When evaluating highly symmetrical molecules for total stereoisomerism, why does the simplistic unsymmetrical rule fundamentally fail, mathematically resulting in a severe overcount of isomers?

Question

When evaluating highly symmetrical molecules for total stereoisomerism, why does the simplistic unsymmetrical $2^n$ rule fundamentally fail, mathematically resulting in a severe overcount of isomers?

Prashant Kotian · Accepted Answer

Correct answer: A,B,C. The formula assumes every possible structural permutation generates a physically distinct mirror-image enantiomer. In symmetrical molecules, certain permutations create conformations containing an internal Plane of Symmetry. The mirror image of this conformation is physically superimposable on the original, meaning they are the exact same molecule (a meso compound). This collapses an expected pair (2) into a single isomer (1), invalidating the rule.

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