Cyclopentadiene

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.

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Properties

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]

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.

See also

• Aromaticity
• Methylcyclopentadiene and pentamethylcyclopentadiene, also found as ligands in organometallic chemistry

References

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). http://www.orgsyn.org/orgsyn/pdfs/CV9P0186.pdf
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