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This question is from GRB Problems In Organic Chemistry For Jee Advanced.

Identify the relation between these two compounds:

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(a) Homomers

(b) Enantiomers

(c) Diastereomers

(d) Different compounds

The answer given is (b) Enantiomers. However, I feel that it should be (c) Diastereomers.

Enantiomers are non-superimposable mirror images of each other. But as far as I can see, these two compounds are not mirror images of each other, and should be diastereomers.

What is the correct answer to this question?

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    $\begingroup$ First of all, they are poor drawings! If you assume that the uppermost group, Ph or H, is syn to the methano bridge, then they are diastereomers. $\endgroup$
    – user55119
    Jan 5 at 15:37

1 Answer 1

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Bicyclo[2.2.1]heptane (red box) has two planes of symmetry. It is achiral. Placing a phenyl ring at $\ce{C2}$ in A and B removes the symmetry and renders both structures chiral. A has the phenyl syn to the methano bridge while B has the phenyl syn to an ethano bridge. They have the same atom connectivity but different arrangements in space. They are diastereomers of each other.

Now "slide the phenyl group forward to $\ce{C3}$". This operation gives the enantiomers of A and B, namely, C and D, respectively. If you are not convinced, rotate C and D about a vertical axis in the plane of the page by $180^\circ$ in a counterclockwise direction such that $\ce{C1}$ and $\ce{C4}$ change positions, C and D will mirror A and B, respectively. Note that the R/S descriptors of enantiomers mirror one another but those of diastereomers do not.

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