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Question #43985
The Question
Compound \textbf{X} (1-deoxy-D-fructose) is a carbohydrate derivative that possesses a \ce{CH3-C(=O)-} moiety in its acyclic form. However, when \textbf{X} is treated with excess \ce{I2} and \ce{NaOH} (aq) at 298 K, it fails to yield the characteristic yellow precipitate of iodoform (\ce{CHI3}). Which of the following statements best explains this anomalous chemical behavior?
Options
- A.
The alkaline medium induces a rapid intramolecular Cannizzaro reaction, reducing the C2 carbonyl to an alcohol.
- B.
The thermodynamic stability of the cyclic hemiketal minimizes the acyclic methyl ketone concentration, while base-catalyzed enolization preferentially yields a C2-C3 enediol that isomerizes to an unreactive 3-ketose.
- C.
The \ce{I2/NaOH} reagent selectively oxidizes the terminal methyl group to a carboxylate, preventing the formation of the triiodomethyl intermediate.
- D.
The cyclic hemiketal undergoes a rapid base-catalyzed retro-aldol cleavage, yielding exclusively fragments that lack the requisite \ce{CH3-C(=O)-} moiety.
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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
Haloform Reaction Logic|Reducing Sugar Criteria|D-Glucose Anomeric Equilibrium
Why This Question Is Tricky
Common Mistakes
The 'Chemical Exception' being tested is the failure of a methyl ketone to undergo the haloform reaction when embedded in a carbohydrate framework.
Students typically assume any \ce{CH3-C(=O)-} group will yield \ce{CHI3}.
The hidden variables are the thermodynamic stability of the cyclic hemiketal (lowering reactive concentration) and the preferential formation of the more stable C2-C3 enediol, which irreversibly isomerizes the molecule to an unreactive 3-ketose before tri-iodination can occur.
Distractor D is particularly deceptive because retro-aldol cleavage of \textbf{X} would actually yield 1-hydroxyacetone, which DOES contain a methyl ketone and would give a positive test.
Quick Facts About This Question
Question Information
| Subject | Chemistry |
| Chapter | Aldehydes, Ketones, and Carboxylic Acids - Class 12 |
| Concept | Haloform Reaction Logic |
| Difficulty | 8 |
| Cognitive Load | 7 |
| Target Solve Time | 160 sec |
| Access | Free |