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JEE Advanced levelSingle correct optionDifficulty 3 of 3g3245

Cyclopentadiene (pKa about 16) is a far stronger carbon acid than the compound shown (pKa about 36). Which explanation is correct?

  1. AThe compound shown has no CH group, so it has no acidic hydrogen.
  2. BThe anion formed from cyclopentadiene is a planar cyclic ion with 6 pi electrons (aromatic), whereas the anion from the compound shown would have 8 pi electrons in a cycle (antiaromatic) and gains no such stabilisation.
  3. CThe anion from the compound shown is aromatic but strained, while that from cyclopentadiene is not aromatic.
  4. DSeven-membered rings cannot lose a proton from an sp carbon.

Answer: B

The compound shown is cycloheptatriene, whose only sp carbon is a CH group. Removing a proton gives a cycloheptatrienyl anion with 8 pi electrons ( with ), which would be antiaromatic and is not stabilised. Cyclopentadiene gives the cyclopentadienyl anion with 6 pi electrons (, ), which is aromatic, explaining the 20 pKa unit difference.
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