Cyclopentadiene

Cyclopentadiene is the chemical compound with the formula C5H6. This colorless liquid organic chemical has an odor reminiscent of turpentine or camphor. 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 these are uncommon.

Properties
The hydrogen atoms in cyclopentadiene undergo rapid [1,5]-sigmatropic shifts as indicated by the variable temperature1H NMR spectra recorded at various temperatures.

Reactivity of cyclopentadiene
Cyclopentadiene readily undergoes Diels-Alder reactions with dienophiles, such as 1,4-benzoquinone.

The compound is weakly acidic, which is unusual for a hydrocarbon. This relatively high acidity is explained by the stability of the aromatic, anionic, 5-membered ring (cyclopentadienyl anion, C5H5−). Derivatives of this anion are often depicted as salts, although the free anion does not exist in any appreciable extent in solution. Deprotonation can be achieved, e.g. by alkali metal bases. The anion serves as a weak nucleophile in organic synthesis and combines with numerous anhydrous halides of the transition metals to form cyclopentadienyl complexes, such as metallocenes.

Uses
Aside from its use as a synthetic intermediate, cyclopentadiene itself has few applications because it reverts so readily to dicyclopentadiene. Cyclopentadiene reacts with linseed oil to facilitate "drying" upon contact with oxygen.