Question 20:
A spatial arrangement of substituent groups that are free to assume different positions in space, without breaking any bonds (due to freedom in bond rotation) is called:
Molecular Conformation Bond Rotation
Spatial arrangements of substituents via free rotation around single bonds define conformations in molecules. The correct term is option (B).
Correct Answer
(B) Conformation
Conformations arise from rotation about single bonds without breaking covalent linkages, creating different 3D shapes like staggered ethane vs. eclipsed.
Key Concept
Single bond rotation barrier (~3-5 kcal/mol) allows interconversion at room temperature. Stable conformations minimize steric repulsion; Newman projections visualize this.
Option Analysis
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(A) Rotation: Incorrect; describes the motion mechanism, not the resulting spatial arrangement.
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(B) Conformation: Correct; different spatial isomers interconvertible by bond rotation without bond breakage.
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(C) Conjugation: Incorrect; refers to p-orbital overlap in π systems (e.g., benzene), affecting stability via delocalization.
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(D) Config: Incorrect; likely shorthand for configuration, which requires bond breaking (e.g., R/S chirality).
| Option | Term | Requires Bond Breakage? | Status |
|---|---|---|---|
| A | Rotation | N/A (motion) | Wrong |
| B | Conformation | No | Correct |
| C | Conjugation | No (but π overlap) | Wrong |
| D | Config | Yes | Wrong |
Exam Preparation Tips
Distinguish: conformation (rotation, low energy barrier) vs. configuration (bond break, high barrier). Ethane staggered is most stable; butane antiperiplanar preferred.
