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JEE Main levelSingle correct optionDifficulty 2 of 3st064

Assertion (A): meso-Tartaric acid is optically inactive although it has two stereocentres. Reason (R): the rotation caused by one stereocentre is cancelled by the equal and opposite rotation of the other, because the molecule has an internal plane of symmetry.

  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

A meso compound has an internal mirror plane (or an inversion centre) in some accessible conformation, so the molecule is superimposable on its mirror image. The two halves are mirror images of each other, so their contributions to the rotation are equal and opposite. A is true and R correctly explains it.
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