Tests for Amino Group [—NH3]: Procedure And Results Interpretation

Amines

Amines may be considered as substitution products of ammonia. When one hydrogen atom of ammonia is replaced with an alkyl or aryl group,the resulting amine is called primary amine (R—NH₂). When two hydrogen atoms of ammonia molecule are replaced with two alkyl or aryl groups, the resulting compound is called secondary amine. The replacement of all the three hydrogen atoms of ammonia with alkyl and aryl groups gives tertiary amine.

There are 5 tests that can be carried out for amino functional group:

• Solubility Test
• Litmus Test
• Carbylamine
• Nitrous Acid Test
• Azo-Dye Test

Materials Required

1. Aniline
2. Potassium hydroxide
3. Sodium nitrite solution
4. Chloroform
5. Dilute hydrochloric acid
6. Concentrated hydrochloric acid
7. Sodium hydroxide solution
8. Benzene sulfonyl chloride
9. β-Naphthol
10. Test tubes
11. Test tube holder
12. Bunsen burner

Solubility Test

Amines are the organic compounds that have appreciable basicity and so they dissolve in mineral acids  like hydrochloric acid.

Procedure

1. Take the given organic compound to be tested in a clean test tube.
2. Add 2 to 3 drops of concentrated hydrochloric acid.
3. Shake well the solution and observe the solubility.

Note: If it is soluble in mineral acid then it may be an amine.

 Litmus Test

Amines have appreciable basicity and turn red litmus blue.Amines are basic in nature.This test is also not a confirmation test for amines.

Procedure

1. Take the moist red litmus paper.
2. Place one to two drops of given organic compound on it.
3. Observe the change in the colour of red litmus paper.

Note: The change in the colour of red litmus paper shows the given organic compound is a base. (may be amine)

Carbylamine Test

This test is also called isocyanide test, because when amines are treated with chloroform in the presence of alkali, isocyanide is formed. Both aliphatic and aromatic amines gives positive for this test. Secondary and tertiary amines do not give this test.

Procedure

1. Take potassium hydroxide solid in a clean test tube and add 2ml of ethanol to it.
2. Warm the test tube until the potassium hydroxide dissolves completely in it.
3. To this add a few drops of chloroform and shake well.
4. To the above mixture add a small amount of given organic compound to be tested and warm gently.
5. Observe the odor of the resulting mixture.

Note: An unpleasant odor confirms the presence of primary amine.

Caution
1. Carpytomine (isocyanide) is highly poisonous so do not inhale its vapours.
2. Destroy carbylamine with cdnc. HCl after the test and then throw into sink.

Nitrous Acid Test

This test can be used to identify primary, secondary and tertiary amines. An aromatic primary amine reacts with nitrous acid forms a diazonium salt and at higher temperatures the formed diazonium compounds undergoes decomposition.

Procedure

1. Dissolvethe given organic compound in concentrated hydrochloric acid and water in a clean test tube.
2. Cool the solution in ice bath for 0-5^oC.
3. To the cooled solution add sodium nitrite dropwise and mix well.
4. Test the solution for the presence of free nitrous acid.
5. Add excess of nitrous acid solution if there is an absence of nitrous acid.

Note: Appearance of bubbles shows the presence of primary amines, yellow oily liquid shows the presence of secondary amines and formation of nitrite salts conforms the presence of tertiary amines.

Azo-Dye Test

This test is performed in order to confirm the presence of aromatic amines. Aromatic amines forms a diazonium salts when treated with HNO₂. These diazo compounds couple with phenols like β-naphthols forms an orange azo dye which is insoluble in water.

Procedure

1. Simultaneously take three test tubes and mark it as A, B and C.
2. In test tube A dissolve the given organic compound in 2ml of dilute hydrochloric acid.
3. In test tube B prepare aqueous solution of sodium nitrite solution.
4. In test tube C dissolve naphthol in dilute sodium hydroxide.
5. Place all the three test tubes in an ice bath.Now add the contents in test tube B to the mixture in test tube A.
6. The resulting mixture is added to the contents in test tube C.
7. Stir the mixture with a glass rod and observe the changes in the test tube.

Note: The presence of red orange dye conforms the presence of aromatic amines.

Distingushing Test for Primary , Secondary and Tertiary Amines

 Hinsberg Test

The reagent used in this test is benzene sulfonyl chloride. Amines reacts with benzene sulfonyl chloride in the alkaline medium.

Procedure

1. Take the given organic compound to be tested in a clean test tube.
2. Add 2ml of sodium hydroxide solution and 1ml of benzene sulfonyl chloride to it.
3. Shake the mixture well and add water to dilute the solution.
4. Cool the mixture under tap water and observe the formation of precipitate.
5. If precipitate formed then treat it with concentrated hydrochloric acid.

Results

1. Tertiary amine. Precipitate in the test-tube. It dissolves in cone. HCl.
2. Secondary amine. Precipitate in the test-tube. It does not dissolve in cone. HCl.
3. Primary amine. No precipitate (clear solution). On addition of cone. HCl, insoluble material is seen.

Summary Of Tests For Amino Functional Group