Cyclopentadiene

Cyclopentadiene
Skeletal formula of cyclopentadiene	Spacefill model of cyclopentadiene
	Ball and stick model of cyclopentadiene
Names
	Preferred IUPAC name Cyclopenta-1,3-diene
	Other names 1,3-Cyclopentadiene; Pyropentylene
Identifiers
CAS Number	542-92-7 ;
3D model (JSmol)	Interactive image;
Abbreviations	CPD, HCp
Beilstein Reference	471171
ChEBI	CHEBI:30664 ;
ChemSpider	7330 ;
ECHA InfoCard	100.008.033
EC Number	208-835-4;
Gmelin Reference	1311
MeSH	1,3-cyclopentadiene
PubChem CID	7612;
RTECS number	GY1000000;
UNII	5DFH9434HF ;
CompTox Dashboard (EPA)	DTXSID0027191 ;
	InChI InChI=1S/C5H6/c1-2-4-5-3-1/h1-4H,5H2 Key: ZSWFCLXCOIISFI-UHFFFAOYSA-N ; 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	66.103 g·mol−1
Appearance	Colourless liquid
Odor	irritating, terpene-like
Density	0.802 g/cm3
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 (53 kPa)
Acidity (pKa)	16
Conjugate base	Cyclopentadienyl anion
Magnetic susceptibility (χ)	−44.5×10−6 cm3/mol
Refractive index (nD)	1.44 (at 20 °C)
Structure
Molecular shape	Planar
Dipole moment	0.419 D
Thermochemistry
Heat capacity (C)	115.3 J/(mol·K)
Std molar; entropy (S⦵298)	182.7 J/(mol·K)
Std enthalpy of; formation (ΔfH⦵298)	105.9 kJ/mol
Hazards
NFPA 704 (fire diamond)	2 3 0
Flash point	25 °C (77 °F; 298 K)
Autoignition; temperature	640 °C (1,184 °F; 913 K)
	Lethal dose or concentration (LD, LC):
LC50 (median concentration)	14,182 ppm (rat, 2 h); 5091 ppm (mouse, 2 h)
	NIOSH (US health exposure limits):
PEL (Permissible)	TWA 75 ppm (200 mg/m3)
REL (Recommended)	TWA 75 ppm (200 mg/m3)
IDLH (Immediate danger)	750 ppm
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). verify (what is ?) Infobox references

Cyclopentadiene is an organic compound with the formula C₅H₆.^[6] It is often abbreviated CpH because the cyclopentadienyl anion is abbreviated Cp⁻.

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 anion (Cp⁻), an important ligand in cyclopentadienyl complexes in organometallic chemistry.^[7]

Production and reactions

Cyclopentadiene production is usually not distinguished from dicyclopentadiene since they interconvert. They are obtained from coal tar (about 10–20 g/t) and by steam cracking of naphtha (about 14 kg/t).^[8] To obtain cyclopentadiene monomer, commercial dicyclopentadiene is cracked by heating to around 180 °C. The monomer is collected by distillation and used soon thereafter.^[9] It advisable to use some form of fractionating column when doing this, to remove refluxing uncracked dimer.

Sigmatropic rearrangement

The hydrogen atoms in cyclopentadiene undergo rapid [1,5]-sigmatropic shifts. The hydride shift is, however, sufficiently slow at 0 °C to allow alkylated derivatives to be manipulated selectively.^[10]

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

Cyclopentadiene is a highly reactive diene in the Diels–Alder reaction because minimal distortion of the diene is required to achieve the envelope geometry of the transition state compared to other dienes.^[11] Famously, cyclopentadiene dimerizes. The conversion occurs in hours at room temperature, but the monomer can be stored for days at −20 °C.^[8]

Deprotonation

The compound is unusually acidic (pK_a = 16) for a hydrocarbon, a fact explained by the high stability of the aromatic cyclopentadienyl anion, C
₅H⁻
₅. Deprotonation can be achieved with a variety of bases, typically sodium hydride, sodium metal, and butyl lithium. Salts of this anion are commercially available, including sodium cyclopentadienide and lithium cyclopentadienide. They are used to prepare cyclopentadienyl complexes.

Metallocene derivatives

Metallocenes and related cyclopentadienyl derivatives have been heavily investigated and represent a cornerstone of organometallic chemistry owing to their high stability. 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:^[12] As typical example, nickelocene forms upon treating nickel(II) chloride with sodium cyclopentadienide in THF.^[13]

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.^[14]

Organic synthesis

It was the starting material in Leo Paquette's 1982 synthesis of dodecahedrane.^[15] The first step involved reductive dimerization of the molecule to give dihydrofulvalene, not simple addition to give dicyclopentadiene.

