What is the Test for Amino Groups Hinsberg and Carbylamine Reaction Explained
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 in Organic Compounds
1. What is the test for amino groups?
The most common test for amino groups (–NH2) is the carbylamine test, which produces a foul-smelling isocyanide when a primary amine is heated with chloroform and alcoholic potassium hydroxide.
- Reagents: CHCl3 (chloroform) and KOH (alc.)
- Observation: Formation of a pungent-smelling isocyanide (carbylamine)
- General reaction: R–NH2 + CHCl3 + 3KOH → R–NC + 3KCl + 3H2O
2. How does the carbylamine test detect primary amines?
The carbylamine test detects primary amines by converting them into foul-smelling isocyanides (R–NC) on heating with chloroform and alcoholic KOH.
- Only 1° amines (R–NH2) respond to this test.
- Secondary (2°) and tertiary (3°) amines do not form isocyanides.
- Example: C2H5NH2 + CHCl3 + 3KOH → C2H5NC + 3KCl + 3H2O
3. What is the Hinsberg test for amines?
The Hinsberg test is used to distinguish primary, secondary, and tertiary amines using benzenesulfonyl chloride in alkaline medium.
- Reagent: C6H5SO2Cl (benzenesulfonyl chloride) + NaOH
- 1° amine: Forms soluble sulfonamide (soluble in alkali).
- 2° amine: Forms insoluble sulfonamide (insoluble in alkali).
- 3° amine: Does not react; remains insoluble but dissolves in dilute acid.
4. What is the nitrous acid test for amino groups?
The nitrous acid test distinguishes primary, secondary, and tertiary amines based on their reaction with nitrous acid (HNO2).
- Nitrous acid is generated in situ: NaNO2 + HCl → HNO2 + NaCl
- 1° aliphatic amine: Liberates nitrogen gas (N2).
- 2° amine: Forms yellow oily N-nitrosamine.
- 3° amine: Forms soluble ammonium salt (no gas).
5. How do you test for aromatic amino groups?
Aromatic primary amino groups are tested by diazotization followed by a coupling reaction to form a colored azo dye.
- Reagents: NaNO2 + HCl at 0–5°C
- Example: C6H5NH2 + HNO2 + HCl → C6H5N2+Cl− + 2H2O
- The diazonium salt couples with phenol or aniline to give a brightly colored azo dye.
6. What is the difference between primary, secondary, and tertiary amines in chemical tests?
Primary, secondary, and tertiary amines differ in their reactions in the carbylamine, Hinsberg, and nitrous acid tests.
- 1° amines (R–NH2): Positive carbylamine test; release N2 with HNO2.
- 2° amines (R2NH): Negative carbylamine test; form N-nitrosamines with HNO2.
- 3° amines (R3N): Do not react in carbylamine test; form soluble salts with acids.
7. Can you give an example of a chemical equation for the carbylamine test?
An example of the carbylamine reaction is the formation of ethyl isocyanide from ethylamine.
- Reaction: C2H5NH2 + CHCl3 + 3KOH → C2H5NC + 3KCl + 3H2O
- Conditions: Heat with alcoholic KOH
- Observation: Extremely unpleasant smell
8. Why is the carbylamine test not given by secondary and tertiary amines?
The carbylamine test is not given by secondary and tertiary amines because they lack the required hydrogen atom on the nitrogen necessary to form an isocyanide intermediate.
- 1° amines have two N–H bonds.
- 2° amines have only one N–H bond.
- 3° amines have no N–H bond.
9. What is the role of alcoholic KOH in the carbylamine test?
In the carbylamine test, alcoholic KOH acts as a strong base to generate dichlorocarbene from chloroform, which then reacts with the primary amine.
- KOH deprotonates CHCl3 to form a reactive intermediate.
- This intermediate reacts with R–NH2 to form R–NC.
- The reaction must be heated in alcoholic medium.
10. How can you confirm the presence of an amino group in an unknown organic compound?
You can confirm the presence of an amino group (–NH2) by performing qualitative tests such as the carbylamine, Hinsberg, and nitrous acid tests.
- Carbylamine test: Foul-smelling isocyanide confirms 1° amine.
- Hinsberg test: Differentiates 1°, 2°, and 3° amines.
- Nitrous acid test: Evolution of N2(g) confirms primary aliphatic amine.
