Test for Amino Groups

What are Amines?

Amines are regarded as derivatives of ammonia in which one, two, or all three hydrogen atoms are replaced by alkyl or aryl groups. Amines constitute an important class of organic compounds. They occur widely throughout both plants and animals. They are found among proteins, vitamins, alkaloids, hormones, etc. synthetic examples include polymers, drugs, dyestuffs, etc. these amines find extensive uses. For example, quinine is an important antimalarial drug, adrenaline and ephedrine are used for increasing blood pressure, novacin used as an anesthetic in dentistry. Quaternary ammonium salts are used as surfactants. Here, we will discuss the functional group amine test, qualitative test for aliphatic amines, and aromatic amines.

Classification of Amines

The amines are classified as primary, secondary, or tertiary according to one, two, or three hydrogen atoms of ammonia molecule are replaced by alkyl or aryl groups in ammonia molecule. If one hydrogen atom of ammonia is replaced by an alkyl or aryl group, we get RNH2 or ArNH2, a primary amine. If two hydrogen atoms of ammonia are replaced by an alkyl or aryl group, we get secondary amine. If three hydrogen atoms of ammonia are replaced by an alkyl or aryl group, we get tertiary amine. 

Qualitative Test for Amines

The following tests are used for the identification of amino groups.

1. Carbylamine Test

This test is also known as the isocyanide test. Aliphatic and aromatic primary amines when warmed with chloroform and an alcoholic solution of KOH, forms isocyanide or carbylamine which have a very unpleasant or foul smell.

R-NH2 + CHCl3 + 3KOH (alc.) → R-NC + 3KCl + 3H2O

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Secondary and tertiary amines do not give this test. This reaction is a  test for primary amine. Therefore, this test can be used to distinguish between primary amines from secondary and tertiary amines.

2. Libermann’s Nitroso Reaction

Secondary aliphatic and aromatic amines react with nitrous acid slowly in the cold to form yellow oily nitroso amines. The yellow oily nitrosamine gives a green solution when warmed with phenol and conc. Sulphuric acid. On dilution with water, the colour changes to greenish-blue to violet on the addition of sodium hydroxide. The overall reaction is called Libermann’s nitroso reaction. This test used for secondary amines. 

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3. Reaction with Aryl Sulphonyl Chloride

Benzene sulphonyl chloride which is also known as Hinsberg’s reagent reacts with primary and secondary amines to form sulphonamides. 

• Primary amines react with benzene sulphonyl chloride to give N-alkyl benzene sulphonamide. The hydrogen attached to nitrogen in sulphonamide is strongly acidic due to the presence of a strongly electron-withdrawing sulphonyl group. Therefore, it is soluble in alkali. On acidification, it gives an insoluble material.

• Secondary amines react with benzene sulphonyl chloride to form N, N-dialkyl benzene sulphonamide. Since the sulphonamide does not contain any hydrogen atom attached to a nitrogen atom, so it is not acidic. Hence it is not soluble in alkali.

• Tertiary amines do not react with benzene sulphonyl chloride.

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Since different amines react differently with benzene sulphonyl chloride, this reaction can be used for the distinction between primary, secondary, and tertiary amines and also for the separation of their mixtures.

4. Reaction with Carbon Disulphide

Primary amines react with carbon disulfide to form dithioalkyl carbamic acids which decompose on heating with mercuric chloride to give alkyl isothiocyanates. These have a characteristic smell like mustard oil. This reaction is called the Hoffmann mustard oil reaction and is used as a test for primary amines.

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5. Azo Dye Test for Amines

Aromatic amines react with diazonium salts to form azo compounds in an acidic medium called dyes. This test is called the azo dye test and this reaction is known as coupling or diazo reaction.

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6. Oxidation with Caro’s Acid or H2O2

This test is used for testing an aromatic amino group. Aromatic amines, because of high electron density on the benzene ring,  are readily oxidized on exposure to air or oxidizing agents forming a complex coloured product.

C₆H₅NH₂ + K₂Cr₂O₇ + H₂SO₄ → a black product called aniline black.

However, controlled oxidation of aniline with potassium dichromate and sulphuric acid gives p-benzoquinone.

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7. Halogenation Reaction

This test can be used for testing an aromatic amino group. Aniline reacts with bromine water readily to give a white precipitate of 2,4,6-tribromoaniline. 

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8. Solubility Test

Amines are polar organic compounds with one lone pair. Availability of lone pair in amines makes it basic in nature, so they can be dissolved in mineral acids. This test is not a confirmatory test for amines.

The chemical reaction that shows the basic nature of amines is given below.

C6H5NH2 + HCl → C6H5NH3+Cl–

9. Litmus Test

As amines are basic in nature, they have a tendency to turn litmus paper to blue. This test does not confirm the presence of the amine group. It confirms the basic nature of the amine group. 

Summary

Did You Know?

• Aliphatic amines of low molecular weight are used as solvents.

• Amines are used as intermediates in drug manufacture and as reagents in organic synthesis.

• Aromatic amines are used for the manufacture of polymers, dyes, and as intermediates for additives in the rubber industry.