Cyclopentadiene

Cyclopentadiene

Skeletal formula of cyclopentadiene Spacefill model of cyclopentadiene
Ball and stick model of cyclopentadiene
Names
Systematic IUPAC name Cyclopenta-1,3-diene
Other names Pentole Pyropentylene 1,3-Cyclopentadiene
Identifiers
CAS Number	542-92-7 Y
Abbreviations	CPD, HCp
Beilstein Reference	471171
ChEBI	CHEBI:30664 Y
ChemSpider	7330 Y
EC Number	208-835-4
Gmelin Reference	1311
Jmol 3D model	Interactive image
MeSH	1,3-cyclopentadiene
PubChem	7612
RTECS number	GY1000000
UNII	5DFH9434HF Y
InChI InChI=1S/C5H6/c1-2-4-5-3-1/h1-4H,5H2 Y Key: ZSWFCLXCOIISFI-UHFFFAOYSA-N Y InChI=1/C5H6/c1-2-4-5-3-1/h1-4H,5H2 Key: ZSWFCLXCOIISFI-UHFFFAOYAI
SMILES C1C=CC=C1
Properties
Chemical formula	C5H6
Molar mass	66.10 g·mol−1
Appearance	Colourless liquid
Odor	irritating, terpene-like
Density	0.786 g cm−3
Melting point	−90 °C; −130 °F; 183 K
Boiling point	39 to 43 °C; 102 to 109 °F; 312 to 316 K
Solubility in water	insoluble
Vapor pressure	400 mmHg
Acidity (pKa)	16
Basicity (pKb)	-2
Structure
Molecular shape	Planar
Thermochemistry
Specific heat capacity (C)	115.3 J K−1 mol−1
Std molar entropy (So298)	182.7 J K−1 mol−1
Vapor pressure	{{{value}}}
Related compounds
Related hydrocarbons	Benzene Cyclobutadiene Cyclopentene
Related compounds	Dicyclopentadiene
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Cyclopentadiene is an organic compound with the formula C₅H₆. This colorless liquid 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 can be restored by heating to give the monomer.

The compound is mainly used for the production of cyclopentene and its derivatives. It is popularly used as a precursor to the cyclopentadienyl ligand (Cp) in cyclopentadienyl complexes in organometallic chemistry.

Production and reactions

Cyclopentadiene production is usually not distinguished from dicyclopentadiene since they are interconverted. They are obtained from coal tar (about 10 – 20 g/ton) and by steam cracking of Naphtha (about 14 kg/ton). To obtain cyclopentadiene monomer, commercial dicyclopentadiene is cracked by heating to ~ 180 °C. The monomer is collected by distillation, and used soon thereafter.

Sigmatropic rearrangement

The hydrogen atoms in cyclopentadiene undergo rapid [1,5]-sigmatropic shifts as indicated by ¹H NMR spectra recorded at various temperatures. Even more fluxional are the derivatives C₅H₅E(CH₃)₃ (E = Si, Ge, Sn), wherein the heavier element migrates from carbon to carbon with a low activation barrier.

Diels–Alder reactions

Famously, cyclopentadiene dimerizes via a reversible Diels–Alder reaction. The conversion occurs in hours at room temperature, but the monomer can be stored for days at −20 °C. Cyclopentadiene is a highly reactive diene in the Diels–Alder reaction because less distortion of the diene termini is required to achieve the transition state geometry compared to other dienes.

Deprotonation

Main article: Cyclopentadienyl

The compound is unusually acidic (pK_a 16) for a hydrocarbon, a fact explained by the high stability of the aromatic cyclopentadienyl anion, C₅H₅⁻. Deprotonation can be achieved with a variety of bases, typically sodium hydride or even sodium metal. Simple compounds of this anion (such as the commercially available sodium cyclopentadienide) are often depicted as salts, although the free anion is not present in appreciable quantities in solution, more complex coordination compounds are known as cyclopentadienyl complexes.

Metallocene derivatives

Main article: metallocene

File:Protonated rhodocene.svg

[(η⁵-C₅H₅)Rh(η⁴-C₅H₆)], an 18-valence electron mixed-hapticity rhodocene derivative that can form when the rhodocene monomer is protonated.

Metallocenes and related cyclopentadienyl derivatives have been heavily investigated and represent a cornerstone of organometallic chemistry owing to their high stability. Indeed, the first metallocene characterised, ferrocene, was prepared the way many other metallocenes are prepared: by combining alkali metal derivatives of the form MC₅H₅ with dihalides of the transition metals: As typical example, nickelocene forms upon treating nickel(II) chloride with sodium cyclopentadiene in THF.

NiCl₂ + 2 NaC₅H₅ → Ni(C₅H₅)₂ + 2 NaCl

Organometallic complexes that include both the cyclopentadienyl anion and cyclopentadiene itself are known, one example of which is the rhodocene derivative produced from the rhodocene monomer in protic solvents.

Uses

Cyclopentadiene is mainly useful as a precursor to cyclopentene and related monomers such as ethylidenenorbornene. Such species are used in the production of specialty polymers. Cyclopentadienyl complexes serve as reagents in organic synthesis. It was also used as the starting material in Leo Paquette's 1982 dodecahedrane synthesis. The first step involved reductive dimerization of the molecule to give dihydrofulvalene, not simple addition to give dicyclopentadiene.

File:DodecahedranePrecursorSynthesis.png

The start of Paquette's 1982 dodecahedrane synthesis. Note the dimerisation of cyclopentadiene in step 1 to dihydrofulvalene.

Abbreviation

The commonly used abbreviation of the cyclopentadienyl anion is Cp. The abbreviation played a part in the naming of copernicium: the original proposal for the element's symbol was also Cp, but because of the abbreviation for this anion and the fact that lutetium was originally named cassiopeium and had Cp for the symbol as well, the symbol for copernicium was changed to Cn.

See also

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

External links

• International Chemical Safety Card 0857
• NIOSH Pocket Guide to Chemical Hazards