Discussion
What is the relationship between Ha and Hb in the product P formed in the following reaction ?
Answer: B
Ha and Hb are enantiotopic in P.
Methoxide removes HCl from the side chain (E2, taking the hydrogen on the isopropyl carbon), turning CH(Cl)-CH(CH3)2 into CH=C(CH3)2, while the ring CH2 carrying Ha and Hb is untouched. The chlorine-bearing stereocentre disappears, and P, a cyclopentadiene with a conjugated 2-methylpropenyl group, is achiral and essentially planar.
The ring CH2 lies in that plane with Ha above it and Hb below. The molecular plane is a mirror plane that exchanges them, but no rotation axis does. Protons exchanged only by reflection are enantiotopic: replacing one or the other gives a pair of enantiomers.
They would be diastereotopic if a stereocentre remained in P, as in the starting chloride. They are not homotopic because no rotation interchanges them, and vicinal describes atoms on adjacent carbons, which these geminal protons are not.
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