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

Uses

Aside from serving as a precursor to cyclopentadienyl-based catalysts, the main commercial application of cyclopentadiene is as a precursor to comonomers. Semi-hydrogenation gives cyclopentene. Diels–Alder reaction with butadiene gives ethylidene norbornene, a comonomer in the production of EPDM rubbers.

Derivatives

Structure of t-Bu₃C₅H₃, a prototypical bulky cyclopentadiene^[16]

Cyclopentadiene can substitute one or more hydrogens, forming derivatives having covalent bonds:

Most of these substituted cyclopentadienes can also form anions and join cyclopentadienyl complexes.

References

^ ^a ^b ^c ^d ^e ^f ^g NIOSH Pocket Guide to Chemical Hazards. "#0170". National Institute for Occupational Safety and Health (NIOSH).
^ William M. Haynes (2016). CRC Handbook of Chemistry and Physics [Physical Constants of Organic Compounds]. Vol. 97. CRC Press/Taylor and Francis. p. 276 (3-138). ISBN 978-1498754286.
^ ^a ^b ^c William M. Haynes; David R. Lide; Thomas J. Bruno, eds. (2016). CRC handbook of chemistry and physics : a ready-reference book of chemical and physical data (2016-2017, 97th ed.). Boca Raton, Florida: CRC Press. ISBN 978-1-4987-5428-6. OCLC 930681942.
^ Faustov, Valery I.; Egorov, Mikhail P.; Nefedov, Oleg M.; Molin, Yuri N. (2000). "Ab initio G2 and DFT calculations on electron affinity of cyclopentadiene, silole, germole and their 2,3,4,5-tetraphenyl substituted analogs: structure, stability and EPR parameters of the radical anions". Phys. Chem. Chem. Phys. 2 (19): 4293–4297. Bibcode:2000PCCP....2.4293F. doi:10.1039/b005247g.
^ "Cyclopentadiene". Immediately Dangerous to Life or Health Concentrations (IDLH). National Institute for Occupational Safety and Health (NIOSH).
^ Scharpen, LeRoy H.; Laurie, Victor W. (1965-10-15). "Structure of Cyclopentadiene". The Journal of Chemical Physics. 43 (8): 2765–2766. Bibcode:1965JChPh..43.2765S. doi:10.1063/1.1697207. ISSN 0021-9606.
^ Hartwig, J. F. (2010). Organotransition Metal Chemistry: From Bonding to Catalysis. New York, NY: University Science Books. ISBN 978-1-891389-53-5.
^ ^a ^b Hönicke, Dieter; Födisch, Ringo; Claus, Peter; Olson, Michael. "Cyclopentadiene and Cyclopentene". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a08_227. ISBN 978-3-527-30673-2.
^ Moffett, Robert Bruce (1962). "Cyclopentadiene and 3-Chlorocyclopentene". Organic Syntheses; Collected Volumes, vol. 4, p. 238.
^ ^a ^b Corey, E. J.; Weinshenker, N. M.; Schaaf, T. K.; Huber, W. (1969). "Stereo-controlled synthesis of prostaglandins F-2a and E-2 (dl)". Journal of the American Chemical Society. 91 (20): 5675–5677. doi:10.1021/ja01048a062. PMID 5808505.
^ Levandowski, Brian; Houk, Ken (2015). "Theoretical Analysis of Reactivity Patterns in Diels–Alder Reactions of Cyclopentadiene, Cyclohexadiene, and Cycloheptadiene with Symmetrical and Unsymmetrical Dienophiles". J. Org. Chem. 80 (7): 3530–3537. doi:10.1021/acs.joc.5b00174. PMID 25741891.
^ Girolami, G. S.; Rauchfuss, T. B.; Angelici, R. J. (1999). Synthesis and Technique in Inorganic Chemistry. Mill Valley, CA: University Science Books. ISBN 0-935702-48-2.
^ Jolly, W. L. (1970). The Synthesis and Characterization of Inorganic Compounds. Englewood Cliffs, NJ: Prentice-Hall. ISBN 0-13-879932-6.
^ Kolle, U.; Grub, J. (1985). "Permethylmetallocene: 5. Reactions of Decamethylruthenium Cations". J. Organomet. Chem. 289 (1): 133–139. doi:10.1016/0022-328X(85)88034-7.
^ Paquette, L. A.; Wyvratt, M. J. (1974). "Domino Diels–Alder reactions. I. Applications to the rapid construction of polyfused cyclopentanoid systems". J. Am. Chem. Soc. 96 (14): 4671–4673. Bibcode:1974JAChS..96.4671P. doi:10.1021/ja00821a052.
^ Reiners, Matthis; Ehrlich, Nico; Walter, Marc D. (2018). "Synthesis of Selected Transition Metal and Main Group Compounds with Synthetic Applications". Inorganic Syntheses. Vol. 37. p. 199. doi:10.1002/9781119477822.ch8. ISBN 978-1-119-47782-2. S2CID 105376454.