Which hydrocarbon is the most acidic?

Acyclohexane
Bcyclopenta-1,3-diene
Ccyclohepta-1,3,5-triene
Dcyclohexa-1,3-diene

Answer: B

Loss of a proton from cyclopenta-1,3-diene gives the cyclopentadienyl anion, a planar ring with 6 pi electrons, which is aromatic and strongly stabilised (pKa about 16). The other hydrocarbons give anions that are not aromatic, with pKa of 35 or above.

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