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Tests for aldehydes and ketones are done by performing oxidation tests. Aldehydes and ketones are important functional groups in organic chemistry. The general formula of aldehydes is RCH(=O) and of ketones is R2C(=O). Different tests can be performed to identify the presence as well as to distinguish between aldehydes and ketones. The two main test used to detect aldehydes and ketones are
- 2,4-Dinitrophenylhydrazine Test
- Sodium Bisulfate Test
| Table of Content |
Key Terms: Aldehydes, Ketones, Oxidation Reactions, Double Bond, Addition Reactions, Sodium Bisulfate test, 2,4-Dinitrophenylhydrazine Test
Aim
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To identify whether the given organic compound contains ketone or aldehyde functional groups.
Requirements
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Chemicals required for tests for aldehydes and ketones:
- Schiff's reagent
- Fehling's solution
- 2,4-Dinitrophenyl hydrazine reagent
- Silver nitrate solution
- Ammonium hydroxide solution
- Sulfuric acid
- Chromic acid
- Sodium bisulfate
Apparatus required for tests for aldehydes and ketones:
- Beakers
- Test tube holder
- Test tube stand
- Test tubes
Also Read:
| Related Articles | ||
|---|---|---|
| Oxidation and Reduction | Nucleophilic Addition Reaction | Etard Reaction |
| Decomposition Reaction | Displacement Reaction | Physical properties of Aldehydes, Ketones and Carboxylic acids |
Theory
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Aldehydes and ketones are compounds having low molecular mass. Because of that, they are highly volatile. These functional groups undergo two types of reactions:
- Oxidation reactions
- Double bond addition reactions
In aldehydes, the carbonyl group is attached to an aromatic or aliphatic radical on one side and a hydrogen-atom on the other. The only exception to this is formaldehyde in which hydrogen atoms are present on both sides. Aldehydes are easy to oxidize. They form carboxylic acids on oxidation.
In ketones, two aromatic or aliphatic groups are attached to the carbonyl group. Ketones are not easy to oxidize because of the presence of two aromatic or aliphatic groups on either side.
Tests for Identification of Aldehydes & Ketones
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Aldehydes and ketones can be identified only with oxidation reactions and the addition of double bonds. The two tests that can be used for the identification of aldehydes and ketones are:
- 2,4-Dinitrophenylhydrazine Test
- Sodium Bisulfate Test
2,4-Dinitrophenylhydrazine Test
Aldehydes and ketones react with the reagent 2,4-Dinitrophenyl Hydrazine to give a yellow-orange precipitate.
Chemical Reaction
Procedure
- Dissolve the compound in ethanol.
- To this add alcoholic 2,4-Dinitrophenyl Hydrazine.
- Shake this reaction mixture well.
- Check whether precipitate is formed and note the colour of that precipitate.
Observation and Conclusion
The formation of yellow-orange precipitate confirms the presence of a carbonyl group in the given organic compound.
Sodium Bisulfate Test
Aldehydes and ketones react with sodium bisulfate (NaHSO3) to form an aldehyde bisulfite or ketone bisulfite. The products are well crystallized and perfectly soluble in water.
Reaction
Procedure
- Take a clean test tube and pour a small quantity of saturated solution of sodium bisulfate into it.
- To the same test tube, add 1 ml of the compound that is to be analyzed.
- Shake the test tube well and leave it undisturbed in the test tube stand for 15 to 20 minutes.
- Check whether a crystalline precipitate is formed.
Observation and Conclusion
The formation of a crystalline precipitate confirms the presence of the carbonyl group in the given organic compound.
Tests for Distinguishing Between Aldehydes & Ketones
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Aldehydes and ketones react differently with different reagents. Aldehydes are easier to oxidize than ketones. This reactivity difference can be used to distinguish between aldehydes and ketones. The following tests can be used for this purpose:
- Schiff's Test
- Fehling's Test
- Tollen's Test
- Chromic Acid Test
- Sodium Nitroprusside Test
Schiff's Test
Freshly prepared Schiff's reagent is treated with aldehydes and ketones to give a pink coloured complex.
Schiff’s Reagent
Schiff's reagent is prepared by passing the sulfur dioxide gas in a solution of the fuchsin dye or Rosaline hydrochloride having molecular formula C20H20N3·HCl. The reagent is colourless.
Procedure
- Take a clean and dry test tube and in it take a small quantity of the compound that is to be analyzed.
- To this test tube, add 2 to 3 drops of the Schiff's reagent.
- Keep an eye out for the colour change.
Observation and Conclusion
If the aldehyde functional group is present, then the product complex takes a pink, magenta, or red colour. This happens because aldehydes can extract sulfurous acid from Schiff's reagent which makes the product complex pink in colour.
Schiff’s Test Result
If the ketone functional group is present, then the product complex either remains colourless or takes a light pink colour.
Read More: Methods of Preparation of Diazonium Salts
Fehling's Test
This test is only for the confirmation of the presence of an aldehyde functional group.
An aldehyde is treated with Fehling's solution. The Cu2+ in the solution gets reduced to Cu+ forming a red coloured precipitate. The aldehyde is reduced to form an acid.
Reaction
Procedure
- Take a clean test tube and in it take a small quantity of the compound that is to be analyzed.
- To the same test tube, add Fehling's solution.
- Lightly heat this test tube.
- Observe the colour change in the test tube.
Observation and Conclusion
The formation of red coloured precipitate confirms the presence of the aldehyde functional group in the given organic compound.
