Test for Amines: Amines are the class of organic compounds that contain the \( – {\text{N}}{{\text{H}}_2}\) functional group. The general structure of amine is \({\text{R}} – {\text{N}}{{\text{H}}_2},\) where \({\text{R}}\) is an alkyl group (aliphatic amine) or \({\text{AR}} – {\text{N}}{{\text{H}}_2}\) for aromatic amines where \({\text{Ar}}\) is an aryl group. Amines can be classified as primary, secondary, and tertiary amines. Primary amines consist of a nitrogen atom bonded to one hydrocarbon atom and two hydrogen atoms.

Secondary amines consist of two hydrocarbon atoms and one hydrogen atom. Tertiary amines are amines in which hydrocarbon atoms replace all the hydrogen atoms. In this article, we will learn about different types of tests conducted to check if a compound has amine or not.

Test for Amines

An amine functional group contains a lone pair on a nitrogen atom. Amines are structurally similar to ammonia in that nitrogen can link up to three hydrogen atoms. It is also distinguished by a number of features depending on carbon connection.

The following tests were performed to determine the presence of an amino group in an organic molecule:

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

Different Types of Qualitative Test for Amines

To test the presence of the amino group in organic compounds, the following tests are performed:

1. Solubility and Litmus Test

Amines are organic compounds that are basic in nature, so they dissolve in mineral acids like hydrochloric acid. They turn red litmus paper blue. But this test is not a confirmation test for amines.

\({{\text{C}}_2}{{\text{H}}_5}{\text{N}}{{\text{H}}_2} + {{\text{H}}_2}{\text{O}} \to {{\text{C}}_2}{{\text{H}}_5}{\text{NH}}_3^ + + {\text{O}}{{\text{H}}^ – }\)

2. Carbylamine Test

The Carbylamine test is also known as the isocyanide test. Amines, on treating with chloroform in the presence of alkali, isocyanide is formed. Both aliphatic and aromatic primary amines respond to this test. However, secondary and tertiary amines do not give this test.

The Carbylamine test is given by the chemical reaction mentioned below:

\({\text{R}} – {\text{N}}{{\text{H}}_2} + 3{\text{KOH}} + {\text{CHC}}{{\text{l}}_3} \to {\text{R}} – {\text{NC}}\left({{\text{isocyanide}}} \right) + 3{\text{KCl}} + 3{{\text{H}}_2}{\text{O}}\)

Observations: The presence of primary amine is confirmed by the release of a foul smell.

3. Nitrous Acid Test

Nitrous acid is a qualitative test for identifying aliphatic and aromatic primary, secondary, and tertiary amines. Aromatic primary amines react with nitrous acid to form diazonium salts that undergo decomposition at higher temperatures.

Primary \(\left({{1^ \circ }} \right)\) aliphatic amines react with nitrous acid with the evolution of nitrogen gas, which is seen as bubbles.
\({\text{R}} – {\text{N}}{{\text{H}}_2} + {\text{HN}}{{\text{O}}_2}\xrightarrow{{{\text{NaN}}{{\text{O}}_2} +{\text{HCl}}}}\left[{{\text{R}} – \overset{ + }{\mathop {{{\text{N}}_2}}} \overset{ – }{\mathop {{\text{Cl}}}} } \right]\xrightarrow{{{{\text{H}}_2}}}{\text{ROH}} + {{\text{N}}_2} + {\text{HCl}}\)
A yellow oily nitrosamine is formed when Secondary \(\left({{2^ \circ }} \right)\) amines react with nitrous acid.
\({{\text{R}}_2} – {\text{NH}} + {\text{HONO}} \to {{\text{R}}_2}{\text{N}} – {\text{NO}} + {{\text{H}}_2}{\text{O}}\)
Soluble nitrite salts are formed when Tertiary \(\left({{3^ \circ }} \right)\) amines react with nitrous acid.
\({{\text{R}}_3}{\text{N}} + {\text{HONO}} \to {{\text{R}}_3}{\text{N}}{{\text{H}}^ + }{\text{NO}}_2^ – \)

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. The isothiocyanates have a characteristic smell of mustard oil; hence it is known as Hoffmann mustard oil reaction and is used as a test for primary amines.

5. 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.

6. Azo-Dye Test

Aromatic amines give a positive azo-dye test. Aromatic amines form diazonium salts when treated with \({\text{HN}}{{\text{O}}_2}.\) These diazo compounds undergo a coupling reaction with phenols like \(\beta \)-naphthols to form an orange azo dye compound which is insoluble in water.

The chemical reaction is given below:

Observation: The formation of a red-orange dye confirms the presence of aromatic amines.

Distinguishing Test for Primary, Secondary and Tertiary Amines

Hinsberg Test

Hinsberg test is used to identify the detection of primary, secondary and tertiary amines. Benzene sulfonyl chloride is the reagent used in Hinsberg’s test. Amines react with benzene sulfonyl chloride in an alkaline medium.

Primary \(\left({{1^ \circ }} \right)\) amines react with benzene sulfonyl chloride to produce substituted sulfonamide with an acidic hydrogen atom. It dissolves in a basic medium.
A secondary \(\left({{2^ \circ }} \right)\) amine forms a substituted sulphonamide that does not consist of an acidic hydrogen atom. Hence, these compounds are insoluble in alkalis.
A tertiary \(\left({{3^ \circ }} \right)\) amine forms a substituted sulphonamide which is insoluble in alkali because it does not have acidic hydrogen.

