Which of the following is Hinsberg reagent?

Benzenesulfonyl chloride

The correct answer is option 3. Benzenesulfonyl chloride.

The Hinsberg test is a qualitative analysis used to distinguish between primary, secondary, and tertiary amines based on their reactivity with a sulfonyl chloride reagent. The reagent used in this test is benzenesulfonyl chloride \((C_6H_5SO_2Cl)\), which is the Hinsberg reagent. The Hinsberg test involves the reaction of benzenesulfonyl chloride with an amine in the presence of a base, typically aqueous \(NaOH\). The test distinguishes between primary, secondary, and tertiary amines based on the formation and solubility of sulfonamide derivatives.

1. Primary Amines \((R-NH_2)\):

Primary amines react with benzenesulfonyl chloride to form N-alkylbenzenesulfonamides. These sulfonamides are soluble in the basic medium (NaOH) but precipitate out when the solution is acidified.

2. Secondary Amines \((R_2-NH)\):

Secondary amines react with benzenesulfonyl chloride to form N,N-dialkylbenzenesulfonamides. These sulfonamides are insoluble in both the basic medium and after acidification.

3. Tertiary Amines \((R_3-N)\):

Tertiary amines do not react with benzenesulfonyl chloride because they lack an available hydrogen on the nitrogen to form the sulfonamide. They remain unreacted and do not form any precipitate.

Summary of the Hinsberg Test:

Primary amines: Form a soluble sulfonamide in NaOH, precipitates upon acidification.

Secondary amines: Form an insoluble sulfonamide in both NaOH and acid.

Tertiary amines: Do not react and remain in the aqueous phase.

Benzenesulfonyl chloride is chosen for this test because its sulfonyl chloride group \((SO_2Cl)\) is highly reactive with amines, forming stable sulfonamide products. The differences in solubility of these sulfonamides in basic and acidic conditions make it easy to distinguish between primary, secondary, and tertiary amines. Thus, among the options provided, benzenesulfonyl chloride is the Hinsberg reagent used for this important test in organic chemistry.

Therefore, the correct answer is Benzenesulfonyl chloride.