CBSE · Class 12 · Chemistry · AminesExplain the chemical methods used for the distinction between primary, secondary, and tertiary amines. Give the relevant chemical equations and structural changes involved.

Distinction Between Primary, Secondary, and Tertiary Amines

\nPrimary ($1^\circ$), secondary ($2^\circ$), and tertiary ($3^\circ$) amines can be distinguished using various chemical tests. The most prominent methods include Hinsberg's test and the reaction with nitrous acid.

1. Hinsberg's Test (Benzenesulphonyl Chloride Test)\nBenzenesulphonyl chloride ($\text{C}_6\text{H}_5\text{SO}_2\text{Cl}$), also known as Hinsberg's reagent, reacts differently with $1^\circ$, $2^\circ$, and $3^\circ$ amines:

• Primary Amines: Primary amines react with benzenesulphonyl chloride to form $N$-alkylbenzenesulphonamide. The hydrogen attached to nitrogen in this sulphonamide is strongly acidic due to the electron-withdrawing sulphonyl group. Therefore, this product is soluble in alkali (like $\text{NaOH}$) to form a clear solution.
• Secondary Amines: Secondary amines react to form $N,N$-dialkylbenzenesulphonamide. Since there is no hydrogen atom attached to the nitrogen atom in this product, it is acidic-free and insoluble in alkali solutions.
• Tertiary Amines: Tertiary amines do not react with benzenesulphonyl chloride at all, as they lack a replaceable hydrogen atom on the nitrogen atom. They remain insoluble in the reaction mixture and can be separated easily.

2. Reaction with Nitrous Acid ($\text{HNO}_2$)\nNitrous acid, prepared in situ from sodium nitrite and hydrochloric acid at $0-5^\circ\text{C}$, reacts distinctly with the three classes of amines:

• Primary Aliphatic Amines: They react to form unstable aliphatic diazonium salts, which immediately decompose to evolve nitrogen gas with the quantitative formation of alcohols.
• Secondary Amines: They react with nitrous acid to form yellow-colored oily nitrosamines ($N$-nitrosoamines), which are insoluble in water.
• Tertiary Amines: They dissolve in nitrous acid to form stable salts due to salt formation through the lone pair on the nitrogen atom, without evolving nitrogen gas.