# 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 is used as an anaesthetic in dentistry. Quaternary ammonium salts are used as surfactants. Here, we will discuss the functional group amine teast, 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

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.

1. 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 is used for secondary amines.

1. 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.

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.

1. 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.

$CS_2+3Cl_2\rightarrow CCl_4+S_2Cl_2$

1. 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.

1. Oxidation with Caro’s Acid or H2O2

This test is used for testing an aromatic amino group. Aromatic amines, because of the high electron density on the benzene ring,  are readily oxidised on exposure to air or oxidising 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.

1. 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.

1. Solubility Test

Amines are polar organic compounds with one lone pair. The availability of lone pairs 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-

1. 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

 S. No Test Primary Amines Secondary Amines Tertiary Amines Anilline 1 Carbylamine test Formation of isocyanide results in a foul smell. No reaction No reaction Formation of isocyanide results in a foul smell. 2 Libermann's Nitroso Reaction No reaction Forms yellow oily nitroso amines. No reaction No reaction 3 Reaction with Aryl Sulphonyl Chloride The product formed is soluble in alkali. The product formed is not soluble in alkali. No reaction The product formed is soluble in alkali. 4 Reaction with Carbon Disulphide Formation of isothiocyanates results in the mustard oil smell. No reaction No reaction Formation of isothiocyanates results in the mustard oil smell. 5 Azo Dye Test for Amines No reaction No reaction No reaction Produces yellow and orange dyes. 6 Oxidation with Caro's Acid or H2O2 No reaction No reaction No reaction Produces aniline black. 7 Halogenation Reaction No reaction No reaction No reaction Produces white ppt. 8 Solubility Test Soluble in acid Soluble in acid Soluble in acid Sparingly soluble in acid. 9 Litmus Test Turns litmus blue Turns litmus blue Turns litmus blue Turns litmus blue

### Did You Know?

A few of the interesting points about the amines are listed below.

• 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.

## FAQs on Test for Amino Groups

1. What is the 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. 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.

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

2. How Can You Differentiate Between Primary, Secondary, and Tertiary Amines?

primary, secondary, and tertiary amines can be distinguished by performing Hinsberg’s test.

• 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.

3. Write Two Tests that Can be Used to Distinguish Between Ethanamine and Aniline?

Two tests that can be used to distinguish between ethanamine and aniline are given below:

1. Oxidation with Caro’s Acid or H2O2 - This test is used for testing an aromatic amino group. Aromatic amines, because of the high electron density on the benzene ring,  are readily oxidised on exposure to air or oxidising agents forming a complex coloured product. While ethanamine will not give this reaction.

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

1. 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. While ethanamine will not give any reaction.

4. What is the name of the test that is used to differentiate between ethylamine and diethylamine?

Hinsberg's test is used to determine the difference between ethylamine and diethylamine. The reaction produces a clear solution (potassium salt) when aqueous potassium hydroxide and benzene sulphonyl chloride are treated with ethylamine.

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