Which of the following is known as Hinsberg's reagent?

Benzenesulphonyl chloride

The correct answer is Option (1) → Benzenesulphonyl chloride

Hinsberg's reagent is benzenesulphonyl chloride (C₆H₅SO₂Cl).

It is used in the Hinsberg test, which is a chemical test used to distinguish between primary, secondary, and tertiary amines.

In this test, an amine reacts with benzenesulphonyl chloride in the presence of aqueous alkali (NaOH or KOH) to form different products depending on the type of amine.

The reaction behavior differs because primary and secondary amines contain N–H bonds, while tertiary amines do not.

Primary amines form N-alkylbenzenesulphonamide, which is soluble in alkali due to the presence of an acidic hydrogen.

Secondary amines form dialkylbenzenesulphonamide, which does not contain acidic hydrogen and therefore remains insoluble in alkali.

Tertiary amines do not react with Hinsberg reagent under these conditions.

Thus, benzenesulphonyl chloride is known as Hinsberg's reagent.

Option-wise Explanation

Option 1: Benzenesulphonyl chloride

This is correct. Benzenesulphonyl chloride (C₆H₅SO₂Cl) is the reagent used in the Hinsberg test to differentiate primary, secondary, and tertiary amines.

Option 2: N-Ethylbenzenesulphonamide

This is incorrect. This compound is actually a product formed when a primary amine reacts with Hinsberg reagent, not the reagent itself.

Option 3: Diethylbenzenesulphonamide

This is incorrect. This compound is a product formed when a secondary amine reacts with Hinsberg reagent.

Option 4: Benzenesulphonyl bromide

This is incorrect. Although it is chemically similar, the reagent specifically used in the Hinsberg test is benzenesulphonyl chloride, not the bromide derivative.