Cyclopentadiene

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Cyclopentadiene
Image:Cyclopentadiene.png
General
Systematic name	1,3-cyclopentadiene
Other names	cyclopentadien, pentole, pyropentylene
Molecular formula	C₅H₆
SMILES	C1C=CC=CC1
Molar mass	66.10 g/mol
Appearance	colourless liquid
CAS number	[542-92-7]
Properties
Density and phase	0.81 g/cm³, liquid
Solubility in hexane, benzene, toluene	miscible
Melting point	-85 °C (188 K)
Boiling point	41 °C (314 K)
Structure
Molecular shape	envelope structure
Dipole moment	? D
Hazards
MSDS	External MSDS
EU classification	not listed
NFPA 704
Flash point	25 °C
RTECS number	GY1000000
Supplementary data page
Structure and properties	n, ε_r, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Related compounds
Related hydrocarbon	Benzene Cyclobutadiene
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, 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 to form dicyclopentadiene through a Diels-Alder reaction if stored at room temperature. This dimer can be cracked and separated into the monomer by distillation at atmospheric pressure.

Deprotonation yields the five-membered aromatic hydrocarbon cyclopentadienyl anion, C₅H₅⁻, which can combine with a number of different transition metals to form cyclopentadienyl complexes, such as metallocenes.

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

Monomeric cyclopentadiene's methylene hydrogens rapidly undergo [1,5] sigmatropic shifts and occupy positions at all carbons; at room temperature the NMR spectrum appears as a single peak at 4.8 ppm. At cooler temperatures, these resolve into peaks at 4.0, 5.2, and 5.4 ppms, which reflect the static structure drawn in most depictions of cyclopentadiene.

1 Organometallic chemistry
2 See also
3 References
4 Other sources
5 External links

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Organometallic 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.Template:Ref

In rare cases, the Cp unit can bond through three carbon atoms, like in [(η³Cp)WCp(CO)₂]; or through one carbon atom, as in [(η¹-Cp)FeCp(CO)₂]. The Greek letter η (eta) is used to denote the number of carbon atoms bound to the metal center, such as with a Cp ring. η¹ means one carbon, η³ means three carbons, and η⁵ means five carbons.Template:Ref

Other metals form bent structures, such as zirconocene dichloride, [ZrCp₂Cl₂], which is a catalyst for ethylene polymerization.

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References

Template:Note Crabtree, R. H. (2001). The Organometallic Chemistry of the Transition Metals (3rd Edn.) New York: John Wiley and Sons.
Template:Note Shriver, D.; Atkins, P. (1999). Inorganic Chemistry, New York: W. H. Freeman.

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Other sources

Girolami, G. S.; Rauchfuss, T. B.; & Angelici, R. J. (1999). Synthesis and Technique in Inorganic Chemistry, A Laboratory Manual (3rd Edn.) Sausalito, CA: University Science Books.
Streitwieser, A.; Heathcock, C. H.; Kosower, E. M. (1998). Introduction to Organic Chemistry (4th Edn.) Upper Saddle River, NJ: Prentice Hall.
ChemFinder "Dicyclopentadiene" Accessed 2005-04-13.

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