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Question #44990

Difficulty: 8/10 | 🟢 Free

The Question

D-Glyceraldehyde undergoes a Kiliani-Fischer chain extension via reaction with $\ce{HCN/KCN}$ followed by complete hydrolysis using aqueous $\ce{H2SO4}$ at $373 \text{ K}$ to yield a mixture of epimeric aldonic acids. The unbranched aliphatic analog, butanoic acid, has a $pK_a$ of $4.82$ at $298 \text{ K}$ . Considering the intramolecular electronic effects in the newly formed aldonic acids, which of the following represents their most accurate estimated $pK_a$ value at $298 \text{ K}$ ?

Options

A.
4.82
B.
3.75
C.
1.25
D.
5.90

Check Your Answer

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

Kiliani-Fischer Chain Extension|Nucleophilic Acyl Substitution|Carboxylic Acid Strength

Why This Question Is Tricky

Common Mistakes

This question tests the student's ability to bridge organic synthesis (Kiliani-Fischer chain extension) with physical organic chemistry (electronic effects on acidity).

The 'Hidden Variable' is the structural nature of the product: students often memorize that hydrolysis of a nitrile yields a carboxylic acid and erroneously assign it a standard aliphatic $pK_a$ ( $\approx 4.8$ ).

The trap relies on failing to account for the strong $-I$ effect of the newly adjacent $\alpha$ -hydroxy group (originating from the cyanohydrin intermediate), which drastically stabilizes the carboxylate conjugate base and lowers the $pK_a$ to the $3.5 - 3.9$ range.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Biomolecules - Class 12
Concept	Kiliani-Fischer Chain Extension
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free