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Question #44450

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The Question

An aliphatic amine \ce{A} (\ce{C8H19N}) undergoes the Hinsberg test with benzenesulfonyl chloride (\ce{PhSO2Cl}) in a biphasic system of $1.0\text{ M}$ aqueous \ce{KOH} ( $\text{pH} = 14.0$ ) and diethyl ether (equal volumes) at $298\text{ K}$ . After equilibration, the layers are separated. Acidification of the aqueous layer yields a negligible amount of precipitate. Evaporation of the ether layer yields a large amount of crystalline solid \ce{B}. Solid \ce{B} is insoluble in water, but dissolves completely in aqueous \ce{NaOH} when ethanol is added as a co-solvent, confirming the presence of an acidic proton. Given that the $\text{p}K_a$ of the sulfonamide \ce{N-H} proton is $10.0$ and the partition coefficient of the neutral sulfonamide between ether and water is $P = 10^{5.0}$ , which of the following correctly identifies \ce{A} and the dominant species of \ce{B} in the ether layer before evaporation?

Options

A.
\ce{A} is octan-1-amine; \ce{B} is the neutral sulfonamide \ce{PhSO2NH(C8H17)}. The high partition coefficient ( $P = 10^5$ ) shifts the equilibrium, resulting in a distribution ratio $D \approx 10$ favoring the organic phase.
B.
\ce{A} is octan-1-amine; \ce{B} is the potassium salt \ce{PhSO2N^{-}(C8H17) K+}. The strong base ( $\text{pH} = 14$ ) ensures $>99.9\%$ deprotonation, and the resulting lipophilic ion-pair partitions into the ether layer.
C.
\ce{A} is $N$ -methylheptan-1-amine; \ce{B} is the neutral sulfonamide \ce{PhSO2N(CH3)(C7H15)}. The secondary amine forms a product lacking an acidic proton, which inherently partitions into the ether layer ( $D = 10^5$ ).
D.
\ce{A} is octan-1-amine; \ce{B} is the neutral sulfonamide \ce{PhSO2NH(C8H17)}. The aqueous \ce{KOH} ( $\text{pH} = 14$ ) is insufficiently basic to deprotonate the sulfonamide ( $\text{p}K_a = 10.0$ ), leaving it entirely in the neutral form.

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Explanation

How the Examiner Thinks

Exception_Exposure

Learn to identify edge cases and boundary exceptions that JEE examiners hide in math problems.

Concepts Behind This Question

Solvent Effects on Reactivity|Hinsberg Reagent Selectivity|Basicity-Solvation Competition

Why This Question Is Tricky

Common Mistakes

This question tests the 'Anomalous Hinsberg Test' for long-chain primary amines, a classic trap in qualitative organic analysis.

Students rote-learn that primary amines form water-soluble salts in aqueous KOH.

However, they overlook the competing phase-partition equilibrium in a biphasic system.

By providing the pKa and partition coefficient, the problem forces students to quantitatively evaluate the distribution ratio (D).

The hidden mechanism is Le Chatelier's principle across phases: the massive thermodynamic preference of the neutral sulfonamide for the organic phase (P = 10^5) outcompetes the strong base's tendency to deprotonate and hold it in the aqueous phase (a factor of 10^4).

This leads to a false negative for a primary amine, requiring mathematical proof to identify the dominant species.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Organic Chemistry – Some Basic Principles and Techniques - Class 11 Part 2
Concept	Solvent Effects on Reactivity
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free