Test for Amines - Practice Questions & MCQ

Libermann Nitroso Test
While phenol is reacted with NaNO₂ and concentrated H₂SO₄, it provides a deep green or blue colour which changes to red on dilution with water. while generated alkaline along with NaOH original green or blue colour is restored. This reaction is termed as Liebermann's nitroso reaction and is employed as a test of phenol. Libermann's nitroso test is used as a test for secondary amines.

Hoffmann's Test
Separation of primary, secondary and tertiary amines by Hoffmann's method. The mixture of three amines is treated with diethyl oxalate. The primary amine forms a solid oxamide, a secondary amine gives a liquid oxamic ester while tertiary amine does not react.

Hinsberg's Test
The Hinsberg test which is used to distinguish between primary, secondary, and tertiary amines, is based upon sulfonamide formation. In this test, an amine is reacted with benzene sulfonyl chloride. If a product forms, the amine is either a primary or secondary amine, because tertiary amines do not form stable sulfonamides. If this sulfonamide that formed is dissolved in aqueous sodium hydroxide solution, it is a primary amine. But if is insoluble in aqueous sodium hydroxide then it is a secondary amine. The sulfonamide of a primary amine is soluble in an aqueous base because it still possesses an acidic hydrogen on the nitrogen, which can be lost to form a sodium salt.

Hoffmann's Mustard Oil Reaction
When ethylamine is heated with carbon disulphide and mercuric chloride, ethyl isothiocyanate is formed which has smell like mustard oil.

When any primary amine(aliphatic or aromatic) is heated with chloroform and alcoholic potassium hydroxide solution, isocyanide(carbylamine) is formed which has a very unpleasant smell. This test is called carbylamine test or isocyanide test. The reactions occur as follows:

Alkylation
Amines undergo alkylation with RX and undergo complete methylation and this is called exhaustive methylation, but with Me₂SO₄ amines undergo monomethylation. 1^o and 2^o amines are also methylated by heating HCHO and excess of HCOOH at 100^oC. This reaction is known as Eschweiler-Clarke methylation. The reaction occurs as follows:

Acylation
1^o and 2^o aliphatic and aromatic amines react with acid chlorides (RCOCl), anhydrides and esters by SN² reaction is called acylation reaction. The reaction is carried out in the presence of a base stronger than amine, such as pyridine, which removes HCl so formed and shifts the equilibrium to the product side. The reaction occurs as follows:

Nitrous acid(HNO₂) is obtained in situ by the reaction of sodium nitrite (NaNO₂) with dil. HCl. The reaction occurs as follows:

1^o aliphatic amines react with HNO₂ to form aliphatic diazonium salts, which, being unstable, liberate N₂ gas quantitatively and alcohols. Quantitative evolution of N₂ is used in the estimation of amino acids and proteins. The reaction occurs as follows:

Mechanism