-85 °C (188 K)
41 °C (314 K)
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 "cracks" to give the monomer upon heating.
The compound is the precursor to the popular cyclopentadienyl-ligand ('Cp') in cyclopentadienyl complexes in organometallic chemistry. Both cyclopentadiene and dicyclopentadiene can serve as ligands as well, but such complexes are uncommon.
Production and reactions
Cyclopentadiene production is usually not distinguished from dicyclopentadiene since they are interconverted. They are obtained from coal tar (about 10 – 20 g/ton) and steam cracking from naphtha (about 14 kg/ton).
The hydrogen atoms in cyclopentadiene undergo rapid [1,5]-sigmatropic shifts as indicated by 1H NMR spectra recorded at various temperatures.
Famously, cyclopentadiene dimerizes via a reversible Diels-Alder reaction. The rate is convenient at room temperature, and the monomer can be stored for days at -20 °C.
Cyclopentadiene readily undergoes other Diels-Alder reactions with dienophiles such as 1,4-benzoquinone. Cycloaddition of O2 gives the bicyclic peroxide.
The compound is unusually acidic (pKa 16) for a hydrocarbon, a fact explained by the high stability of the aromatic cyclopentadienyl anion, C5H5−. Simple compounds of this anion (such as the commercially available sodium cyclopentadienide) are often depicted as salts, although the free anion is not present in appreciable quantities in solution.
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