Compare the two protons H₁ and H₂ shown in the following compound and identify which proton is more acidic.

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Removing H₁ gives the cyclopentadienyl anion : structure

Removing H₂ gives a vinylic carbanion : structure

The cyclopentadienyl anion has 6 π electrons delocalised over all five ring carbons - it is aromatic (Hückel 4n+2 with n = 1) and exceptionally stable (cyclopentadiene pKa ~ 15). The vinylic anion has no such stabilisation (pKa ~ 45). H₁ is far more acidic.

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