Cyclopentadiene

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 cyclopentadienyl complexes, such as metallocenes.

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