Cyclopentadiene

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

Quick facts: Names, Identifiers, Properties, Structure, Th... ▼
Cyclopentadiene
Cyclopentadiene.png
Skeletal formula of cyclopentadiene
Cyclopentadiene-3D-vdW.png
Spacefill model of cyclopentadiene
Cyclopentadiene-3D-balls.png
Names
Preferred IUPAC name
Cyclopenta-1,3-diene
Other names
1,3-Cyclopentadiene[1]
Pyropentylene[2]
Identifiers
3D model (JSmol)
Abbreviations CPD, HCp
471171
ChEBI
ChemSpider
ECHA InfoCard 100.008.033 OOjs_UI_icon_edit-ltr-progressive.svg
EC Number
  • 208-835-4
1311
MeSH 1,3-cyclopentadiene
RTECS number
  • GY1000000
UNII
  • InChI=1S/C5H6/c1-2-4-5-3-1/h1-4H,5H2 Yes_check.svgY
    Key: ZSWFCLXCOIISFI-UHFFFAOYSA-N Yes_check.svgY
  • InChI=1/C5H6/c1-2-4-5-3-1/h1-4H,5H2
    Key: ZSWFCLXCOIISFI-UHFFFAOYAI
  • C1C=CC=C1
Properties
C5H6
Molar mass 66.103 g·mol−1
Appearance Colourless liquid
Odor irritating, terpene-like[1]
Density 0.802 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
insoluble[1]
Vapor pressure 400 mmHg (53 kPa)[1]
Acidity (pKa) 16
Conjugate base Cyclopentadienyl anion
−44.5×10−6 cm3 mol−1
1.44 (at 20 °C)[3]
Structure
Planar[4]
0.419 D[3]
Thermochemistry
115.3 J·mol−1·K−1
182.7 J·mol−1·K−1
105.9 kJ·mol−1[3]
Hazards
Flash point 25 °C (77 °F; 298 K)
Lethal dose or concentration (LD, LC):
14,182 ppm (rat, 2 hr)
5091 ppm (mouse, 2 hr)[5]
NIOSH (US health exposure limits):
PEL (Permissible)
 TWA 75 ppm (200 mg/m3)[1]
REL (Recommended)
 TWA 75 ppm (200 mg/m3)[1]
IDLH (Immediate danger)
 750 ppm[1]
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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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]