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Question #43958
The Question
Consider the chiral amino ketone, (2S)-2-(dimethylamino)-1-phenylpropan-1-one (Compound \textbf{X}). Compound \textbf{X} is subjected to two different reduction pathways:
\textbf{Pathway 1:} Reduction with \ce{LiAlH4} in dry diethyl ether at to yield major product \textbf{P}.
\textbf{Pathway 2:} Compound \textbf{X} is treated with 1 equivalent of anhydrous \ce{HCl} in ether to form a salt, followed by reduction with \ce{NaBH4} in ethanol to yield major product \textbf{Q}.
Assuming standard stereochemical models, identify the absolute configurations of the newly formed chiral center (C1) and the existing chiral center (C2) in the major products \textbf{P} and \textbf{Q} respectively.
Options
- A.
\textbf{P}: (1S, 2S); \textbf{Q}: (1R, 2S)
- B.
\textbf{P}: (1R, 2S); \textbf{Q}: (1S, 2S)
- C.
\textbf{P}: (1S, 2S); \textbf{Q}: (1S, 2S)
- D.
\textbf{P}: (1R, 2S); \textbf{Q}: (1R, 2S)
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
Reduction Selectivity (Hydrides)|Grignard Reaction Pathways|Inductive Effect Trends
Why This Question Is Tricky
Common Mistakes
The question tests the student's ability to distinguish between Chelation control and Felkin-Anh control in the reduction of chiral -amino ketones.
The 'hidden variable' is the basicity of the amine group.
In neutral/basic conditions (Pathway 1), the amine lone pair chelates with the lithium ion, locking the conformation and directing hydride attack to give the syn (1S, 2S) product.
In Pathway 2, protonation of the amine destroys its ability to chelate.
The reaction defaults to the polar Felkin-Anh model, where the bulky ammonium group dictates an anti-attack, yielding the anti (1R, 2S) product.
Students who ignore the acid-base chemistry will incorrectly assume chelation for both pathways.
Quick Facts About This Question
Question Information
| Subject | Chemistry |
| Chapter | Aldehydes, Ketones, and Carboxylic Acids - Class 12 |
| Concept | Reduction Selectivity (Hydrides) |
| Difficulty | 8 |
| Cognitive Load | 7 |
| Target Solve Time | 160 sec |
| Access | Free |