📚 Aldehydes and Ketones: GCSE Chemistry Key Points | 醛和酮：GCSE化学考点精讲
Aldehydes and ketones are two closely related families of organic compounds that share the reactive carbonyl group. Understanding their structure, naming, oxidation and chemical tests is essential for GCSE Chemistry. This article breaks down every key point you need, with clear English–Chinese explanations to help you master the topic.
醛和酮是两类密切相关的有机化合物，它们都含有活泼的羰基。理解它们的结构、命名、氧化反应及化学检验对GCSE化学至关重要。本文用清晰的中英双语解释每一个考点，帮助你彻底掌握这一主题。
1. What are Aldehydes and Ketones? | 什么是醛和酮？
ched to at least one hydrogen atom. Because of this, the carbonyl group always sits at the end of the carbon chain.
醛是一类有机化合物，其羰基（C=O）至少与一个氢原子相连。正因为如此，羰基总是位于碳链的末端。
Ketones have the carbonyl group bonded to two carbon-containing groups. This places the carbonyl group within the carbon chain, never at the very end. The simplest ketone is propanone (acetone), CH₃COCH₃.
酮的羰基则与两个含碳基团相连。这使得羰基位于碳链内部，而不会出现在最末端。最简单的酮是丙酮，CH₃COCH₃。
Both families share the carbonyl functional group, but the position of this group gives them different chemical properties, especially in oxidation reactions.
这两个家族都拥有羰基官能团，但该基团的位置不同，赋予了它们不同的化学性质，尤其是在氧化反应中。
2. Carbonyl Functional Group & Structure | 羰基官能团与结构
carbonyl group consists of a carbon atom double-bonded to an oxygen atom: C=O. The carbon– on, making the carbon atom partially positive (δ+) and the oxygen partially negative (δ−).
羰基由碳原子与氧原子通过双键连接而成：C=O。碳氧双键具有极性，因为氧的电负性大于碳，使得碳原子带部分正电荷（δ+），氧原子带部分负电荷（δ−）。
In aldehydes, the functional group is often written as –CHO to show the hydrogen attached to the carbonyl carbon. The general formula for saturated aldehydes is CₙH₂ₙO. In ketones, the carbonyl group is between two carbon atoms, giving the same general molecular formula CₙH₂ₙO for saturated ketones. Therefore, aldehydes and ketones with the same number of carbon atoms are functional group isomers.
在醛中，官能团常写作 –CHO，以表示羰基碳上连有一个氢原子。饱和醛的通式为 CₙH₂ₙO。在酮中，羰基位于两个碳原子之间，饱和酮也具有相同的通式 CₙH₂ₙO。因此，碳原子数相同的醛和酮互为官能团异构体。
3. Naming Rules for Simple Aldehydes and Ketones | 简单醛与酮的命名规则
For aldehydes, count the longest carbon chain that includes the –CHO group. The ending is -al. The carbonyl carbon is always number 1, so no locant is needed. For example, methanal (HCHO), ethanal (CH₃CHO) and propanal (CH₃CH₂CHO).
对于醛，选择包含 –CHO 基团的最长碳链，词尾为 -al。羰基碳总是位于1号位，因此无需标出位次。例如：methanal（甲醛，HCHO）、ethanal（乙醛，CH₃CHO）和 propanal（丙醛，CH₃CH₂CHO）。
For ketones, the ending is -one. Number the chain from the end nearest to the carbonyl group to give the carbonyl carbon the lowest possible number. Propanone (CH₃COCH₃) needs no number because there is only one position. Butanone can be written as CH₃COCH₂CH₃, with the carbonyl at position 2 (butan-2-one).
酮以 -one 结尾。从靠近羰基的一端开始给碳链编号，使羰基碳的位次尽可能小。丙酮（CH₃COCH₃）无需标出位次，因为只有一种可能。丁酮可写作 CH₃COCH₂CH₃，羰基在2号位，故称为 butan-2-one。
4. Physical Properties | 物理性质
Lower aldehydes and ketones are volatile liquids with distinctive, often pungent smells. Methanal is a gas at room temperature, while ethanal and propanone are liquids.
低级醛和酮是易挥发的液体，具有特殊的、常有刺激性的气味。甲醛在室温下为气体，而乙醛和丙酮是液体。
They have boiling points higher than alkanes of similar relative molecular mass because of dipole–dipole attractions between polar carbonyl groups. However, their boiling points are lower than the corresponding alcohols, as they cannot form intermolecular hydrogen bonds (they lack an –OH group).
它们的沸点比相对分子质量相近的烷烃高，这是因为极性羰基之间存在偶极-偶极作用力。但它们的沸点低于相应的醇，因为它们无法形成分子间氢键（缺少 –OH 基团）。
Short-chain aldehydes and ketones are miscible with water. The carbonyl oxygen can form hydrogen bonds with water molecules. Solubility decreases as the hydrocarbon chain lengthens.
