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Question #44911

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The Question

An aqueous solution of L-Alanine ( $pK_{a1} = 2.34$ , $pK_{a2} = 9.69$ , $pI = 6.01$ ) is subjected to two separate experiments at 298 K:

Experiment I: Treated with excess benzenesulfonyl chloride ( $PhSO_2Cl$ ) in an aqueous buffer at pH = 4.0. Experiment II: Treated with excess $PhSO_2Cl$ in aqueous $NaOH$ (pH = 12.0), followed by careful acidification to pH = 1.0.

Identify the major organic species recovered from the amino acid in Experiment I and Experiment II, respectively.

Options

A.
Exp I: $CH_3CH(NH_3^+)COO^-$ ; Exp II: $PhSO_2NHCH(CH_3)COOH$
B.
Exp I: $PhSO_2NHCH(CH_3)COOH$ ; Exp II: $CH_3CH(NH_3^+)COO^-$
C.
Exp I: $CH_3CH(NH_3^+)COOH$ ; Exp II: $PhSO_2NHCH(CH_3)COO^-$
D.
Exp I: $PhSO_2NHCH(CH_3)COO^-$ ; Exp II: $PhSO_2NHCH(CH_3)COOH$

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Correct Answer

Step-by-Step Solution

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

Hinsberg Reagent Selectivity|Amino Acid Zwitterion pH Sensitivity

Why This Question Is Tricky

Common Mistakes

This question tests the 'Hidden Variable' of pH-dependent nucleophilicity in amino acids.

Students often mechanically apply the Hinsberg test to any molecule containing an 'amino' group, forgetting that in aqueous media, amino acids exist as zwitterions.

At a pH below the $pK_a$ of the amino group, the nitrogen is protonated ( $-NH_3^+$ ) and completely non-nucleophilic.

The trap lies in assuming primary amines always react with benzenesulfonyl chloride regardless of the pH, failing to account for the isoelectric point and acid-base equilibria.

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