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JEE Advanced levelSingle correct optionDifficulty 3 of 3st184

Let X = (1R,3S)-1-bromo-3-chlorocyclohexane, Y = (1S,3R)-1-bromo-3-chlorocyclohexane and Z = (1S,3S)-1-bromo-3-chlorocyclohexane. Which statement is correct?

  1. AX is cis and meso, so X and Y are identical
  2. BX and Y are diastereomers, and X and Z are enantiomers
  3. CX is cis, X and Y are enantiomers, and X and Z are diastereomers
  4. DX is trans, X and Y are enantiomers, and X and Z are diastereomers

Answer: C

With different substituents (Br, Cl) there is no internal mirror plane, so the cis isomers are a chiral pair, (1R,3S) and (1S,3R), and the trans isomers a second pair, (1R,3R) and (1S,3S) (CIP check). X and Y have opposite descriptors at both centres, so they are enantiomers (not identical). Z differs from X at one centre only, so it is the trans diastereomer of X.
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