Title: Cyclopentadiene
CAS Registry Number: 542-92-7
CAS Name: 1,3-Cyclopentadiene
Molecular Formula: C5H6
Molecular Weight: 66.10
Percent Composition: C 90.85%, H 9.15%
Literature References: Obtained from the distillates produced in carbonization of coal, esp from the foreruns of coke-oven light oil: Ward, US 2211038 (1940); process involving the liquefaction of coke-oven gas: Horclois, Chim. Ind. (Paris), special issue April 1934, pp 357-363. Also obtained during the cracking of petr hydrocarbons: Tropsch et al., Ind. Eng. Chem. 30, 169 (1938); cf. Dedussenko, J. Gen. Chem. USSR 7, 1467 (1937); Chem. Zentralbl. 109, I, 793 (1938). Synthesis by passing vaporized cyclopentane over activated alumina and oxides of molybdenum, chromium, or vanadium: Grosse et al., Ind. Eng. Chem. 32, 309 (1940); US 2157202; US 2157203; US 2157939. Lab prepn by depolymerization of dicyclopentadiene: Moffett, Org. Synth. coll. vol. IV, 238 (1963). Toxicity study: Smyth et al., Arch. Ind. Hyg. Occup. Med. 10, 61 (1954). Review and discussion of structure: Wilson, Wells, Chem. Rev. 34, 1 (1944). Review: M. Fefer, A. B. Small in Kirk-Othmer Encyclopedia of Chemical Technology vol. 7 (Wiley-Interscience, New York, 3rd ed., 1979) pp 417-429.
Properties: Liquid. d40 0.8235; d410 0.8131; d420 0.8021; d425 0.7966; d430 0.7914. mp -85°. bp760 41.5-42.0°. nD16 1.44632. Absorption spectum: Pickett et al., J. Am. Chem. Soc. 63, 1073 (1941). Insol in water. Miscible with alc, ether, benzene, carbon tetrachloride. Sol in carbon disulfide, aniline, acetic acid, liquid petrolatum. Cyclopentadiene polymerizes to dicyclopentadiene on standing. Polymerization is accelerated by the presence of peroxides or trichloroacetic acid. The dimer is cryst, mp 32.5°, with a camphor-like odor, having the structure of a partially hydrogenated indene contg a bridged methylene group. It is a more convenient form in which to handle cyclopentadiene, and is easily depolymerized by distilling at atmospheric pressure. LD50 of dimer orally in rats: 0.82 g/kg (Smyth).
Melting point: mp -85°; mp 32.5°
Boiling point: bp760 41.5-42.0°
Index of refraction: nD16 1.44632
Density: d40 0.8235; d410 0.8131; d420 0.8021; d425 0.7966; d430 0.7914
Toxicity data: LD50 of dimer orally in rats: 0.82 g/kg (Smyth)
CAUTION: Potential symptoms of overexposure are irritation of eyes and nose. See NIOSH Pocket Guide to Chemical Hazards (DHHS/NIOSH 97-140, 1997) p 86.
Use: Manuf resins; in organic synthesis as the diene in the Diels-Alder reaction producing sesquiterpenes, synthetic alkaloids, camphors.
Cyclopentamine Cyclopentane Cyclopentanol Cyclopentanone Cyclopenthiazide

Skeletal formula of cyclopentadiene Spacefill model of cyclopentadiene
Ball and stick model of cyclopentadiene
Abbreviations CPD, HCp
CAS number 542-92-7 YesY
PubChem 7612
ChemSpider 7330 YesY
EC number 208-835-4
MeSH 1,3-cyclopentadiene
ChEBI CHEBI:30664 YesY
RTECS number GY1000000
Beilstein Reference 471171
Gmelin Reference 1311
Jmol-3D images Image 1
Molecular formula C5H6
Molar mass 66.10 g mol−1
Appearance Colourless liquid
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
Acidity (pKa) 16
Basicity (pKb) -2
Molecular shape Planar[2]
heat capacity C
115.3 J K−1 mol−1
Std molar
entropy So298
182.7 J K−1 mol−1
Flash point 25 °C; 77 °F; 298 K
Related compounds
Related hydrocarbons Benzene
Related compounds Dicyclopentadiene
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Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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.[3]