Test for Amino Groups

The amino group is one of the most important functionalities in organic synthesis and in nature. Amines are a class of organic compounds that contain the functional group -NH2. The structure of amine is R-NH2, where R in the case of aliphatic amine is an alkyl group or for aromatic amines is an aryl group.

Aim:

To find the presence of amine functional group in the given organic compound.

Theory:

Amines are classified as primary, secondary and tertiary amines. Primary amines is the one in which the nitrogen atom is bonded to one hydrocarbon atom and two hydrogen atoms. Secondary amines is bonded with two hydrocarbon atom and one hydrogen atom. Tertiary amines are amines in which all the hydrogen atoms are replaced by hydrocarbon atoms.

Classification of Amines

The following are the tests carried to identify the presence of amino group present in the organic compound.

  1. Solubility test
  2. Litmus test
  3. Carbylamine test
  4. Nitrous acid test
  5. Azo-dye test
  6. Hinsberg test

(a) Solubility Test:

Amines are organic compounds which is basic in nature, so they dissolve in mineral acids like hydrochloric acid. But this test is not a confirmation test for amines.

The chemical reaction is given below.

C6H5NH2 + HCl → C6H5NH3+Cl

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

(b) Litmus Test:

Amines are basic in nature. So it turns red litmus paper blue. This test is also not a confirmation test for amines.

The chemical reaction is given below.

CH3-CH2-NH2 + H2O → CH3-CH2-NH3+ + OH

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

(c) 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.

The chemical reaction is given below.

R-NH2 + 3KOH + CHCl3 → RNC(isocyanide) + 3KCl + 3H2O

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

(d) 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.

C6H5NH2 + HNO2 → C6H5-N=N+Cl(diazonium compound)

Primary aliphatic amines react with nitrous acid to give nitrogen gas, which is seen as bubbles.

R-NH2 + HONO → R-OH + H2O + N2

Secondary amines react with nitrous acid to form a yellow oily nitrosamine.

R2-NH + HONO → R2N-NO + H2O

Tertiary amines react with nitrous acid to form soluble nitrite salts.

R3N + HONO → R3NH+ON

Note: Nitrosamines are highly carcinogenic substances.

(e) 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.

The chemical reaction is given below.

C6H5NH2 + HNO2 → C6H5-N=N+Cl

Azo-Dye Test

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

Distinguishing Test for Primary, Secondary and Tertiary Amines

(f) Hinsberg Test:

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

Primary amines reacts with benzene sulfonyl chloride to produce substituted sulfonamide which contains an acidic hydrogen and dissolve in basic medium.

Hinsberg Test for Primary Amines

A secondary amine forms a substituted sulfonamide which is insoluble in alakali because it does not have acidic hydrogen.

Hinsberg Test for Secondary Amines

A tertiary amine forms a substituted sulfonamide which is insoluble in alkali because it does not have an acidic hydrogen.

Hinsberg Test for Tertiary Amines

Note:

  • Tertiary amines: Precipitate in the test tube but dissolves in concentrated HCl.
  • Secondary amines: Precipitate in test tube but does not dissolve in concentrated hydrochloric acid.
  • Primary amines: No reaction or precipitate, on adding concentrated hydrochloric acid insoluble material is seen.

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

Procedure:

(a) Solubility Test:

  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.

(b) Litmus Test:

  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.

(c) Carbylamine Test:

  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.

(d) Nitrous Acid Test:

  1. Dissolvethe given organic compound in concentrated hydrochloric acid and water in a clean test tube.
  2. Cool the solution in ice bath for 0-5oC.
  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.

(e) Azo-Dye Test:

  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.

(f) Hinsberg Test:

  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.

Observations:

Solubility test If it is soluble in mineral acid then it may be an amine
Litmus test It turns red litmus paper blue
Carbylamine test An unpleasant odor confirms the presence of primary amine.
Nitrous acid test 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 The presence of red orange dye conforms the presence of aromatic amines.
Hinsberg test Primary amines: No reaction or precipitate, on adding concentrated hydrochloric acid insoluble material is seen.

Secondary amines: Precipitate in test tube but does not dissolve in concentrated hydrochloric acid.

Tertiary amines: Precipitate in the test tube but dissolves in concentrated HCl.

Results and Discussions:

The given organic compound contains ___________ [aliphatic or aromatic (primary/secondary/tertiary)] amines.

Precautions:

  1. Isocyanides are highly poisonous, do not smell the gas directly from the mouth of the test tube.
  2. Temperature for azo dye test should be maintained between 0-5oC.

Viva Questions:

1. Name the test used to distinguish between ethylamine and diethylamine.

Ans: The test conducted by Hinsberg is used to recognize the difference between ethylamine and diethylamine. When the aqueous potassium hydroxide and benzene sulphonyl chloride are treated with ethyl amine, it provides a clear solution (potassium salt) as the output of the reaction.

2. Why are diazonium chloride soluble in water?

Ans: Aryl diazonium salts are ionic crystalline solids that are colorless. That is why it is soluble in polar solvents like water.

3. How isocyanides are destroyed in carbylamine test?

Ans: Isocyanides are destroyed by adding concentrated hydrochloric acid and thrown the mixture into sink.

4. Name the test used to distinguish between primary, secondary and tertiary amines?

Ans: Hinsberg test is used to distinguish between primary, secondary and tertiary amines.

5. Name the reagent used in Hinsberg test?

Ans: The reagent used in Hinsberg test is benzene sulfonyl chloride.