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Question #44912

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The Question

Consider the reaction of the following four amines with benzenesulfonyl chloride (Hinsberg reagent) in aqueous $NaOH$ at $298\text{ K}$ : (I) Cyclohexylamine (II) Aniline (III) 4-Nitroaniline (IV) 2-Nitroaniline Let $k_1, k_2, k_3,$ and $k_4$ be their respective initial rate constants for the nucleophilic attack on the sulfur atom. Which of the following correctly represents the decreasing order of their rate constants, and what is the primary chemical rationale for the slowest reacting species?

Options

A.
$k_1 > k_2 > k_3 > k_4$ ; (IV) is the slowest due to the combined ortho-effect (steric hindrance to the approaching electrophile) and strong -I/-M effects of the $-NO_2$ group.
B.
$k_1 > k_2 > k_4 > k_3$ ; (III) is the slowest because the -M effect is most effective at the para position, maximizing the resonance stabilization of the nitrogen lone pair.
C.
$k_2 > k_1 > k_3 > k_4$ ; (IV) is the slowest because intramolecular hydrogen bonding completely prevents the deprotonation step required to form the soluble sulfonamide salt.
D.
$k_1 > k_2 > k_3 > k_4$ ; (IV) is the slowest because the ortho $-NO_2$ group forces the $-NH_2$ group out of the benzene ring plane (Steric Inhibition of Resonance), deactivating the lone pair.

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Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Mechanism_Trap

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Concepts Behind This Question

Hinsberg Reagent Selectivity|Aniline Resonance Pitfalls|Ortho-Effect in Anilines

Why This Question Is Tricky

Common Mistakes

Tests the student's ability to differentiate between thermodynamic stability and kinetic nucleophilicity, specifically focusing on the 'ortho-effect' in anilines.

Students often assume all primary amines react readily with Hinsberg reagent.

Furthermore, it traps students who blindly apply Steric Inhibition of Resonance (SIR); if SIR were dominant here, the amine would become more nucleophilic (like an aliphatic amine).

The true exception is that the ortho-nitro group acts as a physical barricade (steric hindrance) against the bulky sulfonyl chloride, compounding its strong -I/-M electron-withdrawing effects.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Amines - Class 12
Concept	Hinsberg Reagent Selectivity
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free