Test for Aldehydes and Ketones
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An important class of organic compounds containing the carbonyl group comprises aldehydes and ketones. Aldehyde has the RCH(=O) structure, while R2C(=O) has the structure of a ketone.

In this article, we will study the following concepts in detail.

  1. Test for aldehydes and ketones, 

  2. How aldehyde and ketone can be distinguished by various tests.


Aim

Identify the presence of functional group aldehydes or ketones in the given organic compound.


Materials Required

The test of aldehyde and ketone-

  1. Schiff’s reagent

  2. Silver nitrate solution

  3. Fehling’s solutions A

  4. Fehling’s solutions B

  5. Dilute ammonium hydroxide solution

  6. 2,4-Dinitrophenylhydrazine reagent

  7. Chromic acid

  8. Sulfuric acid

  9. Sodium bisulfite

  10. Test tubes

  11. Test tube holder

  12. Beaker


Theory 

Aldehydes and ketones are volatile compounds with low molecular weights. Aldehyde and ketone identification is based on two types of reactions, the double bond addition reaction, and the oxidation reaction. 


In aldehydes, a hydrogen atom and aliphatic or aromatic radicals are attached to the carbonyl group. Formaldehyde is an exceptional case where two hydrogen atoms are attached to the carbonyl that is present in formaldehyde. In ketones, two aliphatic or aromatic groups are affixed to the carbonyl group.


Tests to Detect the Presence of Aldehyde and Ketone

1. 2,4-dinitrophenyl Hydrazine Test

Aldehyde and ketone react with a 2,4-dinitrophenyl hydrazine test to give a yellow to orange colour precipitate. The aldehyde test reaction is given below-

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2. Sodium Bisulfite Test

When aldehydes and ketone react with Sodium bisulfite forms well crystalline precipitate.

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Aldehyde and Ketone can be Distinguished By

1. Schiff’s Test

In some dye formulation reactions, such as the reaction between sodium bisulfite and fuchsine, the Schiff reagent is the product produced. It is used to detect an aldehyde's presence. A magenta-coloured dye with a chemical formula C20H20N3·HCl, which is decolourized by sodium bisulfate, is fuchsine or rosaniline hydrochloride. A pink colour indicates the presence of an aldehyde group.


2. Fehling’s Test

The Fehling test consists of a solution that in laboratories is normally freshly prepared. The solution occurs in two different forms known as Fehling's A and Fehling's B. Fehling's A is a blue copper(II) sulfate solution. A clear liquid consisting of potassium sodium tartrate (Rochelle salt) and a powerful alkali, normally sodium hydroxide, is Fehling's B. Solutions A and B are prepared separately.


The deep blue ingredient is the bis(tartrate) complex of Cu2+. Cu2+ is reduced to Cu+ when the aldehyde compound is treated with Fehling's solution, and the aldehyde is reduced to acids. A red precipitate is formed during the reaction.

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3. Tollen’s Test

The Tollens test is a chemical test used to distinguish sugar reducers from non-sugar reducers. This test is known as the silver mirror test. Aldehydes react by giving a grey-black precipitate or a silver mirror to the Tollens reagent. A freshly prepared reagent from Tollen should always be used. In the Tollens reagent, aldehydes are oxidized to the corresponding acid, and silver is reduced from the +1 oxidation state to its elemental form. Ketones usually do not respond to this test.

RCHO + 2[Ag(NH3)2]OH → R-COONH4+ 3NH3 + H2O + 2Ag↓(silver mirror)


4. Test with Chromic Acid

A green to blue precipitate is given by aldehydes reacting with chromic acid. With chromic acid, ketones do not react. Some of the primary and secondary alcohols are also tested.

R-CHO + 2CrO3 + 3H2SO4 → 3R-C(O)-OH + 3H2O + Cr2(SO4)3(green colour)


5. Sodium Nitroprusside Test:

Ketones only give this test and not aldehyde. An anion is formed when ketone reacts with an alkali, further this anion reacts with sodium nitroprusside to form a coloured complex. The red colour shows the presence of ketones. 

