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Other names pentole, pyropentylene, CPD
CAS number 542-92-7
RTECS number GY1000000
Molecular formula C5H6
Molar mass 66.10 g/mol
Appearance colourless liquid
Density 0.81 g/cm³, liquid
Melting point

-85 °C (188 K)

Boiling point

41 °C (314 K)

Molecular shape envelope structure
Flash point 25 °C
Related Compounds
Related hydrocarbon Benzene
Related compounds Dicyclopentadiene
Except where noted otherwise, data are given for
materials in their standard state
(at 25 °C, 100 kPa)

Infobox disclaimer and references

Cyclopentadiene is the chemical compound with the formula C5H6. This colorless liquid organic chemical 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 these are uncommon.



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

Reactivity of cyclopentadiene

Cyclopentadiene readily undergoes Diels-Alder reactions with dienophiles, such as 1,4-benzoquinone.[2]

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.[3]


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.

See also


  1. ^ Streitwieser, A.; Heathcock, C. H.; Kosower, E. M. (1998). Introduction to Organic Chemistry (4th Edn.) Upper Saddle River, NJ: Prentice Hall.
  2. ^ Masaji Oda, Takeshi Kawase, Tomoaki Okada, and Tetsuya Enomoto "2-Cyclohexene-1,4-dione" Organic Syntheses, Collected Volume 9, p.186 (1998).
  3. ^ Girolami, G. S.; Rauchfuss, T. B. and Angelici, R. J., Synthesis and Technique in Inorganic Chemistry, University Science Books: Mill Valley, CA, 1999.ISBN 0935702482
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Cyclopentadiene". A list of authors is available in Wikipedia.
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