Discussion
The molecule that gets inter-converted into its enantiomer, upon ring flipping in its chair conformation, is ____________.
Answer: A
The molecule is cis-1,2-dimethylcyclohexane.
In a chair, a cis-1,2 pair must be one axial and one equatorial. Ring flipping turns the axial methyl equatorial and the equatorial one axial, and the new chair is the non-superimposable mirror image of the old one. So each chair is chiral, but the two chairs are enantiomers that interconvert rapidly at room temperature, which is why the compound cannot be resolved and behaves as an achiral (meso) compound.
The other options do not do this. In cis-1,3-dimethylcyclohexane both chairs (diaxial and diequatorial) contain a mirror plane, so neither is chiral. In cis-1,4-dimethylcyclohexane the flip gives an identical, achiral chair. In trans-1,2-dimethylcyclohexane the flip converts the diequatorial chair into the diaxial chair of the same enantiomer, a diastereomeric conformer, not its mirror image.
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