Testing for Aldehydes and Ketones | 醛和酮的检验

📚 Testing for Aldehydes and Ketones | 醛和酮的检验

Aldehydes and ketones both contain the carbonyl group, but they can be distinguished by their different responses to oxidation and other test-tube reactions. This article covers the key tests required for Cambridge A-Level Chemistry, including 2,4-DNP, Tollens’ reagent, Fehling’s and Benedict’s solutions, the iodoform test, and acidified dichromate oxidation.

醛和酮都含有羰基，但它们对氧化反应和其他试管反应的不同表现可将二者区分。本文涵盖剑桥 A-Level 化学要求掌握的关键检验，包括 2,4-DNP、托伦试剂、斐林和本尼迪克特试剂、碘仿反应以及酸性重铬酸盐氧化。

1. The Carbonyl Group: A Shared Functional Group | 羰基：共有的官能团

Aldehydes and ketones both contain the carbonyl functional group, >C=O, but differ in the groups attached to the carbonyl carbon. In an aldehyde, the carbonyl carbon is bonded to at least one hydrogen atom, giving the general formula RCHO. In a ketone, the carbonyl carbon is bonded to two alkyl or aryl groups, giving the general formula RCOR’. This small structural difference has a major effect on reactivity, especially towards oxidation.

醛和酮都含有羰基官能团 >C=O，但羰基碳上所连的基团不同。醛的羰基碳至少连有一个氢原子，通式为 RCHO；酮的羰基碳连有两个烷基或芳基，通式为 RCOR’。这个微小的结构差异对反应活性，尤其是氧化反应，影响很大。

Simple examples include ethanal, CH₃CHO, which is an aldehyde, and propanone, CH₃COCH₃, which is a ketone. Both have the carbonyl group, but only ethanal has a hydrogen atom directly attached to the carbonyl carbon.

简单例子包括乙醛 CH₃CHO（醛）和丙酮 CH₃COCH₃（酮）。两者都含羰基，但只有乙醛的羰基碳上直接连有氢原子。

2. Why We Need Two Stages of Testing | 为什么需要两步检验

Because both aldehydes and ketones contain the carbonyl group, they share some reactions, such as addition with 2,4-dinitrophenylhydrazine. However, aldehydes are easily oxidised to carboxylic acids, whereas ketones resist oxidation under normal laboratory conditions. The tests below therefore first confirm the presence of a carbonyl group, then distinguish aldehydes from ketones.

由于醛和酮都含有羰基，它们有一些共同反应，例如与 2,4-二硝基苯肼的加成。但醛容易被氧化为羧酸，而酮在常规实验条件下不易被氧化。因此，下面的检验首先确认羰基的存在，然后区分醛和酮。

A logical testing sequence is: (i) confirm the carbonyl group with 2,4-DNP; (ii) test for an aldehyde with Tollens’ or Fehling’s reagent; (iii) if required, identify ethanal or methyl ketones with the iodoform test.

合理的检验顺序是：(i) 使用 2,4-DNP 确认羰基；(ii) 使用托伦试剂或斐林试剂检验醛；(iii) 如有需要，使用碘仿反应识别乙醛或甲基酮。

3. 2,4-DNP Test: Detecting the Carbonyl Group | 2,4-DNP检验：检测羰基

The 2,4-dinitrophenylhydrazine (2,4-DNP) test, also called Brady’s test, gives an orange or yellow precipitate with both aldehydes and ketones. A few drops of the unknown compound are added to a solution of 2,4-DNP in dilute acid. The precipitate is a hydrazone formed by a condensation reaction, showing that a carbonyl group is present. The equation can be written as:

2,4-二硝基苯肼（2,4-DNP）检验，也称 Brady 检验，与醛和酮都会生成橙色或黄色沉淀。将几滴未知化合物加入 2,4-DNP 的稀酸溶液中，生成的沉淀是缩合反应生成的腙，表明存在羰基。反应可写为：

R₂C=O + H₂N-NH-C₆H₃(NO₂)₂ → R₂C=N-NH-C₆H₃(NO₂)₂ + H₂O

Because this test is positive for both aldehydes and ketones, it cannot distinguish between them. It is used as the first stage: confirm the carbonyl group before carrying out a specific oxidation test.

由于该检验对醛和酮都呈阳性，因此不能区分二者。它用作第一阶段：先确认羰基，再进行特定的氧化检验。

The solid hydrazone derivative can be filtered, purified, and its melting point measured. This melting point can be compared with data tables to identify the original aldehyde or ketone, which is useful in practical work.

生成的固体腙衍生物可以过滤、纯化并测定熔点。将熔点与数据表对比，可鉴定原来的醛或酮，这在实验工作中非常有用。

4. Tollens’ Reagent: The Silver Mirror Test | 托伦试剂：银镜反应

Tollens’ reagent is [Ag(NH₃)₂]⁺ in aqueous ammonia, prepared by adding dilute ammonia to silver nitrate until the brown silver oxide precipitate just redissolves. Aldehydes reduce the colourless [Ag(NH₃)₂]⁺ complex to metallic silver, which deposits as a shiny silver mirror on the inner wall of a clean test tube. Ketones give no reaction, so no silver mirror appears.

托伦试剂是氨水中的 [Ag(NH₃)₂]⁺，由稀氨水加入硝酸银溶液中，直到生成的棕色氧化银沉淀恰好重新溶解制得。醛将无色的 [Ag(NH₃)₂]⁺ 络离子还原为金属银，在洁净试管内壁形成光亮的银镜。酮不发生反应，因此不会出现银镜。

RCHO + 2[Ag(NH₃)₂]⁺ + 3OH⁻ → RCOO⁻ + 2Ag(s) + 4NH₃ + 2H₂O

The observation is a silver mirror or grey-black precipitate of silver if the tube is not clean. This is a specific test for aldehydes; simple ketones do not reduce Tollens’ reagent under these conditions.

观察结果是银镜，若试管不洁净则可能生成灰色或黑色银沉淀。这是醛的特效检验；简单酮在该条件下不能还原托伦试剂。

Safety note: Tollens’ reagent should be prepared fresh and discarded safely after use, because explosive silver nitride can form if the solution is stored for long periods.

安全提示：托伦试剂应现配现用，并在使用后安全处理，因为长期存放可能生成具有爆炸性的氮化银。

5. Fehling’s and Benedict’s Tests: Copper Reduction | 斐林与本尼迪克特试剂：铜的还原

Fehling’s solution contains copper(II) ions complexed with tartrate ions in alkaline solution. Benedict’s solution contains copper(II) ions complexed with citrate ions. On warming with an aldehyde, the blue Cu²⁺ complex is reduced to an orange-red precipitate of copper(I) oxide, Cu₂O. Ketones do not reduce these reagents, so the solution remains blue.

斐林试剂含有在碱性溶液中与酒石酸根离子络合的铜(II)离子。本尼迪克特试剂含有与柠檬酸根离子络合的铜(II)离子。与醛一起温热时，蓝色的 Cu²⁺ 络离子被还原为橙红色的氧化亚铜 Cu₂O 沉淀。酮不能还原这些试剂，因此溶液保持蓝色。

RCHO + 2Cu²⁺ + 5OH⁻ → RCO

Published by TutorHao | A-Level Chemistry Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）