Question

Describe the method of identification of primary, secondary and tertiary amines. Write the chemical equations of these reactions also.

Hint:

Hinsberg's test is a simple and reliable way to distinguish between primary, secondary, and tertiary amines. Always check solubility in alkali after reaction with benzenesulfonyl chloride.

Answer 1

The Hinsberg's Test is the standard method for distinguishing primary, secondary and tertiary amines. It uses benzenesulfonyl chloride (C$_6$H$_5$SO$_2$Cl) in the presence of aqueous alkali (NaOH/KOH).

Step 1: Reaction with primary amine.
Primary amines react with benzenesulfonyl chloride to give sulfonamides which are soluble in alkali due to the presence of an acidic hydrogen atom. \[ RNH_2 + C_6H_5SO_2Cl \;\longrightarrow\; R{-}NH{-}SO_2C_6H_5 \] This product dissolves in alkali, forming a soluble salt.

Step 2: Reaction with secondary amine.
Secondary amines react to form sulfonamides which have no acidic hydrogen and hence remain insoluble in alkali. \[ R_2NH + C_6H_5SO_2Cl \;\longrightarrow\; R_2N{-}SO_2C_6H_5 \] The product is insoluble in alkali.

Step 3: Reaction with tertiary amine.
Tertiary amines do not react with benzenesulfonyl chloride under these conditions. They remain unreacted and can be recovered unchanged. Conclusion:
- Primary amines: give soluble sulfonamide salts.
- Secondary amines: give insoluble sulfonamides.
- Tertiary amines: do not react.