In the Hinsberg test used to distinguish between primary, secondary, and tertiary amines, how does a primary amine behave upon reaction with benzenesulfonyl chloride (C6H5SO2Cl) and subsequent addition of aqueous potassium hydroxide?

Answer and explanation

Correct answer: A. Forms N-alkylbenzenesulfonamide that dissolves in KOH due to an acidic sulfonamide proton

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  1. Step 1: Benzenesulfonyl chloride (, Hinsberg's reagent) reacts with primary amines to form an N-alkylbenzenesulfonamide:
  2. Step 2: The hydrogen attached to nitrogen in this sulfonamide is strongly acidic because of the powerful electron-withdrawing nature of the adjacent sulfonyl () group.
  3. Step 3: Because of this acidic hydrogen, the sulfonamide dissolves readily in aqueous to form a clear soluble potassium salt: .
  4. Step 4: Secondary amines form N,N-dialkylbenzenesulfonamides lacking an acidic hydrogen, which remain insoluble in , while tertiary amines do not react at all.
Answer
It forms an N-alkylbenzenesulfonamide that dissolves completely in aqueous KOH due to the presence of an acidic hydrogen on the sulfonamide nitrogen
Why not the others: 'It forms a precipitate that remains insoluble in aqueous KOH' describes secondary amines; 'It does not react...' describes tertiary amines; 'It decomposes to liberate nitrogen gas...' describes nitrous acid reaction.

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