Cyclopentadiene is an organic compound with the formula C₅H₆. This colorless liquid has a strong and unpleasant odor. At room temperature, this cyclic diene dimerizes over the course of hours to give dicyclopentadiene via a Diels–Alder reaction. This dimer can be restored by heating to give the monomer.

The compound is mainly used for the production of cyclopentene and its derivatives. It is popularly used as a precursor to the cyclopentadienyl ligand (Cp) in cyclopentadienyl complexes in organometallic chemistry.

Cyclopentadiene production is usually not distinguished from dicyclopentadiene since they are interconverted. They are obtained from coal tar (about 10–20 g/tonne) and by steam cracking of Naphtha (about 14 kg/tonne). To obtain cyclopentadiene monomer, commercial dicyclopentadiene is cracked by heating to ~ 180 °C. The monomer is collected by distillation, and used soon thereafter.

The hydrogen atoms in cyclopentadiene undergo rapid [1,5]-sigmatropic shifts as indicated by ¹H NMR spectra recorded at various temperatures. Even more fluxional are the derivatives C₅H₅E(CH₃)₃ (E = Si, Ge, Sn), wherein the heavier element migrates from carbon to carbon with a low activation barrier.

Famously, cyclopentadiene dimerizes via a reversible Diels–Alder reaction. The conversion occurs in hours at room temperature, but the monomer can be stored for days at −20 °C. Cyclopentadiene is a highly reactive diene in the Diels–Alder reaction because less distortion of the diene termini is required to achieve the transition state geometry compared to other dienes.

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