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JEE Advanced levelSingle correct optionDifficulty 3 of 3st081

Assertion (A): in the meso stereoisomers of 2,3,4-trichloropentane the central carbon C3 is a stereogenic centre even though it carries two constitutionally identical groups. Reason (R): the two constitutionally identical branches at C3 have opposite chirality (one R, one S), so exchanging two substituents at C3 gives a different stereoisomer.

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

Answer: A

In the meso forms C2 is R and C4 is S, so the two branches on C3 are mirror-image (enantiomorphic) groups. Swapping H and Cl on C3 then converts one meso diastereomer into the other, so C3 is stereogenic (pseudoasymmetric, r or s). Both statements are true and R is the explanation.
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