Question

Which reagent is used to distinguish between primary, secondary and tertiary amines ?

• A. $\mathrm{C_6H_5SO_2Cl}$
• B. $\mathrm{C_6H_5COCl}$
• C. $\mathrm{CHCl_3}$ + ethanolic KOH
• D. $\mathrm{NaOH + I_2}$

Hint:

Hinsberg's reagent gives three different results, one for each class of amine.

Answer 1

The Correct Option is A

Concept: To tell three different things apart, the reagent must give three different results, one for each. The reagent that does this for amines is Hinsberg's reagent.

Step 1: Name the reagent
Hinsberg's reagent is benzenesulphonyl chloride, $\mathrm{C_6H_5SO_2Cl}$. It reacts in a different way with each type of amine.

Step 2: See the three different results
A primary amine gives a product that still has an N−H bond left, so it dissolves in KOH. A secondary amine gives a product with no N−H bond, so it stays as a solid and does not dissolve in KOH. A tertiary amine does not react at all. So we get three clearly different observations, which is exactly what we want.

Step 3: Why the other reagents are wrong
The carbylamine test ($\mathrm{CHCl_3}$ with ethanolic KOH) works only for primary amines, so it cannot separate all three. Benzoyl chloride reacts with both primary and secondary, so it also fails. Only $\mathrm{C_6H_5SO_2Cl}$ separates all three.

Answer: Option (A), $\mathrm{C_6H_5SO_2Cl}$ (Hinsberg's reagent).