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Question #44447
The Question
The optically active ketone, (R)-3-phenylbutan-2-one, is subjected to the following reaction sequence: I. Reaction with to form the major cyanohydrin P. II. Treatment of P with excess followed by acidic hydrolysis to yield Q. III. Reduction of Q with in methanol to yield R as the major diastereomer. Which of the following statements correctly describes the stereochemistry of R and its reactivity towards periodic acid ()?
Options
- A.
R is an erythro-diol; its cleavage by HIO4 is exceptionally slow because its most stable conformation has anti-periplanar OH groups.
- B.
R is a threo-diol; its cleavage by HIO4 is fast because its most stable conformation has syn-coplanar OH groups.
- C.
R is an erythro-diol; its cleavage by HIO4 is fast because it easily forms a cyclic periodate ester without steric strain.
- D.
R is a threo-diol; its cleavage by HIO4 is exceptionally slow due to steric hindrance from the phenyl group in the transition state.
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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
Cyanohydrin Stereochemistry|Glycol Periodic Acid Cleavage|Esterification Steric Hindrance
Why This Question Is Tricky
Common Mistakes
The problem tests the stereochemical control in a multi-step synthesis and its kinetic consequences in oxidative cleavage.
Students often assume HIO4 cleavage of acyclic diols is generic and uniformly fast.
However, the reduction of the alpha-hydroxy ketone under non-chelating conditions (polar Felkin-Anh model) yields the erythro-diol.
The hidden variable is that the erythro-diol's most stable conformation places the vicinal OH groups anti-periplanar.
To form the requisite syn-coplanar cyclic periodate ester, the molecule must adopt an eclipsed conformation for its bulky alkyl groups, introducing severe steric strain and making the cleavage exceptionally slow compared to its threo epimer.
Quick Facts About This Question
Question Information
| Subject | Chemistry |
| Chapter | Aldehydes, Ketones, and Carboxylic Acids - Class 12 |
| Concept | Cyanohydrin Stereochemistry |
| Difficulty | 8 |
| Cognitive Load | 7 |
| Target Solve Time | 160 sec |
| Access | Free |