Cyclopentadiene

Athanor · official

C₅H₆ · 66.1 g/mol

IUPAC name: cyclopenta-1,3-diene

Also called 1,3-cyclopentadiene, Diels-Alder diene, Cp

Rings Aromatics

Open in Athanor

Drawn from its SMILES with RDKit

DangerHighly Flammable liquid and vapor · Flammable liquid and vapor · Toxic if swallowed or in contact with skin · Toxic if swallowed · Toxic in contact with skin · Harmful in contact with skin · Causes skin irritation · Causes serious eye irritation · Harmful if inhaled · May cause respiratory irritation

Names and identifiers

IUPAC name: cyclopenta-1,3-diene
Synonyms: 1,3-cyclopentadiene, Diels-Alder diene, Cp
+ 37 more
1,3-CYCLOPENTADIENE, cyclopenta-1,3-diene, Pyropentylene, Pentole, R-Pentine, DTXSID0027191, 5DFH9434HF, DTXCID007191, CHEBI:30664, RefChem:5721, 208-835-4, HSDB 2514, EINECS 208-835-4, cyclopentadien, UNII-5DFH9434HF, cyclo pentadiene, cyclo-pentadiene, EC 208-835-4, Phytomenadione Impurity 19, SCHEMBL9377, CYCLOPENTADIENE [MI], SCHEMBL20737, SCHEMBL20738, SCHEMBL227315, SCHEMBL343437, ghl.PD_Mitscher_leg0.1205, SCHEMBL1274165, SCHEMBL4984672, SCHEMBL7877450, SCHEMBL8416113, SCHEMBL8802218, SCHEMBL8927551, CHEMBL3188826, SCHEMBL16702943, 1,3-CYCLOPENTADIENE [HSDB], Tox21_302134, AKOS025295445
Molecular formula: C₅H₆
Empirical formula: C₅H₆
Molecular weight: 66.1 g/mol
Exact mass: 66.04695
CAS number: 542-92-7
PubChem CID: 7612
Canonical SMILES: C1C=CC=C1
Isomeric SMILES: C1C=CC=C1
InChI: InChI=1S/C5H6/c1-2-4-5-3-1/h1-4H,5H2
InChIKey: ZSWFCLXCOIISFI-UHFFFAOYSA-N
Molfile

Names, InChI and the identifiers from PubChem, retrieved 2026-10-14.

