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Two Fischer projections are drawn for a compound with adjacent stereocentres bearing CHO, OH, H and CH3 substituents. One projection is obtained from the other by rotating the entire formula 180 degrees in the plane of the paper. What is the stereochemical relationship between the two representations?

1. Identical compounds
2. Enantiomers
3. Diastereomers
4. Conformers

Correct answer: Identical compounds

Solution

In Fischer projection conventions, rotating the entire projection by 180 degrees in the plane of the paper does NOT change the configuration at any stereocentre. This is because each 90 deg rotation swaps horizontal and vertical substituents at every carbon (inverting configuration), and two such swaps restore the original. Therefore the two structures represent exactly the same compound with identical configurations at all chiral centres. They are identical, not enantiomers or diastereomers.

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