In Hinsberg’s test-

1. Tertiary amines Precipitate in the test tube but dissolve in concentrated \({\text{HCl}}{\text{.}}\)
2. Secondary amines Precipitate in the test tube but do not dissolve in concentrated hydrochloric acid.
3. No reaction or precipitation occurs in the case of primary amines. On adding concentrated hydrochloric acid insoluble substance is visible.

Tests for the amino group (Distinguishing test from other functional groups)
Test	Procedure	Observation
Solubility and Litmus Test	The organic compound is added to dilute mineral acid. One to two drops of the given organic compound are added to moist red litmus paper. The change in the colour of the red litmus paper is observed.	If it is soluble in mineral acid, it may be an amine. It turns red litmus solution blue.
Carbylamine Test	To the solid potassium hydroxide, \(2\,{\text{ml}}\) of ethanol is added. The test tube is warmed until the solid potassium hydroxide completely dissolves. A few drops of chloroform are added to the above solution and shaken well. A small amount of the given organic compound is added to the above solution and warmed gently. The odour of the resulting mixture is observed.	An unpleasant odour confirms the presence of a primary amine.
Nitrous acid test	The given organic compound is dissolved in concentrated hydrochloric acid and water in a clean test tube. The solution is cooled in an ice bath for about \(0 – {5^ \circ }{\text{C}}.\) To the cold solution, sodium nitrite is added dropwise and mixed well. Test the solution for the presence of nitrous acid. Add an excess of the nitrous acid solution if nitrous acid is absent.	Primary amines: Appearance of bubbles. Secondary amines: Appearance of a yellow oily liquid. Tertiary amines: The formation of nitrite salts confirms the presence of tertiary amines.
Azo dye test	Three test tubes A, B and C, are taken. Test tube A: The given organic compound is dissolved in \(2\,{\text{ml}}\) of dilute hydrochloric acid. Test tube B: An aqueous solution of sodium nitrite is prepared. Test tube C: Naphthol is dissolved in dilute sodium hydroxide solution. All three test tubes are placed in an ice bath. The contents in test tube B are added to the mixture in test tube A. The resulting mixture is added to the contents in test tube C. The solution mixture is stirred with a glass rod, and the changes in the test tube are observed.	The appearance of a red-orange dye confirms the presence of aromatic amines.
Tests for distinguishing primary, secondary, and tertiary amines
Hinsberg’s test	The organic compound to be tested is taken in a clean test tube. To it, \(2\,{\text{ml}}\) of sodium hydroxide solution and \(1\,{\text{ml}}\) of benzene sulfonyl chloride is added. Water is added to dilute the solution and is shaken well. The mixture is cooled under tap water, and precipitate formation is observed. If a precipitate is formed, then treat it with concentrated hydrochloric acid.	Primary amines: No reaction or formation of precipitate takes place. On adding concentrated hydrochloric acid, an insoluble substance is seen. Secondary amines: Precipitate formation occurs that does not dissolve in concentrated hydrochloric acid. Tertiary amines: Precipitate formation occurs in the test tube but dissolves in concentrated \({\text{HCl}}{\text{.}}\)

Summary

Naturally occurring amines such as vitamins, proteins, and hormones are vital biological compounds. There are very marginal differences between the primary, secondary and tertiary amines, so it is essential to learn how to distinguish them. In this article, we learned the various methods to differentiate the amino group from the rest of the functional groups. We also learned to differentiate between primary, secondary and tertiary amines.

FAQs on Test for Amines

The frequently asked questions on the test for amines are given below:

Q.1: Which test is used to distinguish between amines?
A: Hinsberg test is used to identify the primary, secondary, and tertiary amines. The reagent used in Hinsberg Test is sulphonyl chloride. In this reaction, the amines act as nucleophiles and attack the electrophile (sulfonyl chloride). The reaction takes place in an alkaline medium. Primary \(\left({{1^ \circ }} \right)\) amines react with benzene sulfonyl chloride to produce substituted sulfonamide that dissolves in a basic medium.
A secondary \(\left({{2^ \circ }} \right)\) amine forms a substituted sulphonamide that is insoluble in alkalis.
A tertiary \(\left({{3^ \circ }} \right)\) amine does not react with benzene sulphonyl chloride.

Q.2: What is the difference between secondary and tertiary amines?
A: The main difference between primary, secondary and tertiary amines is that, in primary amines, one alkyl or aryl group is attached to the nitrogen atom. In secondary amines, two alkyl or aryl groups are attached to the nitrogen atom, whereas, in tertiary amines, three alkyl or aryl groups are attached to the nitrogen.

Q.3: Is pyridine a tertiary amine?
A: Yes, pyridine is a tertiary amine.

Q.4: What kind of amine cannot form hydrogen bonds?
A: Tertiary amines have no hydrogen atoms bonded to the nitrogen atom and therefore are not hydrogen bond donors. Thus, tertiary amines cannot form intermolecular hydrogen bonds.

Q.5: Are amines acidic or basic?
A: Amines are basic and easily react with the hydrogen of electron-poor acids. Amines are one of the only neutral functional groups which are considered basic, which is a consequence of the presence of the lone pair electrons on the nitrogen atom.

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