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JEE Main levelSingle correct optionDifficulty 2 of 3st023

A Fischer projection of butan-2-ol has the vertical chain, top to bottom: , C2, . On C2 the left group is and the right group is . The configuration of C2 is

  1. A
  2. Bcannot be assigned because the chain is written with the ethyl group on top
  3. C
  4. D if the projection is rotated by in the plane

Answer: A

Priorities: . In the projection (left) ethyl (top) methyl (bottom) runs clockwise. Since is on a horizontal bond (pointing at the viewer), the true sense is reversed: anticlockwise, i.e. . A rotation would invert the meaning of the projection and is not allowed.
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