A colourless liquid cyclic alkene, C₅H₆; r.d. 0.8021; m.p. –97.2°C; b.p. 40.0°C. It is prepared as a by-product during the fractional distillation of crude benzene from coal tar. It undergoes condensation reactions with ketones to give highly coloured compounds (fulvenes) and readily undergoes polymerization at room temperature to give the dimer, dicyclopentadiene. Cyclopentadiene itself is not an aromatic compound because it does not have the required number of pi electrons. However, removal of a hydrogen atom produces the stable cyclopentadienyl ion, C₅H₅⁻, which does have aromatic properties. In particular, the ring can coordinate to positive ions in such compounds as ferrocene.

Cyclopentadiene