CH3 + OH–→ CH3COCH2-+ H2O

[Fe(CN)5NO]2- + CH3COCH2- → [Fe(CN)5NO.CH3COCH2]3-

 

Procedure and Observation:

Tests 

Procedure 

Observation

2,4-Dinitrophenyl Hydrazine Test:

  1. In ethanol, dissolve the given organic compound. 

  2. Add the 2,4-dinitrophenyl hydrazine alcoholic reagent to this solution. 

  3. Shake well the mixture. 

  4. If there is a yellow to the orange precipitate formation, then an aldehyde or ketone in the given compound.

An orange-yellow crystal formed.

Sodium Bisulfite Test:

  1. In a clean test tube, take in a saturated solution of sodium bisulfite. 

  2. To be tested, add 1ml of the given organic compound. 

  3. Shake thoroughly and leave for 15-20 minutes. 

  4. If there is a white precipitate form, then the presence of the carbonyl group is confirmed.

Formation of the crystalline precipitate.


Schiff’s Test:

  1. In a clean test tube, take the given organic compound to be examined. 

  2. Add 2-3 drops of Schiff reagent. 

  3. The presence of aldehyde is confirmed when there is instant pink or red color formation.

Pink, red, or orange color formation.

Fehling’s Test:

  1. By combining equal quantities of Fehling's A solution and Fehling’s B solution, Fehling's solution is prepared. 

  2. In a clean test tube, take the given organic compound. 

  3. Add the solution to it and gently heat the solution. 

  4. If a brick-red precipitate occurs, then the aldehyde presence is confirmed.

Formation of the red precipitate.


Tollen’s Test:

  1. In a clean test tube, take 1ml of the silver nitrate solution. 

  2. Add a dilute solution of sodium hydroxide to it, forming a brown precipitate. 

  3. Add the diluted ammonia solution dropwise until the silver oxide brown precipitate dissolves. 

  4. Add the given organic compound to be tested to this freshly prepared Tollen's reagent. 

  5. Place the test tube for around 5 to 10 minutes in a warm bath of water. 

  6. If the presence of a silver mirror is  on the sides of the test tube, the presence of an aldehyde is confirmed.

Formation of a shiny silver mirror.


Test with Chromic Acid:

  1. In a clean test tube, take the given organic compound. 

  2. To the organic compound, add 1ml of chromic acid reagent. 

  3. The existence of a precipitate of a green or blue color suggests the presence of aldehydes.

Formation of the green or blue precipitate.


Sodium Nitroprusside Test:

  1. In a sterile test tube, dissolve sodium nitroprusside in distilled water. 

  2. Add 1ml of the given organic compound. 

  3. Shake well and apply the dropwise solution of sodium hydroxide. 

  4. If the appearance of red color is present, then the ketone presence is confirmed.

The appearance of red color.

 

Results:

The given organic compound has an aldehyde/ ketone functional group present.


Precautions:

  1. To conduct the experiment, the reagents should be freshly prepared. 

  2. Not directly on the flame to heat the reaction mixture. 

  3. Wash the test tube with nitric acid to destroy the silver mirror after running the Tollen test, since it is explosive material.


Did you know?

Formaldehyde has an odor that is unpleasant. It is difficult to handle in a gaseous state due to its reactivity. Therefore, it is dissolved in water for many applications and marketed as a 37 percent to 40 percent aqueous solution called formalin. Proteins are denatured by formaldehyde, making them insoluble in water and resistant to bacterial decay. For this reason, in embalming solutions and in preserving biological specimens, formalin is used.

FAQ (Frequently Asked Questions)

Question: What is the Importance of Aldehydes?

Answer: It is used as fungicide and insecticide for plant and vegetable, tanning, preserving, and embalming, and as a germicide, but its largest use is to manufacture such polymeric materials.

Question: What are the Uses of a Ketone?

Answer:

  1. Ketone acts as an excellent solvent for many forms of plastics and synthetic fibres. 

  2. Acetone serves as a thinner for painting and a remover for nail polish. 

  3. It is also used for medical purposes, such as the method of chemical peeling and acne treatment.

Question: What Foods Contain High Levels of Aldehydes?

Answer: Acetaldehyde-containing food products: milk, fruit juice, pureed fruit (including infant food), preserved vegetables, soy sauce, vinegar products.

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