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JEE Main levelSingle correct optionDifficulty 2 of 3st174

Assertion (A): cis-1,2-Dimethylcyclopropane is optically inactive although it contains two stereocentres. Reason (R): The molecule has an internal plane of symmetry that passes through the CH carbon and the midpoint of the C1-C2 bond.

  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

The cis isomer (1R,2S) has the two methyl groups on the same face of the ring, and the plane through C3 and the midpoint of C1-C2 reflects C1 into C2 (R into S). The molecule is therefore meso and optically inactive. The reason is true and is exactly why the assertion holds.
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