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JEE Advanced levelSingle correct optionDifficulty 3 of 3st102

A staggered Newman projection of butane-2,3-diol is viewed along C2-C3. The front carbon has OH at 12 o'clock, H at 4 o'clock and at 8 o'clock. The rear carbon has OH at 2 o'clock, at 6 o'clock and H at 10 o'clock. The molecule is redrawn as a Fischer projection with at the top (on C1) and at the bottom (on C4). Which description is correct?

  1. A(2S,3S); in the Fischer projection the OH at C2 is on the right and at C3 on the left; optically active
  2. B(2S,3R); in the Fischer projection both OH groups are on the right; meso
  3. C(2S,3S); in the Fischer projection both OH groups are on the right; optically active
  4. D(2R,3S); in the Fischer projection the OH at C2 is on the right and at C3 on the left; meso

Answer: A

Computing the descriptors from the Newman projection gives S at C2 and S at C3, so the compound is (2S,3S), a chiral stereoisomer. For a Fischer projection of 2,3-butanediol with top and bottom, S at C2 needs OH on the right and S at C3 needs OH on the left, so the OH groups are on opposite sides (threo). That agrees with the chiral (S,S) pair. A meso form would need both OH groups on the same side.
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