短链醛和酮可与水互溶。羰基氧原子能与水分子形成氢键。随着碳氢链增长，溶解度下降。
5. Oxidation of Aldehydes to Carboxylic Acids | 醛氧化为羧酸
Aldehydes are easily oxidised to carboxylic acids. The aldehyde group –CHO gains one oxygen atom and becomes –COOH. Oxidising agents such as acidified potassium dichromate(VI) (K₂Cr₂O₇ / H₂SO₄) or Fehling's solution can be used.
醛很容易被氧化成羧酸。醛基 –CHO 获得一个氧原子后转变为 –COOH。常用的氧化剂包括酸化重铬酸钾（K₂Cr₂O₇ / H₂SO₄）或斐林试剂。
uced to the green Cr(III) ion, giving a clear colour change from orange to green. The half‑equation for the aldehyde is: RCHO + [O] → RCOOH. For ethanal: CH₃CHO + [O] → CH₃COOH.
使用酸化重铬酸钾时，橙色的 Cr(VI) 离子被还原为绿色的 Cr(III) 离子，颜色由橙色变为绿色。醛的半反应式为：RCHO + [O] → RCOOH。对于乙醛：CH₃CHO + [O] → CH₃COOH。
3CH₃CHO + Cr₂O₇²⁻ + 8H⁺ → 3CH₃COOH + 2Cr³⁺ + 4H₂O
This reaction is the basis for distinguishing aldehydes from ketones.
该反应是区分醛与酮的基础。
6. Why Ketones Cannot Be Easily Oxidised | 为什么酮难以被氧化
Ketones lack a hydrogen atom directly attached to the carbonyl carbon. Oxidation of a ketone would require breaking a strong carbon–carbon bond, rather than simply inserting an oxygen atom next to a C–H bond. Therefore, ketones resist oxidation by mild oxidising agents such as acidified dichromate, Fehling's or Tollens' reagent.
酮的羰基碳上没有直接相连的氢原子。要使酮被氧化，就需要断裂牢固的碳-碳键，而不像醛那样只需在 C–H 键旁插入一个氧原子。因此，酮难以被如酸化重铬酸钾、斐林试剂或托伦斯试剂这类温和氧化剂氧化。
This is a key practical distinction: aldehydes give a positive result with these oxidising agents, while ketones show no reaction and no colour change.
这是一个重要的实验区别：醛在这些氧化剂中给出阳性结果，而酮不发生反应，也没有颜色变化。
7. Preparation: Controlled Oxidation of Alcohols | 制备：醇的控制氧化
Primary alcohols can be oxidised to aldehydes using acidified potassium dichromate, but the aldehyde must be distilled off immediately to prevent further oxidation to the carboxylic acid. The apparatus is set up for distillation, and the aldehyde product is collected at its boiling point.
伯醇可被酸化重铬酸钾氧化生成醛，但必须立即将醛蒸出，以防进一步氧化为羧酸。实验装置需使用蒸馏装置，并在醛的沸点处收集产物。
CH₃CH₂OH + [O] → CH₃CHO + H₂O
If full oxidation to the carboxylic acid is desired, the reaction is carried out under reflux, allowing the aldehyde intermediate to be oxidised further.
如果希望完全氧化生成羧酸，则需在回流条件下反应，使中间体醛继续被氧化。
Secondary alcohols are oxidised to ketones under similar conditions. Because ketones cannot be oxidised further, reflux can be used without over‑oxidation.
仲醇在类似条件下被氧化为酮。由于酮难以继续被氧化，可使用回流而不用担心过度氧化。
CH₃CH(OH)CH₃ + [O] → CH₃COCH₃ + H₂O
Tertiary alcohols do not undergo oxidation under these conditions because the carbon atom bearing the –OH has no hydrogen to lose.
叔醇在这些条件下不被氧化，因为与 –OH 相连的碳原子上没有可脱去的氢。
8. Fehling's Test for Aldehydes | 斐林试剂检验醛
Fehling's solution is prepared by mixing Fehling's A (aqueous copper(II) sulfate) and Fehling's B (alkaline sodium potassium tartrate) to form a deep‑blue complex of Cu²⁺ ions. The substance to be tested is warmed with the freshly prepared Fehling's solution.
斐林试剂由斐林A液（硫酸铜水溶液）与斐林B液（碱性酒石酸钾钠）混合而成，形成深蓝色的 Cu²⁺ 配合物。将待测物与新配制的斐林试剂一起加热。
An aldehyde reduces the blue Cu²⁺ ions to a brick‑red precipitate of copper(I) oxide, Cu₂O. The aldehyde itself is oxidised to a carboxylate salt in the alkaline medium.
