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Question #44445
The Question
Consider the nucleophilic addition of \ce{HCN} (generated \textit{in situ} from \ce{NaCN} and dilute \ce{H2SO4}) to (3S)-3-phenylbutan-2-one at 298 K. Which of the following statements correctly describes the stereochemical outcome of the major product(s)?
Options
- A.
A racemic mixture of (2R, 3S) and (2S, 3R) isomers is formed in a 1:1 ratio due to equal probability of facial attack.
- B.
A pair of diastereomers is formed in unequal amounts, with (2S, 3S)-2-hydroxy-2-methyl-3-phenylbutanenitrile being the major product.
- C.
A pair of diastereomers is formed in unequal amounts, with (2R, 3S)-2-hydroxy-2-methyl-3-phenylbutanenitrile being the major product.
- D.
A single optically inactive meso compound is formed due to stereospecific anti-addition.
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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
Cyanohydrin Stereochemistry|Nucleophilic Addition Sterics|Optical Activity and Enantiomers
Why This Question Is Tricky
Common Mistakes
This question tests the 'Hidden Variable' of diastereotopic facial selectivity.
A common trap for students is rote-memorizing that the addition of HCN to an unsymmetrical ketone yields a 50:50 racemic mixture of enantiomers.
However, when the ketone possesses an adjacent stereocenter (a chiral environment), the faces of the carbonyl become diastereotopic.
The transition states for attack on either face are diastereomeric and thus have different activation energies.
The Felkin-Anh model must be applied to predict the major diastereomer, rewarding students who understand stereoelectronic control over those who rely on binary textbook rules.
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 |