Cyclopentadiene

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. 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 heated to induce thermal cracking and regenerate the monomer through a retro-Diels–Alder reaction

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.