醛能将蓝色的 Cu²⁺ 离子还原为砖红色的氧化亚铜（Cu₂O）沉淀，同时醛自身在碱性介质中被氧化为羧酸盐。
RCHO + 2Cu²⁺ + 5OH⁻ → RCOO⁻ + Cu₂O + 3H₂O
Ketones give no reaction; the blue colour remains unchanged. This allows a positive identification of an aldehyde.
酮不发生反应，蓝色保持不变。借此可确认醛的存在。
9. Tollens' Silver Mirror Test | 托伦斯银镜反应
Tollens' reagent contains the diamminesilver(I) ion, [Ag(NH₃)₂]⁺. It is prepared by adding sodium hydroxide to silver nitrate, then dissolving the precipitate with dilute ammonia solution. The test must be carried out in a warm water bath using a very clean test tube.
托伦斯试剂含有二氨合银(I)离子，[Ag(NH₃)₂]⁺。制备方法是先向硝酸银中加入氢氧化钠生成沉淀，再用稀氨水溶解沉淀。实验需在温水浴中使用非常洁净的试管进行。
An aldehyde reduces the silver(I) ions to metallic silver, which deposits on the inner wall of the glass, forming a brilliant silver mirror.
醛将银(I)离子还原为金属银，银沉积累在试管内壁上，形成光亮的银镜。
RCHO + 2[Ag(NH₃)₂]⁺ + 3OH⁻ → RCOO⁻ + 2Ag + 4NH₃ + 2H₂O
Ketones give no silver mirror, confirming the absence of an aldehyde group. Tollens' test is often used to distinguish aldehydes from ketones with a dramatic visual effect.
酮不会产生银镜，从而证实不存在醛基。托伦斯试验以其鲜明的视觉效果，常用于区分醛和酮。
10. Acidified Dichromate Test as a Distinction | 酸化重铬酸钾鉴别法
Adding a few drops of acidified potassium dichromate(VI) to a sample and warming gives a rapid visual clue. An aldehyde turns the solution from orange to green (Cr³⁺ formed). A ketone causes no colour change; the solution remains orange.
向样品中滴加数滴酸化重铬酸钾并温热，可提供快速的视觉线索。醛会使溶液由橙色变为绿色（生成 Cr³⁺），而酮则不会引起颜色变化，溶液保持橙色。
This test is less specific than Fehling's or Tollens' because primary and secondary alcohols also reduce dichromate. Therefore, it is usually combined with other tests or used when the sample is known to be a carbonyl compound.
该检验的特异性不如斐林试验或托伦斯试验，因为伯醇和仲醇也能还原重铬酸盐。因此，通常与其他试验结合使用，或用于已知样品为羰基化合物的场合。
11. Everyday Uses of Aldehydes and Ketones | 醛与酮的日常用途
Methanal (formaldehyde) is used as a disinfectant, a preservative for biological specimens, and in the manufacture of polymers such as Bakelite. Ethanal is an intermediate in the synthesis of ethanoic acid, perfumes and plastics.
甲醛（methanal）可用作消毒剂、生物标本防腐剂，并用于制造酚醛树脂（Bakelite）等聚合物。乙醛是合成乙酸、香料和塑料的中间体。
Propanone (acetone) is the simplest ketone and is a highly effective solvent. It dissolves many organic compounds and is used in nail polish removers, degreasers and the production of plexiglass.
丙酮（propanone）是最简单的酮，是一种高效的溶剂。它能溶解许多有机物，广泛用于洗甲水、脱脂剂和有机玻璃的生产中。
Other ketones contribute to fragrances and are used in the pharmaceutical industry. Cyclohexanone is a key raw material for nylon.
其他酮类化合物常用于香料和制药行业。环己酮是生产尼龙的关键原料。
12. Summary: Key Differences Between Aldehydes and Ketones | 总结：醛与酮的关键区别
The table below highlights the most important comparisons you must remember for the GCSE exam.
下表突出了GCSE考试中必须记住的最重要对比。
| Feature | Aldehyde | Ketone |
| Carbonyl position | End of chain | Within chain |
| Functional group | –CHO | >C=O |
| Oxidation | Easily oxidised to carboxylic acid | Resists oxidation |
| Fehling's test | Brick‑red precipitate | No reaction (blue stays) |
| Tollens' test | Silver mirror formed | No reaction |
| Acidified dichromate | Orange → green | Stays orange |
Keep this summary handy when revising and practise writing balanced equations for the oxidation of ethanol to ethanal and for the Fehling's and Tollens' tests. These are common exam questions.
复习时请随时参考这张总结表，并练习书写乙醇氧化为乙醛以及斐林试验和托伦斯试验的配平方程式。这些都是常见的考题。
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