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JEE Advanced levelOne or more correct optionsDifficulty 3 of 3st108

(2R,3R)-2,3-Dibromobutane is drawn as a Fischer projection with at the top and bottom. Which statements are correct?

  1. AThe two Br atoms are on opposite sides of the vertical chain.
  2. BThe Fischer arrangement corresponds to an eclipsed conformation in which the Br/Br dihedral angle is .
  3. CIn the Fischer arrangement the Br/Br dihedral angle is .
  4. DInterchanging Br and H on C3 only gives the meso diastereomer.

Answer: A, B, D

For (R,R), C2 has Br on the left and C3 has Br on the right, so the Br atoms are on opposite sides. In the eclipsed Fischer conformer, groups on opposite sides are apart (not , which applies to same-side groups). Interchanging Br and H at C3 changes (R,R) to (R,S), the meso diastereomer.
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