。
Ketones use the suffix 'one'. Number the chain to give the carbonyl carbon the lowest possible number, and insert this locant before 'one'. For example, propanone (CH₃COCH₃, often simply called acetone), butan-2-one, pentan-3-one. If the ketone group is the only functional group, the ending is 'one'. When higher priority groups are present, 'oxo' is used as a prefix.
酮使用后缀 'one'（中文为“酮”）。给碳链编号时，使羰基碳所在的位次尽可能小，并将该位次标于 'one' 之前。例如：丙酮 (propanone, CH₃COCH₃)、丁-2-酮 (butan-2-one)、戊-3-酮。当酮基不是最优先官能团时，使用 'oxo' 作为前缀（中文以“氧代”表示）。
3. Physical Properties | 物理性质
Lower aldehydes and ketones are polar due to the significant electronegativity difference between carbon and oxygen. They cannot form intermolecular hydrogen bonds with themselves (no O–H bond) but can accept H-bonds from water, making short-chain members soluble. Boiling points are higher than analogous alkanes but lower than corresponding alcohols, which form stronger hydrogen bonds.
低级醛酮因碳氧电负性差异而具有极性。它们自身无法形成分子间氢键（缺少 O–H 键），但可作为氢键受体与水形成氢键，因此短链醛酮可溶于水。其沸点高于相应烷烃，但低于能形成更强氢键的对应醇。
As chain length increases, solubility in water decreases, and boiling points rise due to greater van der Waals forces. Methanal is a gas at room temperature; ethanal and propanone are volatile liquids with characteristic odours.
随着碳链增长，水溶性下降，沸点因范德华力增大而升高。甲 醛在室温下为气体；乙 醛和丙酮为具有特征气味的挥发性液体。
4. Polarity and Reactivity of the Carbonyl Group | 羰基的极性与反应性
The carbonyl group is highly polarised: oxygen bears a partial negative charge (δ⁻), and carbon bears a partial positive charge (δ⁺). This makes the carbonyl carbon electrophilic and susceptible to attack by nucleophiles. The electron-deficient carbon can undergo addition reactions, breaking the π bond but leaving the σ framework intact.
羰基高度极化：氧带部分负电荷 (δ⁻)，碳带部分正电荷 (δ⁺)。这使得羰基碳具有亲电性，易受亲核试剂进攻。缺电子的碳可发生加成反应，π 键断裂而 σ 骨架保持完整。
Aldehydes are generally more reactive than ketones towards nucleophilic addition for two reasons: steric hindrance is lower (one alkyl group vs. two), and the electron-donating inductive effect of extra alkyl groups in ketones stabilises the δ⁺ carbon, reducing electrophilicity.
醛通常比酮更易发生亲核加成，原因有二：醛的空间位阻较小（一个烷基 vs. 两个）；酮中额外的烷基具有给电子诱导效应，稳定了 δ⁺ 碳，降低了亲电性。
5. Nucleophilic Addition Mechanism | 亲核加成反应机理
The general mechanism involves the nucleophile (Nu⁻) attacking the electrophilic carbonyl carbon, moving the π electrons onto oxygen to form a tetrahedral alkoxide intermediate. A subsequent protonation step (by H⁺ or a protic solvent) yields the final alcohol or addition product. This is a two-step process: addition, then protonation.
一般机理为：亲核试剂 (Nu⁻) 进攻亲电的羰基碳，π 电子转移至氧上，形成四面体结构的醇盐中间体。随后质子化（由 H⁺ 或质子溶剂提供质子）得到最终的醇或加成产物。这是一个两步过程：加成，然后质子化。
For example, with NaBH₄ as a source of hydride ions, H⁻ attacks the carbonyl, forming an alkoxide that is then protonated to give an alcohol. This reduction mechanism is commonly tested in IB and AQA exams; students must be able to draw curly arrows from the hydride ion to the carbonyl carbon and from the π bond to the oxygen.
例如，以 NaBH₄ 提供氢负离子，H⁻ 进攻羰基，形成醇盐，继而质子化得到醇。这种还原机理是 IB 和 AQA 考试中的常见考点；学生必须能够画出从氢负离子指向羰基碳的弯箭头，以及从 π 键指向氧的弯箭头。
6. Addition of Hydrogen Cyanide | 与氰化氢的加成
droxynitriles (cyanohydrins). The reaction is catalysed by cyanide ions (CN⁻) from KCN, with acid maintaining an equilibrium supply of HCN. The mechanism: CN⁻ attacks the carbonyl carbon, the oxygen gains a negative charge, and then protonation yields the –OH group.
