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Question #44010

Difficulty: 8/10 | 🟢 Free

The Question

Compound \textbf{X} is the most abundant naturally occurring D-ketohexose. It gives a positive Tollens' test due to its transformation in the alkaline medium via a 1,2-enediol intermediate \textbf{Y}. \textbf{X} also reacts with excess phenylhydrazine to form an osazone \textbf{Z}.

Determine the value of the expression: $(p \times q) + (r \times s) + t$

Where: $p$ = number of chiral carbons in the acyclic form of \textbf{X}. $q$ = number of chiral carbons in the intermediate \textbf{Y}. $r$ = number of moles of phenylhydrazine consumed per mole of \textbf{X} to completely form \textbf{Z}. $s$ = number of chiral carbons in \textbf{Z}. $t$ = number of D-aldohexoses that yield the exact same osazone \textbf{Z}.

Options

A.
17
B.
20
C.
21
D.
23

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

Tollens vs. Fehling Specificity|Osazone Formation Logic|Epimerization vs Anomerization

Why This Question Is Tricky

Common Mistakes

This question tests the mechanistic understanding of carbohydrate reactions.

The primary trap is the Lobry de Bruyn-van Ekenstein transformation: students often memorize that 'ketones do not give Tollens' but forget that alpha-hydroxy ketones (like fructose) epimerize to aldoses in the alkaline medium of the Tollens reagent via an enediol intermediate.

Furthermore, it tests the exact stoichiometry of osazone formation (the hidden oxidation step consuming the second mole of phenylhydrazine) and the stereochemical consequences of these reactions (loss of chirality at C1/C2).

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Aldehydes, Ketones, and Carboxylic Acids - Class 12
Concept	Tollens vs. Fehling Specificity
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free