Tests for Aldehydes and Ketones

Table of Contents

AimTheoryDistinguishing Tests between Aldehydes and KetonesMaterials RequiredApparatus SetupProcedureObservationsResults and DiscussionPrecautions

Aldehydes and ketones constitute an important class of organic compounds containing the carbonyl group. Aldehyde has the structure RCH(=O) while a ketone has the structure of R₂C(=O). Where R may be an alkyl, alkenyl, alkynyl or aryl group.

Aim:

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

Theory:

Aldehydes and ketones of low molecular weights are volatile compounds. Identification of aldehydes and ketones is based on two types of reactions, addition reaction to the double bond and oxidation reaction.

In aldehydes, the carbonyl group is attached to a hydrogen atom and an aliphatic or aromatic radical. Formaldehyde is an exceptional case in which the carbonyl present in formaldehyde is attached to two hydrogen atoms. In ketones, the carbonyl group is attached to two aliphatic or aromatic group.

The following tests are used to identify the presence of aldehydes and ketones.

1. 2,4-dinitrophenyl hydrazine test
2. Sodium bisulfite test

The difference between ketone and aldehyde is the carbonyl group present in aldehydes can be easily oxidised to carboxylic acids whereas the carbonyl group in ketones are not oxidised easily. This difference in reactivity is the basis for the distinction of aldehydes and ketones. They are generally distinguished by the following tests.

1. Schiff’s test
2. Fehling’s test
3. Tollen’s test
4. Test with chromic acid
5. Sodium nitroprusside test

(a) 2,4-Dinitrophenyl Hydrazine Test:

Aldehydes and ketones react with 2,4-dinitrophenylhydrazine give a yellow to orange precipitate.

The chemical reaction is given below.

Note: Formation of orange-yellow crystals indicates the presence of carbonyl group.

(b) Sodium Bisulfite Test:

Aldehydes and ketones combine with sodium bisulfite to for well-crystallized water-soluble products known as “aldehyde bisulfite” and “ketone bisulfite”.

The chemical reaction is given below.

Note: Formation of crystalline precipitate confirms carbonyl group.

Distinguishing Tests between Aldehydes and Ketones:

(c) Schiff’s Test:

Schiff’s reagent is prepared by passing sulfur dioxide into a solution of the dye fuchsin. The solution becomes colourless due to the formation of an additional product. Aldehydes abstract sulfurous acid from the Schiffs reagent and restores the pink colour. The colouration is due to the formation of complex compound. Ketones, in general, do not respond to this reaction.

The reaction should not be subjected to heat. Some ketones give a light pink colour with Schiff’s reagent therefore light pink colour formation is not a positive test.

Note: Appearance of pink, red or magenta colour indicates the presence of aldehyde group.

(d) Fehling’s Test:

Feling’s solution is a complex compound of Cu²⁺. When aldehyde compound is treated with Fehling’s solution Cu²⁺ is reduced to Cu⁺ and the aldehyde is reduced to acids. During the reaction, a red precipitate is formed.

Aromatic aldehydes do not respond to Fehling’s test. An aqueous solution of the compound may be used instead of an alcoholic solution. Formic acid also give this test.

Note: Appearance of red precipitate confirms the presence of an aldehydic group.

(e) Tollen’s Test: (Silver Mirror Test)

This test is also called the silver mirror test. Tollens reagent consists of silver ammonia complex in ammonia solution. Aldehydes reacts with Tollens reagent gives a grey-black precipitate or a silver mirror. Always a freshly prepared Tollen’s reagent should be used. Aldehydes are oxidised to the corresponding acid and silver in Tollens reagent is reduced from +1 oxidation state to its elemental form. Generally ketones do not respond to this test.

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

Apart from aldehydes some other compounds also respond to Tollen’s test, but the presence of aldehydes is conformed when the given substance shows a positive test for Tollens test but if the given compound passes 2,4-dinitrophenylhydrazine test.

Note: The appearance of shiny silver mirror confirms the presence of aldehydes.

(f) Test with Chromic Acid:

Aldehydes react with chromic acid gives a green to blue precipitate. Ketones do not react with chromic acid. Some of the primary and secondary alcohols also give this test but they do not give dinitrophenylhydrazine test.

The chemical reaction is given below.

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

Note: The appearance of green or blue colour precipitate confirms the presence of aldehydes.

(g) Sodium Nitroprusside Test:

Ketone responds to this test. Ketone reacts with alkali forms an anion further it reacts with sodium nitroprusside forms a coloured complex ion. Aldehydes do not respond to this test.

The chemical reaction is given below.

CH₃COCH₃ + OH^–→ CH₃COCH₂^– + H₂O

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

Note: The appearance of red colouration shows the presence of ketone.

Materials Required:

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

Apparatus Setup:

Procedure:

(a) 2,4-Dinitrophenyl Hydrazine Test:

1. Dissolve the given organic compound in ethanol.
2. To this solution add alcoholic reagent of 2,4-dinitrophenyl hydrazine.
3. Shake the mixture well.
4. If there is a formation of yellow to orange precipitate then the given compound is an aldehyde or ketone.
5. An orange precipitate is obtained from carbonyl compounds in which the C=O groups is conjugated with C=C.

(b) Sodium Bisulfite Test:

1. Take a saturated solution of sodium bisulfite in a clean test tube.
2. Add 1ml of the given organic compound to be tested.
3. Shake well and leave it for 15-20 minutes.
4. If there is a formation of white precipitate then the presence of the carbonyl group is confirmed.

(c) Schiff’s Test:

1. Take the given organic compound to be tested in a clean test tube.
2. Add 2-3 drops of Schiff’s reagent.
3. If there is instant pink or red colour formation then the presence of aldehyde is confirmed.

(d) Fehling’s Test:

1. Fehling’s solution is prepared by mixing equal amounts of Fehling’s A and Feling’s B solution.
2. Take the given organic compound in a clean test tube.
3. Add Fehling’s solution to it and heat the solution gently.
4. If a brick-red precipitate appears then the presence of aldehyde is conformed.

(e) Tollen’s Test: (Silver Mirror Test)

1. Take 1ml of silver nitrate solution in a clean test tube.
2. Add dilute sodium hydroxide solution to it, a brown precipitate forms.
3. Add dilute ammonia solution dropwise till the brown precipitate of silver oxide dissolves.
4. To this freshly prepared Tollen’s reagent add the given organic compound to be tested.
5. Place the test tube in a warm water bath for about 5 to 10 minutes.
6. If there is the appearance of a silver mirror in the sides of the test tube conforms the presence of an aldehyde.

(f) Test with Chromic Acid:

1. Take the given organic compound in a clean test tube.
2. Add 1ml of chromic acid reagent to the given organic compound.
3. The appearance of a green or blue colour precipitate indicates the presence of aldehydes.

(g) Sodium Nitroprusside Test:

1. Dissolve sodium nitroprusside in distilled water in a clean test tube.
2. Add 1ml of the given organic compound to be tested.
3. Shake well and add sodium hydroxide solution dropwise.
4. If there is the appearance of red colour then the presence of ketone is conformed.

Observations:

Results and Discussion:

The given organic compound has ___________ (aldehyde/ketone) functional group.

Precautions:

1. The reagents should be freshly prepared to perform the test.
2. Not to heat the reaction mixture directly on the flame.
3. After performing the Tollen’s test wash the test tube with nitric acid to destroy the silver mirror, because it’s an explosive substance.

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