HCN 跨 C=O 双键加成，生成羟腈（氰醇）。反应由 KCN 提供的氰根离子 (CN⁻) 催化，酸用于维持 HCN 的平衡供应。机理：CN⁻ 进攻羰基碳，氧带负电荷，然后质子化产生 –OH 基团。
This reaction is synthetically useful because the nitrile group can be hydrolysed or reduced, extending the carbon chain. Safety note: hydrogen cyanide is extremely toxic; great care is required in the laboratory. For aldehydes and asymmetric ketones, the product is a racemic mixture if the carbonyl carbon becomes chiral, as nucleophilic attack can occur from either face.
此反应在合成上很有用，因为腈基可进一步水解或还原，实现碳链增长。安全提示：氰化氢剧毒，实验室需格外小心。对于醛和不对称酮，若羰基碳变为手性中心，产物为外消旋混合物，因为亲核进攻可从平面两侧发生。
7. Reaction with 2,4-Dinitrophenylhydrazine (2,4-DNPH) | 与 2,4-二硝基苯肼的反应
2,4-DNPH is the classic reagent for detecting the presence of a carbonyl group. Both aldehydes and ketones react with 2,4-DNPH in an acid-catalysed condensation–elimination reaction to form a yellow, orange, or red precipitate of the corresponding 2,4-dinitrophenylhydrazone.
2,4-DNPH 是检测羰基存在的经典试剂。醛和酮均可在酸催化下与 2,4-DNPH 发生缩合–消除反应，生成相应的 2,4-二硝基苯腙，呈现黄色、橙色或红色沉淀。
The derivative has a sharp melting point, which can be compared with literature values to identify the original carbonyl compound. This is a key test across both IB and AQA syllabuses; students should write the balanced equation and describe the observation: a bright orange precipitate forms.
该衍生物具有明确的熔点，可与文献值对比以鉴定原始羰基化合物。这是 IB 和 AQA 大纲中的核心检验；学生应能写出配平方程式并描述观察结果：生成亮橙色沉淀。
8. Oxidation of Aldehydes: Silver Mirror and Fehling's Tests | 醛的氧化：银镜反应与斐林试验
Aldehydes are easily oxidised to carboxylic acids, whereas ketones resist mild oxidation. This difference is the basis for qualitative tests.
醛易于氧化为羧酸，而酮则不易被温和氧化剂氧化。这一差异是定性检验的基础。
Tollens' n the test tube wall. Equation: RCHO + 2[Ag(NH₃)₂]⁺ + 2OH⁻ → RCOO⁻ + 2Ag(s) + 4NH₃ + 2H₂O.
托伦斯试剂含 [Ag(NH₃)₂]⁺，与醛共热时，Ag⁺ 被还原为金属银，在试管壁上形成特征的银镜。方程式：RCHO + 2[Ag(NH₃)₂]⁺ + 2OH⁻ → RCOO⁻ + 2Ag↓ + 4NH₃ + 2H₂O。
Fehling's solution (Cu²⁺ complexed with tartrate in alkaline medium) oxidises aliphatic aldehydes, producing a brick-red precipitate of Cu₂O. Aromatic aldehydes do not react. Ketones give no reaction with either Tollens' or Fehling's reagent.
斐林试剂（碱性酒石酸根合铜 (II) 离子）可氧化脂肪族醛，生成砖红色 Cu₂O 沉淀。芳香醛不反应。酮与托伦斯试剂和斐林试剂均不反应。
Tollens': RCHO + 2[Ag(NH₃)₂]⁺ + 2OH⁻ → RCOO⁻ + 2Ag↓ + 4NH₃ + 2H₂O
Fehling's: RCHO + 2Cu²⁺ + 5OH⁻ → RCOO⁻ + Cu₂O↓ + 3H₂O
9. Iodoform (Triiodomethane) Reaction | 碘仿反应
Methyl ketones (CH₃CO–R) and compounds with a CH₃CH(OH)– group (which can be oxidised to a methyl ketone under the reaction conditions) give a positive iodoform test. The reagent is iodine in aqueous sodium hydroxide. The reaction involves halogenation of the methyl group, followed by cleavage to give the yellow precipitate triiodomethane (CHI₃), which has a distinctive antiseptic smell.
甲基酮 (CH₃CO–R) 以及含 CH₃CH(OH)– 基团的化合物（在反应条件下可被氧化为甲基酮）会呈现阳性碘仿反应。试剂为碘的氢氧化钠溶液。反应包括甲基的卤化，随后断裂生成黄色沉淀三碘甲烷 (CHI₃)，具有独特的消毒水气味。
This test is specific to ethanol, ethanal, and methyl ketones. Ketones such as propanone and butan-2-one give a positive result, while ketones lacking a methyl group adjacent to the carbonyl, like pentan-3-one, do not. It is a useful distinction test within the carbonyl family.