Physical properties

TemperaturePressureDensity

Appearance: Colorless liquid[6]
6 values
From Hazardous Substances Data Bank (HSDB), the most authoritative source here.
Value As written Source
– Cyclopentadiene is a colorless liquid with an irritating, terpene-like odor. Bp: 42.5 °C; Flash point: 77 °F. Density: 0.805 g cm-3. [1] CAMEO Chemicals
– Liquid [2] EPA Chemical Data Reporting (CDR)
– COLOURLESS LIQUID WITH CHARACTERISTIC ODOUR. [3] ILO-WHO International Chemical Safety Cards (ICSCs)
– Colorless liquid with an irritating, terpene-like odor. [4] Occupational Safety and Health Administration (OSHA)
– Colorless liquid with an irritating, terpene-like odor. [5] The National Institute for Occupational Safety and Health (NIOSH)
–shown Colorless liquid [6] Hazardous Substances Data Bank (HSDB)cites Lewis, R.J. Sr.; Hawley's Condensed Chemical Dictionary 15th Edition. John Wiley & Sons, Inc. New York, NY 2007., p. 362
Odor: Irritating terpene-like odor[6]
Melting point: -85 °C (-121 °F, NIOSH 2024)[1]
4 values
4 of 4 sources agree on it (most common value).
Value As written Source
−85 °Cshown -121 °F (NIOSH, 2024) [1] CAMEO Chemicals
−85 °C -85 °C [3] ILO-WHO International Chemical Safety Cards (ICSCs)
−85 °C -121 °F [4] Occupational Safety and Health Administration (OSHA)
−85 °C -121 °F [5] The National Institute for Occupational Safety and Health (NIOSH)
Boiling point: 41.5-42.0 °C[3]
4 values
4 of 4 sources agree on it (most common value).
Value As written Source
42 °C 107 °F at 760 mmHg (NIOSH, 2024) [1] CAMEO Chemicals
41.5 to 42.0 °Cshown 41.5-42.0 °C [3] ILO-WHO International Chemical Safety Cards (ICSCs)
42 °C 107 °F [4] Occupational Safety and Health Administration (OSHA)
42 °C 107 °F [5] The National Institute for Occupational Safety and Health (NIOSH)
Density: Relative density (water = 1): 0.8[3]
4 values
4 of 4 sources agree on it (most common value).
Value As written Source
– 0.8 (NIOSH, 2024) - Less dense than water [1] CAMEO Chemicals
–shown Relative density (water = 1): 0.8 [3] ILO-WHO International Chemical Safety Cards (ICSCs)
– 0.80 [4] Occupational Safety and Health Administration (OSHA)
– 0.80 [5] The National Institute for Occupational Safety and Health (NIOSH)
Solubility in water: Solubility in water: insoluble[3]
4 values
From ILO-WHO International Chemical Safety Cards (ICSCs), the most authoritative source here.
Value As written Source
– Insoluble (NIOSH, 2024) [1] CAMEO Chemicals
– In water, 1,800 mg/L at 25 °C [6] Hazardous Substances Data Bank (HSDB)cites PMID:6376602 Amoore JE, Hautala E; J Appl Toxicol 3: 272-90 (1983)
–shown Solubility in water: insoluble [3] ILO-WHO International Chemical Safety Cards (ICSCs)
– Insoluble [5] The National Institute for Occupational Safety and Health (NIOSH)
Vapor pressure: Reported, but no value passed the checks.
The 4 values and why
400 mmHg (NIOSH, 2024) (no temperature given; source [1])
Vapor pressure, kPa at 20 °C: 45.1 (calculated) (could not be read; source [3])
400 mmHg (no temperature given; source [4])
400 mmHg (no temperature given; source [5])
Flash point: 25 °C (77 °F, OPEN CUP)[6]
4 values
3 of 4 sources agree on it (most common value).
Value As written Source
25 °C 77 °F (NIOSH, 2024) [1] CAMEO Chemicals
25 °Cshown 77 °F (OPEN CUP) [6] Hazardous Substances Data Bank (HSDB)cites NIOSH. NIOSH Pocket Guide to Chemical Hazards & Other Databases CD-ROM. Department of Health & Human Services, Centers for Disease Prevention & Control. National Institute for Occupational Safety & Health. DHHS (NIOSH) Publication No. 2005-151 (2005)
−20 °C -20 °C [3] ILO-WHO International Chemical Safety Cards (ICSCs)
25 °C 77 °F (open cup) [4] Occupational Safety and Health Administration (OSHA)
1 more value was reported but failed the checks (unreadable or out of range), so it is not shown.
Autoignition temperature: 640 °C[3]
2 values
2 of 2 sources agree on it (most common value).
Value As written Source
640 °C 640 °C [6] Hazardous Substances Data Bank (HSDB)cites Mackison, F. W., R. S. Stricoff, and L. J. Partridge, Jr. (eds.). NIOSH/OSHA - Occupational Health Guidelines for Chemical Hazards. DHHS(NIOSH) Publication No. 81-123 (3 VOLS). Washington, DC: U.S. Government Printing Office, Jan. 1981., p. 2
640 °Cshown 640 °C [3] ILO-WHO International Chemical Safety Cards (ICSCs)

Value	As written	Source
–	Cyclopentadiene is a colorless liquid with an irritating, terpene-like odor. Bp: 42.5 °C; Flash point: 77 °F. Density: 0.805 g cm-3.	[1] CAMEO Chemicals
–	Liquid	[2] EPA Chemical Data Reporting (CDR)
–	COLOURLESS LIQUID WITH CHARACTERISTIC ODOUR.	[3] ILO-WHO International Chemical Safety Cards (ICSCs)
–	Colorless liquid with an irritating, terpene-like odor.	[4] Occupational Safety and Health Administration (OSHA)
–	Colorless liquid with an irritating, terpene-like odor.	[5] The National Institute for Occupational Safety and Health (NIOSH)
–shown	Colorless liquid	[6] Hazardous Substances Data Bank (HSDB)cites Lewis, R.J. Sr.; Hawley's Condensed Chemical Dictionary 15th Edition. John Wiley & Sons, Inc. New York, NY 2007., p. 362

Value	As written	Source
−85 °Cshown	-121 °F (NIOSH, 2024)	[1] CAMEO Chemicals
−85 °C	-85 °C	[3] ILO-WHO International Chemical Safety Cards (ICSCs)
−85 °C	-121 °F	[4] Occupational Safety and Health Administration (OSHA)
−85 °C	-121 °F	[5] The National Institute for Occupational Safety and Health (NIOSH)

Value	As written	Source
42 °C	107 °F at 760 mmHg (NIOSH, 2024)	[1] CAMEO Chemicals
41.5 to 42.0 °Cshown	41.5-42.0 °C	[3] ILO-WHO International Chemical Safety Cards (ICSCs)
42 °C	107 °F	[4] Occupational Safety and Health Administration (OSHA)
42 °C	107 °F	[5] The National Institute for Occupational Safety and Health (NIOSH)

