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JEE Main levelSingle correct optionDifficulty 2 of 3st090

A Fischer projection of lactic acid has at the top, at the bottom, and on the central carbon H on the left and OH on the right. What is its CIP descriptor?

  1. AR
  2. Bcannot be assigned without knowing the optical rotation
  3. CR, because OH is on the right as in D-glyceraldehyde
  4. DS

Answer: D

The priorities are OH, then , then , then H. H is on a horizontal bond. OH (right) to (bottom) to (top) is clockwise, and the sense is reversed because H is horizontal, so the centre is S. The CIP label does not depend on D or L conventions.
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