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Question #44456
The Question
Consider the synthesis of three biphenyl derivatives via the Ullmann coupling reaction (heating the respective aryl halide with Cu powder):
Reaction 1: \ce{1-bromo-2,4-dinitrobenzene ->[Cu, \Delta]} Compound \textbf{X} Reaction 2: \ce{2-bromo-1-methyl-3-nitrobenzene ->[Cu, \Delta]} Compound \textbf{Y} Reaction 3: \ce{2-bromo-1,3-dimethylbenzene ->[Cu, \Delta]} Compound \textbf{Z}
Which of the compounds \textbf{X}, \textbf{Y}, and \textbf{Z} can be resolved into stable enantiomers at 298 K?
Options
- A.
\textbf{X} and \textbf{Y} only
- B.
\textbf{Y} only
- C.
\textbf{Y} and \textbf{Z} only
- D.
\textbf{X}, \textbf{Y}, and \textbf{Z}
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
Alkane Conformation Energy Barriers|Optical Isomerism Logic|Cahn-Ingold-Prelog (CIP) Priority
Why This Question Is Tricky
Common Mistakes
The question tests the dual requirements for atropisomerism in biphenyls: (1) restricted rotation due to steric hindrance (kinetic stability of the enantiomers) and (2) the absence of a plane or center of symmetry (chirality).
Students often fall into the trap of assuming that any biphenyl with four bulky ortho-substituents is automatically resolvable.
Compound \textbf{Z} acts as the 'Lethal Collision' here; despite having a massive rotational energy barrier, it is achiral because each ring is symmetrically substituted, creating internal planes of symmetry in its orthogonal conformation.
Compound \textbf{X} tests the kinetic aspect, showing that chirality alone is insufficient if the rotational barrier is too low to prevent racemization at room temperature.
Quick Facts About This Question
Question Information
| Subject | Chemistry |
| Chapter | Hydrocarbons - Class 11 Part 2 |
| Concept | Alkane Conformation Energy Barriers |
| Difficulty | 8 |
| Cognitive Load | 7 |
| Target Solve Time | 160 sec |
| Access | Free |