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Question #44016

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The Question

An optically active disaccharide $\mathbf{X}$ ( $\ce{C12H22O11}$ ) does not form a silver mirror with Tollens' reagent. Exhaustive methylation of $\mathbf{X}$ followed by acidic hydrolysis yields 2,3,4,6-tetra-O-methyl-D-glucopyranose and 1,3,4,6-tetra-O-methyl-D-fructofuranose. However, when $\mathbf{X}$ is treated with excess 2,4-dinitrophenylhydrazine (2,4-DNP) in a mildly acidic medium (pH $\approx$ 3) at 350 K, it forms a yellow-orange precipitate, consuming $n$ moles of 2,4-DNP per mole of $\mathbf{X}$ (assuming the reaction stops at the hydrazone stage). What is the value of $n$ , and what is the primary reason for the divergent behavior of $\mathbf{X}$ towards Tollens' reagent and 2,4-DNP?

Options

A.
$n = 2$ ; Tollens' reagent is alkaline and cannot cleave the $\alpha-(1 \rightarrow 2)$ glycosidic linkage, whereas the acidic DNP medium hydrolyzes it in situ to release two reactive anomeric carbons.
B.
$n = 1$ ; Tollens' reagent is a weak oxidant unable to open the pyranose ring, whereas the acidic DNP medium catalyzes mutarotation to expose a single reactive aldehyde group.
C.
$n = 2$ ; $\mathbf{X}$ is a reducing sugar whose hemiacetal is sterically hindered from reacting with $[\ce{Ag(NH3)2}]^+$ , but the smaller 2,4-DNP nucleophile can attack both anomeric centers.
D.
$n = 4$ ; The alkaline Tollens' medium stabilizes the cyclic acetal, whereas the acidic DNP medium induces retro-aldol cleavage of $\mathbf{X}$ into four reactive triose fragments.

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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

Reaction with DNP|Tollens vs. Fehling Specificity|Reducing Sugar Criteria

Why This Question Is Tricky

Common Mistakes

This question tests the 'Hidden Variable' of reagent pH and its effect on substrate stability.

Students often memorize the heuristic that 'sucrose is a non-reducing sugar and does not react with carbonyl reagents', correctly applying this to Tollens' and Fehling's tests.

However, they frequently fail to account for the acidic nature of the 2,4-DNP reagent.

The trap exploits this blind spot: the acidic medium induces in situ hydrolysis of the acetal (glycosidic) linkage, unmasking the reactive anomeric carbons.

The methylation data serves as a rigorous confirmation of the sucrose structure, ensuring the student cannot guess the answer without understanding the exact connectivity and its implications for acid-catalyzed hydrolysis.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Aldehydes, Ketones, and Carboxylic Acids - Class 12
Concept	Reaction with DNP
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free