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JEE Main levelSingle correct optionDifficulty 2 of 3g1073

Which of the following hydrocarbons is the most acidic?

  1. Apropene
  2. Bcyclohexa-1,3-diene
  3. Ccyclopenta-1,3-diene
  4. Dcyclohepta-1,3,5-triene

Answer: C

Loss of a proton from cyclopenta-1,3-diene gives the cyclopentadienide anion, which is planar and has 6 electrons (aromatic). The other hydrocarbons give non-aromatic (at best allylic or pentadienylic) anions. Cyclopentadiene is therefore far more acidic, with pKa about 16 compared with over 30 for the others.
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