该检验专一性地适用于乙醇、乙醛和甲基酮。丙酮和丁-2-酮等酮呈阳性，而羰基邻位无甲基的酮（如戊-3-酮）则否。这是羰基化合物家族中有用的区分检验。
10. Reduction of Aldehydes and Ketones | 醛和酮的还原反应
Both aldehydes and ketones can be reduced to alcohols. Common reducing agents include sodium tetrahydridoborate (NaBH₄) in aqueous or alcoholic solution, and lithium tetrahydridoaluminate (LiAlH₄) in dry ether. In IB and AQA contexts, NaBH₄ is the reagent of choice for selective reduction, as LiAlH₄ also reduces carboxylic acids and esters.
醛和酮均可被还原为醇。常用还原剂包括硼氢化钠 (NaBH₄，在水或醇溶液中使用) 和氢化铝锂 (LiAlH₄，在干燥乙醚中使用)。在 IB 和 AQA 体系中，NaBH₄ 是选择性还原的首选试剂，因为 LiAlH₄ 还会还原羧酸和酯。
otation: using [H] to represent the reducing agent. For example, CH₃CHO + 2[H] → CH₃CH₂OH.
醛生成伯醇，酮生成仲醇。机理为氢负离子转移，如第 5 节所述。常用 [H] 表示还原剂。例：CH₃CHO + 2[H] → CH₃CH₂OH。
11. Summary of Distinction Tests | 醛与酮的鉴别总结
| Test | Aldehyde | Ketone |
|---|---|---|
| Tollens' reagent | Silver mirror formed | No reaction |
| Fehling's solution | Brick-red precipitate (aliphatic only) | No reaction |
| Acidified K₂Cr₂O₇ | Orange → green (oxidised) | No reaction (under mild conditions) |
| 2,4-DNPH | Orange precipitate | Orange precipitate |
| Iodoform test | Positive only for ethanal | Positive only for methyl ketones |
This table summarises the key wet-test differences. Note that 2,4-DNPH gives a positive result for both carbonyl classes, serving as a general carbonyl test but not a discriminator.
此表总结了关键的湿法检验差异。注意 2,4-DNPH 对两类羰基均呈阳性，作为通用的羰基检验而非鉴别手段。
12. IB & AQA Exam Tips and Common Pitfalls | IB 和 AQA 考点与常见失分点
For IB Chemistry (Topic 10 and 20) and AQA A-level Chemistry, the following points are frequently assessed:
在 IB 化学（Topic 10 和 20）以及 AQA A-Level 化学中，以下要点经常被考查：
Drawing and interpreting nucleophilic addition mechanisms with curly arrows, including the role of cyanide ions as catalysts.
绘制和解释弯箭头亲核加成机理，包括氰根离子的催化作用。
Explaining why aldehydes are more reactive than ketones using inductive and steric effects.
利用诱导效应和位阻效应解释为何醛比酮更活泼。
Describing the preparation of Tollens' reagent and why it must be freshly prepared (to avoid explosive silver nitride).
描述托伦斯试剂的配制及为何需现配现用（避免生成爆炸性氮化银）。
Identifying unknown carbonyl compounds using a combination of 2,4-DNPH, melting point determination, and oxidation tests.
综合运用 2,4-DNPH、熔点测定和氧化试验鉴定未知羰基化合物。
Writing equations for reduction, oxidation, and addition reactions, using condensed structural formulas and correct balancing.
用简化结构式正确书写还原、氧化和加成反应的方程式，并配平。
Recognising that NaBH₄ is a source of H⁻ and that water or alcohol provides the proton in the second step.
认识 NaBH₄ 是 H⁻ 的来源，水或醇提供第二步的质子。
Linking the iodoform test to the presence of a methyl carbonyl group and to ethanol via oxidation to ethanal.
将碘仿反应与甲基羰基的存在以及乙醇经氧化生成的乙醛联系起来。
Avoid common mistakes: confusing the aldehyde formula with the ketone formula, writing the aldehyde oxidation product as an alcohol, using incorrect arrow pushing in mechanisms, and forgetting that aromatic aldehydes do not react with Fehling's solution.
避免常见错误：混淆醛和酮的通式，将醛的氧化产物误写为醇，机理中弯箭头指向错误，以及忘记芳香醛不与斐林试剂反应。
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