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JEE Advanced levelSingle correct optionDifficulty 3 of 3st151

Assertion (A): (2R,3S)-2,3-dibromobutane is optically inactive, whereas (2R,3S)-2,3-dibromopentane is optically active. Reason (R): In 2,3-dibromobutane the two stereocentres bear identical sets of groups, so the (R,S) isomer has an internal mirror plane; in 2,3-dibromopentane the two ends of the chain differ.

  1. ABoth A and R are true, but R is not the correct explanation of A
  2. BA is true but R is false
  3. CA is false but R is true
  4. DBoth A and R are true, and R is the correct explanation of A

Answer: D

2,3-Dibromobutane is symmetric end-to-end, so (2R,3S) is meso. In 2,3-dibromopentane C2 carries and C3 carries , so no mirror plane or inversion centre can exist and every stereoisomer, including (2R,3S), is chiral. R correctly explains the difference.
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