Tests for Aldehydic and Ketonic Group [—CHO & —CO—]

Aldehydes and Ketones

Aldehydes and ketones have a carbonyl group (C=O) as a functional group. A ketone has two alkyl or aryl groups attached to the carbonyl carbon (RCOR’). The simplest ketone is acetone, which has two methyl groups attached to the carbonyl carbon (CH₃COCH₃).

An aldehyde is similar to a ketone, except that instead of two side groups connected to the carbonyl carbon, they have at least one hydrogen (RCOH). The simplest aldehyde is formaldehyde (HCOH), as it has two hydrogens connected to the carbonyl group. All other aldehydes have one hydrogen bonded to the carbonyl group, like the simple molecule acetaldehyde, which has one hydrogen and one methyl group (HCOCH₃).

The carbonyl carbon in both aldehydes and ketones is electrophilic, meaning that it has a dipole due to the electronegativity of the attached oxygen atom. This makes the carbonyl carbon an ideal target for nucleophiles in a nucleophilic addition reaction. During this reaction, the nucleophile, or electron donor, attacks the carbonyl to form the tetrahedral intermediate. The negatively charged oxygen accepts a hydrogen ion to form a hydroxyl group.

Typically, nucleophiles possess a negative charge or lone pair on a heteroatom, which can take several forms (OH^–, RO^–, CN^–, R₃C^–, RNH₂, ROH). For primary amines (RNH₂), the reaction does not stop at the formation of the tetrahedral intermediate with a hydroxyl group. Rather, an elimination reaction occurs that produces a double-bonded carbon and nitrogen functional group known as an imine. Understanding the reactions that aldehydes and ketones can undergo provides a way to differentiate between these similar organic compound types.

This group has characteristic properties which are shown by both classes of compounds. In aldehydes, the carbonyl group is attached to a hydrogen atom and to an aliphatic or aromatic radical. Formaldehyde is an exception in which the carbonyl group is attached to two hydrogen atoms.

The carbonyl group in aldehydes and ketones is identified by the following tests:
1. 2, 4-dinitrophenyl hydrazine test.
2. Sodium bisulphite 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

Tests for Aldehydic and Ketonic [—CHO and —CO—] Groups

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

2, 4-Dinitrophenyl hydrazine Test (2, 4—DNP Test)

Procedure

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.

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

Sodium bisulphite Test

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

Procedure

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.

The chemical reaction

Formation of crystalline precipitate confirms carbonyl group

Note

1. Formaldehyde, acetaldehyde, benzaldehyde, acetone, methyl ethyl ketone and diethyl ketone give position tests.
2. Aqueous solutions of formaldehyde and acetaldehyde form addition products but as they are highly soluble , precipitates are rarely formed.
3. Acetophenone and benzophenone do not give this test.

Differentiating Tests for Aldehydes

The following tests are given by aldehydes but not by ketones:

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.

Procedure

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.

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

Note:

1. The Schiffs reagent should not be warmed.
2. The Schiff s reagent should not be treated with alkalies. Otherwise the pink colour develops even in the absence of aldehydes.
3. With benzaldehyde the pink colour develops slowly.

 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.

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.

Procedure

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.

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

Note:

1. Many other types of compounds give positive silver mirror test but they do not give 2,4-dinitro phenyl hydrazine test.
2. Formic acid, tartaric acid and many carbohydrates like glucose give silver mirror test.

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.

Procedure

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.

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

Note:
1. Benzaldehyde may or may not give this test as the reaction is very slow.
2. Formic acid also gives this test.

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.

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

Procedure

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.

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

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.

Here is the chemical reaction

CH₃COCH₃ + OH^–→ CH₃COCH₂^– + H₂O

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

Procedure

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.

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

Differentiating Tests for Ketones

The following tests are given by ketones but not by aldehydes:

m-Dinitrobenzene test

Place 0.5 ml of the given liquid (or 0.5 g of solid) in a clean test-tube and add about 0.1 g of finely powdered m-dinitrobenzene. Now add about 1 ml of dilute sodium hydroxide solution and shake.
Appearance of violet colour which slowly fades confirms ketonic group.
Note: Benzophenone does not give this test.

Sodium nitroprusside test

The anion of the ketone formed by an alkali reacts with nitroprusside ion to form coloured ion.

Procedure
Dissolve a crystal of sodium nitroprusside in about 1 ml of distilled water in a clean test-tube and then add 0.5 ml (or 0.5 g) of the given compound. Shake and add sodium hydroxide solution drop wise.
A red colouration indicates the ketonic group.
Note:
1. Benzaldehyde also gives red colour,
2. Benzophenone does not give this test.

Preparation of Reagents
1. 2, 4-Dinitro phenylhydrazine (2, 4-DNP). Dissolve 1 g of 2, 4-dinitrophenyl hydrazine in 50 ml methanol to which 2 ml of concentrated sulphuric acid is added. Filter, if necessary.
2. Sodium bisulphite, NaHSO₃. Dissolve about 30 g sodium bisulphite in 100 ml of distilled water.
3. SchifFs reagent. Dissolve 0.1 g p-rosanaline hydrochloride in 100 ml water and pass sulphur dioxide gas until its red colour is discharged. Filter and use the filtrate.
4. Fehling’s solution A. Dissolve 35 g of crystalline copper sulphate in 500 ml water and add 2 ml cone. H₂S0₄
5. Fehling’s solution B. Dissolve 173 g of Rochelle salt (sodium potassium tartrate) and 60 g sodium hydroxide in 500 ml water.

Summary Of Tests For Aldehydes and Ketones