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.
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Qualitative Test for Amines
The following tests are used for the identification of amino groups.
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.
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.
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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.
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.
\[CS_2+3Cl_2\rightarrow CCl_4+S_2Cl_2\]
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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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.
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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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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-
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?
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 are the common laboratory tests for identifying the amino group in an organic compound?
Several tests are used to identify and differentiate amino groups. The most common confirmatory tests include:
- Carbylamine Test: Specifically for primary amines.
- Hinsberg's Test: Used to distinguish between primary, secondary, and tertiary amines.
- Azo Dye Test: A specific test for aromatic primary amines.
- Libermann’s Nitroso Reaction: Used to identify secondary amines.
2. How does the Carbylamine test (Isocyanide test) confirm the presence of a primary amino group?
The Carbylamine test confirms a primary amino group by its unique reaction. When an aliphatic or aromatic primary amine is warmed with chloroform and an alcoholic solution of potassium hydroxide (KOH), it forms an isocyanide. This product has a highly characteristic and unpleasant or foul smell, which serves as a positive indicator. Secondary and tertiary amines do not undergo this reaction, making the test specific for primary amines.
3. What is the principle of the Hinsberg test for distinguishing between primary, secondary, and tertiary amines?
The principle of the Hinsberg test lies in the differential reactivity of amines with benzene sulphonyl chloride (Hinsberg's reagent).
- A primary amine reacts to form N-alkyl benzene sulphonamide, which has an acidic hydrogen and is therefore soluble in alkali (KOH).
- A secondary amine reacts to form N,N-dialkyl benzene sulphonamide, which has no acidic hydrogen and is insoluble in alkali.
- A tertiary amine does not have a hydrogen atom attached to the nitrogen to be replaced, so it does not react with Hinsberg's reagent.
These distinct outcomes allow for the clear differentiation of the three types of amines.
4. Why is the Azo dye test suitable for aromatic primary amines but not for aliphatic primary amines?
The Azo dye test is effective for aromatic primary amines because they react with nitrous acid (NaNO₂ + HCl) at low temperatures (0-5°C) to form a relatively stable diazonium salt. This stable salt can then couple with a phenol or an aromatic amine to form a brightly coloured azo dye. In contrast, aliphatic primary amines also form diazonium salts, but these are highly unstable and decompose immediately to release nitrogen gas, preventing the coupling reaction and dye formation.
5. Explain the reaction and key observation in Libermann's nitroso test for secondary amines.
In Libermann's nitroso test, a secondary amine (both aliphatic and aromatic) is treated with nitrous acid (prepared from NaNO₂ and HCl). This reaction forms a yellow, oily N-nitrosamine, which is the first key observation. For further confirmation, warming this yellow oil with a crystal of phenol and a few drops of concentrated sulphuric acid produces a deep green or blue solution. Upon dilution with water and adding an alkali like sodium hydroxide, the colour changes to a distinct violet or blue.
6. Why are simple methods like the litmus test or solubility test not considered definitive for identifying amines?
While amines are basic and will turn red litmus paper blue, this property is not unique. Many other classes of organic compounds, such as alkaloids, are also basic. Similarly, their solubility in mineral acids is due to the formation of a salt, a characteristic of bases. Therefore, these tests only confirm the basic nature of the compound, not the specific presence of an amino functional group. Confirmatory tests like the Carbylamine or Hinsberg test are required for a definitive identification.
7. How can you chemically distinguish between aniline (an aromatic amine) and ethanamine (an aliphatic amine)?
Aniline and ethanamine can be distinguished using tests that are specific to aromatic amines. Two effective methods are:
- Azo Dye Test: Aniline will react with NaNO₂/HCl followed by 2-naphthol to form a bright orange-red dye. Ethanamine will not form a dye and will instead produce bubbles of nitrogen gas.
- Bromine Water Test: Aniline reacts readily with bromine water to give a white precipitate of 2,4,6-tribromoaniline. Ethanamine does not react with bromine water in this manner.
