1,2-Cyclopentanedione

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| ImageFile = 1,2-Cyclopentanedione.png

| ImageSize =

| ImageAlt =

| PIN = Cyclopentane-1,2-dione

| OtherNames =

|Section1={{Chembox Identifiers

| CASNo = 3008-40-0

| CASNo_Ref = {{cascite|correct|CAS}}

| UNII_Ref = {{fdacite|correct|FDA}}

| UNII = DQ8L4K1DLJ

| PubChem = 566657

| ChemSpiderID = 492605

| StdInChI=1S/C5H6O2/c6-4-2-1-3-5(4)7/h1-3H2

| StdInChIKey = CIISBNCSMVCNIP-UHFFFAOYSA-N

| SMILES = C1CC(=O)C(=O)C1

}}

|Section2={{Chembox Properties

| C=5|H=6|O=2

| MolarMass =

| Appearance = colorless liquid

| Density = 1.371 g/cm³

| MeltingPtC = 56

| BoilingPt = {{convert|87|-|88|C|F K}}

| BoilingPt_notes = (15 mm Hg)

| Solubility = }}

|Section3={{Chembox Hazards

| MainHazards =

| FlashPt =

| AutoignitionPt = }}

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1,2-Cyclopentanedione is the organic compound with the formula (CH₂)₃(CO)₂. It is one of two isomeric cyclopentanediones, the other being 1,3-cyclopentanedione. It was first prepared by base-induced condensation of di

ethylglutarate with diethyloxalate, followed by hydrolysis of the resulting diketodiester followed by decarboxylation.{{cite journal|doi=10.1002/jlac.19495630105|title=Die Synthese der Reduktinsäure (Synthesis of Reductic Acid)|year=1949|last1=Hesse|first1=Gerhard|last2=Bücking|first2=Eva|journal=Justus Liebigs Annalen der Chemie|volume=563|pages=31–37}}

The enol is predicted to be about 1-3 kcal/mol more stable than the diketo form.{{cite journal|doi=10.1021/acsomega.8b01008|title=DFT Study to Explore the Importance of Ring Size and Effect of Solvents on the Keto–Enol Tautomerization Process of α- and β-Cyclodiones|year=2018|last1=Jana|first1=Kalyanashis|last2=Ganguly|first2=Bishwajit|journal=ACS Omega|volume=3|issue=7|pages=8429–8439|pmid=31458971|pmc=6644555}} The enol structure has been confirmed by X-ray crystallography.{{cite journal|doi=10.1107/S0108270100009550|title=Two crystalline modifications of 2-hydroxycyclopent-2-enone|year=2000|last1=Gerrard|first1=Juliet A.|last2=Meade|first2=Susie J.|last3=Morris|first3=Jonathan C.|last4=Steel|first4=Peter J.|journal=Acta Crystallographica Section C: Crystal Structure Communications|volume=56|issue=10|pages=1251–1253|pmid=11025315|doi-access=}}

Structurally related to 1,2-cyclopentanedione is 2-hydroxy-3-methyl-2-cyclopenten-1-one is a flavor additive, also called cyclotene.