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JEE Advanced levelSingle correct optionDifficulty 3 of 3g3273

The optically active compound shown is treated for a long time with aqueous NaOH. What happens to its optical activity?

  1. AIt is inverted completely to the enantiomer.
  2. BIt is retained, because the stereocentre is beta to the carbonyl and no bond to it is broken during enolisation.
  3. CIt is lost, because the stereocentre becomes planar in the enolate.
  4. DIt becomes a meso compound.

Answer: B

The compound shown is 3-methylcyclohexanone, whose stereocentre is C3, a beta carbon. Base can form enolates toward C2 and C6, which involve only the alpha carbons; the C3 stereocentre keeps its four bonds unchanged, so the configuration at C3 is not affected. The compound therefore stays optically active; racemisation would occur only for an alpha stereocentre, as in 2-methylcyclohexanone.
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