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Cyclopentadiene, with molecular formula C5H6, is a clear, colorless, liquid organic chemical, with an odor reminiscent of turpentine or camphor. It is a cyclic diene which forms a dimer through Diels-Alder reaction under standard conditions.

This dimer can be cracked and separated into the monomer by distillation. Then, removal of a proton leaves the five-membered aromatic cyclopentadienyl anion, C5H5-, which can combine with a number of different transition metals to form organometallic compounds, such as metallocenes.

The delocalisation energy gained by proton loss makes this compound weakly acidic, which is unusual for a hydrocarbon.