Value	As written	Source
–	0.8 (NIOSH, 2024) - Less dense than water	[1] CAMEO Chemicals
–shown	Relative density (water = 1): 0.8	[3] ILO-WHO International Chemical Safety Cards (ICSCs)
–	0.80	[4] Occupational Safety and Health Administration (OSHA)
–	0.80	[5] The National Institute for Occupational Safety and Health (NIOSH)

Value	As written	Source
–	Insoluble (NIOSH, 2024)	[1] CAMEO Chemicals
–	In water, 1,800 mg/L at 25 °C	[6] Hazardous Substances Data Bank (HSDB)cites PMID:6376602 Amoore JE, Hautala E; J Appl Toxicol 3: 272-90 (1983)
–shown	Solubility in water: insoluble	[3] ILO-WHO International Chemical Safety Cards (ICSCs)
–	Insoluble	[5] The National Institute for Occupational Safety and Health (NIOSH)

Value	As written	Source
25 °C	77 °F (NIOSH, 2024)	[1] CAMEO Chemicals
25 °Cshown	77 °F (OPEN CUP)	[6] Hazardous Substances Data Bank (HSDB)cites NIOSH. NIOSH Pocket Guide to Chemical Hazards & Other Databases CD-ROM. Department of Health & Human Services, Centers for Disease Prevention & Control. National Institute for Occupational Safety & Health. DHHS (NIOSH) Publication No. 2005-151 (2005)
−20 °C	-20 °C	[3] ILO-WHO International Chemical Safety Cards (ICSCs)
25 °C	77 °F (open cup)	[4] Occupational Safety and Health Administration (OSHA)

Value	As written	Source
640 °C	640 °C	[6] Hazardous Substances Data Bank (HSDB)cites Mackison, F. W., R. S. Stricoff, and L. J. Partridge, Jr. (eds.). NIOSH/OSHA - Occupational Health Guidelines for Chemical Hazards. DHHS(NIOSH) Publication No. 81-123 (3 VOLS). Washington, DC: U.S. Government Printing Office, Jan. 1981., p. 2
640 °Cshown	640 °C	[3] ILO-WHO International Chemical Safety Cards (ICSCs)

Each value as its source reports it, with the source's number after it (listed under References). Where several sources report a property, the row shows the value most of them agree on, the rule the app uses; open the values to see every one.

Calculated

Heavy atoms: 5
Rings: 1
Aromatic rings: 0
Fraction sp³ carbons (Fsp3): 0.2
H-bond donors: 0
H-bond acceptors: 0
Rotatable bonds: 0
TPSA (Å²): 0
Formal charge: 0
XLogP3: 1.8 PubChem
Complexity: 58 PubChem
Stereocenters: None
Double-bond geometry: None specified
Point group: C2v of one 3D conformer with its hydrogens, by Athanor’s engine (as in the app)

Elemental analysis

C90.85%
H9.15%

Isotope pattern

Each peak is a nominal mass with its abundance-weighted exact mass, as a percentage of the tallest, from IUPAC natural isotope abundances.

Computed from the structure with RDKit 2026.09 when this page was built, beside PubChem's own computed values from its record; where the two differ, both are shown.

Predicted

log P: 1.5 Crippen (Wildman and Crippen 1999)
log S (water solubility): -1.2 ESOL (Delaney 2004), log mol/L
Synthetic accessibility: 3.31 SA score, 1 easy to 10 hard (Ertl and Schuffenhauer 2009)

Drug-likeness: Lipinski’s rule of five

Molecular weight ≤ 500: 66.1
log P ≤ 5 (Crippen): 1.5
H-bond donors ≤ 5: 0
H-bond acceptors ≤ 10: 0

Passes all four rules.

¹³C NMR shifts, predicted

Predictions, not measurements; each names its method. A measured log P, where a source reports one, is under Physical properties.

Safety

Danger

Most widely reported: 372 company reports to ECHA. Source [7].

Flammable
Acute toxicity
Harmful or irritant

H225Highly Flammable liquid and vapor (28.2% of reports)
H226Flammable liquid and vapor (71.8% of reports)
H301+H311Toxic if swallowed or in contact with skin (28% of reports)
H301Toxic if swallowed (92.2% of reports)
H311Toxic in contact with skin (29.6% of reports)
H312Harmful in contact with skin (62.6% of reports)
H315Causes skin irritation
H319Causes serious eye irritation
H332Harmful if inhaled (87.6% of reports)
H335May cause respiratory irritation