Tollen's Test
Tollen's test is also known as the silver mirror test. The ketone functional group does not participate in this test. Therefore, this test is only used to confirm the presence of the aldehyde functional group.
An aldehyde is treated with Tollen's reagent to form a silver mirror or a grey-black precipitate.
Reaction
RCHO + 2(Ag(NH3)2)OH \(\rightarrow\)R-COONH4 + 3NH3 + H2O + 2Ag\(\downarrow\)
Procedure
- Take Tollen's reagent in a clean test tube.
- To this add a small quantity of the compound that is to be analyzed.
- Prepare a warm water bath.
- Plan the test tube containing the reaction mixture in this water bath for 5 to 10 minutes.
- Do not disturb the test tube during this time.
- Observe the colour change in the test tube.
Observation and Conclusion
The formation of a silver mirror or grey-black precipitate confirms the presence of an aldehyde functional group in the given organic compound.
Chromic Acid Test
The ketone functional group does not participate in this test. An aldehyde is treated with chromic acid to give a blue or green coloured precipitate.
Reaction
R-CHO + 2CrO3 + 3H2SO4 \(\to\) 3R-C(O)-OH + 3H2O + Cr2(SO4)3
Procedure
- Take a clean test tube and in it take a small quantity of the compound that is to be analyzed.
- To the same test tube, add 1 mL of chromic acid.
- Observe the formation of precipitate and note its colour.
Observation and Conclusion
The formation of blue or green precipitate confirms the presence of the aldehyde functional group in the given organic compound.
Sodium Nitroprusside Test
Aldehyde functional group does not participate in this test.
A ketone is treated with an alkali to form an anion. This anion is treated with sodium nitroprusside to form a red coloured complex.
Reaction
CH3COCH3 + OH– –→ CH3COCH2– + H2O
(Fe(CN)5NO)2– + CH3COCH2– → (Fe(CN)5NO.CH3COCH2)3-
Procedure
- Take a clean test tube and in it a small quantity of sodium nitroprusside.
- Dissolve the sodium nitroprusside in distilled water.
- To this, add 1 ml of the compound that is to be analyzed.
- Shake the test tube well.
- Then dropwise add a solution of sodium hydroxide.
- Observe the colour change.
Observation and Conclusion
The formation of red precipitate confirms the presence of the ketone functional group in the given organic compound.
Sodium Nitroprusside Test
Precautions
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While conducting tests for aldehydes and ketones following precautions are mandatory:
- All reagents used for performing the tests should be freshly prepared.
- Direct contact between the reaction mixture and the flame should be avoided.
- The silver mirror, if formed during Tollen's test, should be destroyed with the help of nitric acid since it is an explosive material.
Things to Remember
- Aldehydes and ketones show two types of reactions: Oxidation reactions and Double bond addition reactions
- 2,4-Dinitrophenylhydrazine Test and Sodium Bisulfate Test are used for identification of aldehydes and ketones.
- Aldehydes and Ketones can be distinguished by various tests like Schiff's Test, Fehling's Test, Tollen's Test, Chromic Acid Test and Sodium Nitroprusside Test
- All the tests should be performed in clean apparatus and with freshly prepared solutions.
Also Read:
Previous Year Questions
Sample Questions
Ques. How is Fehling's solution prepared? (3 Marks)
Ans. Fehling's solution is prepared by mixing Fehling's A solution and Fehling's B solution. Fehling's A solution is a solution of copper (II) sulfate. It is blue in colour. Fehling's B solution is a solution of a strong alkali such as sodium hydroxide (NaOH) and Rochelle's salt (potassium sodium tartrate). It is a colourless solution. Both Fehling's A and Fehling's B solutions are prepared separately and mixed together at the end to form Fehling's solution.
Ques. How is Tollen's reagent prepared? (3 Marks)
Ans. To prepare the Tollen's reagent, a clean test tube is required. 1 mL of silver nitrate solution is added to this test tube. To the same test tube, a dilute solution of sodium hydroxide is added. On addition, a colour change from colourless to brown is observed. This is due to the formation of brown coloured silver oxide. Again to the same test tube, dropwise add a dilute solution of ammonia till all the silver oxide is dissolved. And thus the Tollen's reagent is formed.
Ques. What is Tollen's reagent? (1 Mark)
Ans. Tollen's reagent is ammoniacal silver nitrate. It has the molecular formula (Ag(NH3)2)NO3
Ques. Give one test for distinguishing between aldehydes and ketones. (2 Marks)
Ans. The silver mirror test can be used for distinguishing between aldehydes and ketones. It is also known as Tollen’s test since it uses Tollen’s reagent. It uses the fact that aldehydes are more readily oxidized than ketones.
Ques. What is Schiff's reagent used for? (2 Marks)
Ans. Schiff's reagent is an important reagent used in many chemical tests. It is usually used to distinguish between aldehyde and ketone functional groups.
Ques. Give uses of ketones. (3 Marks)
Ans. Ketones are used in:
- As solvents in manufacturing synthetic fibers and plastics.
- In nail polish removers
- As tinners in painting
- Treatment of acne
- Chemical peeling
Ques. Why are aldehydes important? (3 Marks)
Ans. Aldehydes are important in a number of places like;
- They are a vital part of insecticides, germicides, and fungicides.
- They are used in embalming, preserving, and also tanning.
- One of the largest uses of aldehydes is in the polymers manufacturing industry.
Ques. Give a list of foods containing a high concentration of aldehydes. (1 Mark)
Ans. Milk, pureed fruit, fruit juice, preserved vegetables, vinegar, soy sauce, etc contain a high concentration of aldehydes.
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