Question

The reagent which is used to distinguish primary, secondary and tertiary amines from the mixture is

• A. Fehling's reagent
• B. Tollens reagent
• C. Lucas reagent
• D. Hinsberg's reagent

Hint:

Instead of working out the full mechanism first, recall that named reagent tests are each tied to one functional group: Fehling and Tollens reagents test for aldehydes, Lucas reagent tests for alcohols. That elimination alone narrows it down. Then confirm by checking how many hydrogens sit on the nitrogen of each amine type and whether the resulting product dissolves in alkali.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
The question asks for a chemical test that can differentiate between primary (1°), secondary (2°), and tertiary (3°) amines. This requires a reagent that reacts differently with each class of amine based on the number of hydrogen atoms attached to the nitrogen.
Step 2: Analyzing the Reagents:
- (A) Fehling's reagent and (B) Tollens' reagent: These are mild oxidizing agents used to distinguish aldehydes from ketones. They do not react with amines in a way that allows for their differentiation.
- (C) Lucas reagent (Anhydrous ZnCl₂ + Conc. HCl): This reagent is used to distinguish between primary, secondary, and tertiary alcohols based on the rate of formation of alkyl chlorides. It does not distinguish between amines.
- (D) Hinsberg's reagent (Benzenesulfonyl chloride, C₆H₅SO₂Cl): This is the classic reagent used for the distinction of primary, secondary, and tertiary amines. The test is based on the formation of sulfonamides. - Primary amines (R-NH₂): Have two H-atoms on N. They react with Hinsberg's reagent to form an N-alkylbenzenesulfonamide. This product has an acidic hydrogen on the nitrogen, making it soluble in aqueous alkali (like KOH or NaOH). - Secondary amines (R₂NH): Have one H-atom on N. They react with Hinsberg's reagent to form an N,N-dialkylbenzenesulfonamide. This product has no acidic hydrogen on the nitrogen, so it is insoluble in alkali. - Tertiary amines (R₃N): Have no H-atoms on N. They do not react with Hinsberg's reagent at all (under these conditions).
Thus, by observing the reaction with Hinsberg's reagent and the subsequent solubility in alkali, one can distinguish between the three classes of amines.
Step 3: Final Answer:
Hinsberg's reagent is used to distinguish primary, secondary, and tertiary amines. Therefore, option (D) is correct.

Answer 2

Concept:

• Classic qualitative organic chemistry tests are named after the reagent used, and each named test is tied to one specific functional group class.
• Matching the reagent name to its known functional group target lets a student shortlist the right answer before working through the full reaction mechanism.

Step 1: Match each reagent option to the functional group it is known for.
Fehling reagent and Tollens reagent are both aldehyde tests, giving a red precipitate or a silver mirror with aldehydes. Neither one reacts usefully with amines, so both are ruled out.

Step 2: Check the alcohol test reagent.
Lucas reagent, anhydrous $ZnCl_2$ dissolved in concentrated HCl, is the standard test for telling primary, secondary, and tertiary alcohols apart through the speed of a cloudy alkyl chloride layer forming. It is built for alcohols, not amines, so this option is ruled out too.

Step 3: Confirm Hinsberg reagent by testing it against each type of amine.
Hinsberg reagent is benzenesulfonyl chloride, $C_6H_5SO_2Cl$. A primary amine has two hydrogens on nitrogen, so it forms an N-alkylbenzenesulfonamide whose remaining N-H is acidic enough to dissolve in aqueous NaOH. A secondary amine has one hydrogen on nitrogen, so it forms an N,N-dialkylbenzenesulfonamide with no acidic N-H left, and this stays insoluble in NaOH. A tertiary amine has no hydrogen on nitrogen at all, so it does not react with Hinsberg reagent.

Step 4: Read off the three outcomes.
Since the three classes of amines give three visibly different results with this one reagent, a base-soluble product, a base-insoluble product, and no reaction at all, Hinsberg reagent alone is enough to identify all three.

Final Answer: Hinsberg reagent (option D).