JEE Advanced 2021 Paper 1 Q02 Chemistry Organic Chemistry Isomerism & Stereochemistry Hard

JEE Advanced 2021 Paper 1 · Q02 · Isomerism & Stereochemistry

Among the following, the conformation that corresponds to the most stable conformation of meso-butane-2,3-diol is:

  1. A. A staggered Newman projection of meso-(2R,3S)-butane-2,3-diol in which the two $-OH$ groups are anti to each other ($180°$ apart), the two $-CH_3$ (Me) groups are gauche, and all three pairs of substituents are staggered — the anti-OH conformer of the meso isomer.
  2. B. A staggered Newman projection of meso-(2R,3S)-butane-2,3-diol in which the two $-OH$ groups are gauche to each other ($60°$ apart), positioned on the same side to allow formation of an intramolecular $O{-}H{\cdots}O$ hydrogen bond, while the two $-CH_3$ groups are anti — the gauche-OH conformer of the meso isomer.
  3. C. A staggered Newman projection in which the two $-CH_3$ groups are gauche and both $-OH$ groups are on the same face, but the arrangement of substituents around both chiral centres corresponds to the (2R,3R) or (2S,3S) enantiomeric pair — this does NOT represent the meso isomer.
  4. D. A staggered Newman projection in which the substituents around C2 and C3 are arranged such that the overall molecule has a $C_2$ rotation axis but no internal mirror plane — this does NOT represent the meso (R,S) isomer.
Reveal answer + step-by-step solution

Correct answer:B

Solution

Meso-butane-2,3-diol has the (2R,3S) configuration. Among the staggered Newman projections of the meso compound, the gauche conformer in which the two -OH groups are 60 degrees apart is stabilised by an intramolecular O-H...O hydrogen bond between the adjacent hydroxyls. Although anti (180 degrees) staggered conformers normally minimise steric strain, here the energetic gain from intramolecular hydrogen bonding outweighs the small gauche strain, so the gauche-OH form is the most stable. Options (C) and (D) do not represent the meso (R,S) isomer at all, leaving (B) as the unique correct choice.

Want to drill deeper? Open this question inside SolveKar AI and tap "Why?" on any step — the AI Mentor reads your full solution context and explains until it clicks. Download SolveKar AI →