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Question #44988

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The Question

An aldopentose $A$ upon oxidation with dilute $\ce{HNO_3}$ yields an optically active aldaric acid. $A$ undergoes Kiliani-Fischer chain extension to produce a mixture of two aldohexoses, $B$ and $C$ . The mixture of $B$ and $C$ is treated with excess phenylhydrazine ( $\ce{PhNHNH_2}$ ) in an acetate buffer at $373\text{ K}$ to yield the osazone product(s) $P$ . Which of the following correctly describes the product(s) $P$ and the reaction stoichiometry?

Options

A.
$P$ consists of a single osazone with 3 chiral centers, and its formation consumes 3 equivalents of $\ce{PhNHNH_2}$ per mole of hexose.
B.
$P$ consists of a racemic mixture of two osazones with 3 chiral centers, consuming 2 equivalents of $\ce{PhNHNH_2}$ per mole of hexose.
C.
$P$ consists of two diastereomeric osazones with 4 chiral centers, consuming 3 equivalents of $\ce{PhNHNH_2}$ per mole of hexose.
D.
$P$ consists of a single osazone with 4 chiral centers, and its formation consumes 2 equivalents of $\ce{PhNHNH_2}$ per mole of hexose.

Check Your Answer

Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Mechanism_Trap

Don't get tricked by the process! Learn the mechanical traps used in competitive exam questions.

Concepts Behind This Question

Kiliani-Fischer Chain Extension|Osazone Formation Logic|Epimerization vs Anomerization

Why This Question Is Tricky

Common Mistakes

This question tests three interconnected traps in carbohydrate chemistry.

First, the 'Stereochemical Trap': Students often assume that starting with a mixture of two diastereomers ( $B$ and $C$ ) will yield a mixture of two diastereomeric products.

The hidden mechanism is that osazone formation destroys chirality at C-2 via oxidation, causing C-2 epimers to converge into a single product.

Second, the 'Stoichiometric Trap': Students frequently guess that only 2 equivalents of phenylhydrazine are consumed because only two phenylhydrazone moieties appear in the final structure, forgetting the equivalent used as an oxidizing agent.

Third, the 'Structural Trap': Miscounting chiral centers by forgetting that C-1 and C-2 become planar $sp^2$ carbons in the osazone.

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