Cyclopentadiene

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Template:Chembox header | Cyclopentadiene
Image:Cyclopentadiene.png
Template:Chembox header | General
Systematic name	1,3-Cyclopentadiene
Other names	cyclopentadien, pentole, pyropentylene
Molecular formula	C5H6
SMILES
Molar mass	66.10 g/mol
Appearance	colourless liquid
CAS number	[542-92-7]
Template:Chembox header | Properties
Density and phase	0.81 g/m3, liquid
Solubility in hexane, benzene, toluene
Melting point	-85 °C (188 K)
Boiling point	+41 °C (314 K)
Template:Chembox header | Structure
Molecular shape	envelope structure
Dipole moment	? D
Template:Chembox header | Hazards
MSDS	External MSDS
Main hazards	toxic, flammable
NFPA 704
Flash point	-32 °C
R/S statement
RTECS number
Template:Chembox header | Supplementary data page
Structure and properties	n, εr, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Template:Chembox header | Related compounds
Related hydrocarbon	benzene cyclobutadiene
Related compounds	aromaticitys dienes
Template:Chembox header | Except where noted otherwise, data are given for materials in their standard state (at 25°C, 100 kPa) Infobox disclaimer and references

Cyclopentadiene, with molecular formula C₅H₆, is a clear, colorless, liquid organic chemical, with an odor reminiscent of turpentine or camphor. It is a cyclic diene which slowly dimerizes into dicyclopentadiene through a Diels-Alder reaction if stored at room temperature. This is a reversible reaction and cyclopentadiene can be recovered by distillation. This should be done always before using the compound.

This dimer can be cracked and separated into the monomer by distillation. Then, removal of a proton leaves the five-membered aromatic cyclopentadienyl anion, C₅H₅^-, which can combine with a number of different transition metals to form cyclopentadienyl complexes, such as metallocenes.

The delocalisation energy gained by proton loss makes this compound weakly acidic, which is unusual for a hydrocarbon.

Metal-organic chemistry

Alkali metal bases can deprotonate cyclopentadiene to form the alkali metal derivatives of the cyclopentadienyl anion. Although these polar species are not really salts, they behave as a source of C₅H₅^-. For example NaC₅H₅ reacts with various metal compounds to form a variety of complexes that are throughout chemistry. The cyclopentadienyl unit, abbreviated Cp, typically bonds to metals through all five carbon atoms, like in ferrocene, FeCp₂; chromocene, CrCp₂; cobaltocene, CoCp₂; and nickelocene, NiCp₂. When the Cp rings are mutually parallel the compound is known as a sandwich complex. This area of organometallic chemistry was first discovered in 1954 with the realization of the structure of ferrocene.³

In rare cases, the Cp unit can bond through three carbon atoms, like in CpWCp(CO)2; or through one carbon atom, like in (cyclopentadiene)FeCp(CO)₂.

The Greek letter eta is used to denote the number of carbon atoms bound to the metal center, such as with a Cp ring. Eta 1 means one carbon, eta 3 means three carbons, and eta 5 means five carbons.⁴

Other metals form bent structures, like Zr(Cp)₂Cl₂, which is a catalyst for ethylene polymerization.³

References

1 Girolami, G. S., Rauchfuss, T. B., & Angelici, R. J. Synthesis and Technique in Inorganic Chemistry, A Laboratory Manual (1999) 3rd ed. Sausalito, CA: University Science Books.

2 Streitwieser, A., Heathcock, C. H., & Kosower, E. M. Introduction to Organic Chemistry (1998) 4th Ed. Revised Printing; Upper Saddle River, NJ: Prentice Hall.

3 Crabtree, R. H. The Organometallic Chemistry of the Transition Metals (2001) 3rd Ed. New York: John Wiley and Sons, Inc.

4 Shriver, D., Atkins, P., Inorganic Chemistry (1999) New York: W. H. Freeman and Company.

5 http://chemfinder.cambridgesoft.com “dicyclopentadiene” Accessed April 13, 2005.de:Cyclopentadien

ja:シクロペンタジエン