Precautionary statements: P210, P233, P240, P241, P242, P243, P261, P262, P264, P264+P265, P270, P271, P280, P301+P316, P302+P352, P303+P361+P353, P304+P340, P305+P351+P338, P316, P317, P319, P321, P330, P332+P317, P337+P317, P361+P364, P362+P364, P370+P378, P403+P233, P403+P235, P405, P501

Exposure limits

PEL-TWA (8-Hour Time Weighted Average): 75 ppm (200 mg/m³)[4]
PEL: TWA 75 ppm (200 mg/m3)[5]
TLV: 0,5 ppm as TWA[3]
TLV-TWA (Time Weighted Average): 0.5 ppm [2018][4]
TLV-STEL (Short Term Exposure Limit): 1 ppm [2018][4]
IDLH: 750 ppm (NIOSH, 2024)[1]

First aid

Excerpt from NIOSH Pocket Guide for Cyclopentadiene: Eye: Irrigate immediately - If this chemical contacts the eyes, immediately wash (irrigate) the eyes with large amounts of water, occasionally lifting the lower and upper lids. Get medical attention immediately. Skin: SOAP WASH promptly - If this chemical contacts the skin, promptly wash the contaminated skin with soap and water. If this chemical penetrates the clothing, promptly remove the clothing and wash the skin with soap and water.[1]
(General first aid procedures) Eye: Irrigate immediately - If this chemical contacts the eyes, immediately wash (irrigate) the eyes with large amounts of water, occasionally lifting the lower and upper lids. Get medical attention immediately. Skin: Soap wash promptly - If this chemical contacts the skin, promptly wash the contaminated skin with soap and water. If this chemical penetrates the clothing, promptly remove the clothing and wash the skin with soap and water.[5]
Fresh air, rest. Refer for medical attention.[3]
Remove contaminated clothes. Rinse and then wash skin with water and soap.[3]

Storage

Fireproof. Store only if stabilized. Cooled. Separated from incompatible materials. See Chemical Dangers.[3]
A harmful contamination of the air can be reached rather quickly on evaporation of this substance at 20 °C.[6]
Fireproof. Separated from strong oxidants, strong acids, potassium hydroxide. Cooled. Store only if stabilized.[6]
Monomer may largely be prevented from dimerizing by storage at -80 °C or below.[6]

Incompatibilities

Reacts vigorously on contact with potassium hydroxide.[6]
Contact with the ethanolic base (eg potassium hydroxide) causes vigorously exothermic resin formation.[6]
Incompatible with nitric acid; oxides of nitrogen; oxygen; sulfuric acid.[6]
Dienes and acetylene derivatives are hypergolic on contact with concn nitric acid, ignition delay being 1 ms. Cyclopentadiene reacts explosively with fuming nitric acid, igniting under nitrogen ...[6]

Safety data sheets on the supplier’s page: Shenghe Chemical, CATO Research Chemicals Inc., BOC Sciences.

Always read the supplier’s safety data sheet before working with it.

Spectra

NMRNMRShiftDB
13C NMRSpectraBase
GC-MSNIST Mass Spectrometry Data Center · NIST Mass Spectrometry Data Center · SpectraBase (CH4)
Other MSHazardous Substances Data Bank (HSDB)
UVHazardous Substances Data Bank (HSDB)
IRHazardous Substances Data Bank (HSDB)

Experimental spectra held by these sources; each link opens the source's own record.

Literature

Reading PubChem, OpenAlex and GSRS…

Bioactivity

Reading ChEMBL…

References

[1]CAMEO Chemicals · Public domain (NOAA) · via PubChem · retrieved 2026-10-14
[2]EPA Chemical Data Reporting (CDR) · Public domain (US EPA) · via PubChem · retrieved 2026-10-14
[3]ILO-WHO International Chemical Safety Cards (ICSCs) · CC BY 4.0 · via PubChem · retrieved 2026-10-14
[4]Occupational Safety and Health Administration (OSHA) · Public domain (US DOL) · via PubChem · retrieved 2026-10-14
[5]The National Institute for Occupational Safety and Health (NIOSH) · Public domain (US CDC) · via PubChem · retrieved 2026-10-14
[6]Hazardous Substances Data Bank (HSDB) · Public domain (US NLM) · via PubChem · retrieved 2026-10-14
[7]European Chemicals Agency (ECHA) · ECHA legal notice: reuse with acknowledgement · via PubChem · retrieved 2026-10-14
[8]NMRShiftDB · Linked only · via PubChem · retrieved 2026-10-14
[9]SpectraBase · Linked only · via PubChem · retrieved 2026-10-14
[10]NIST Mass Spectrometry Data Center · Linked only · via PubChem · retrieved 2026-10-14
·Calculated values: computed by Athanor with RDKit 2026.09 from the structure.
·Literature on PubChem

Data last updated 2026-10-14.