# Acetylacetone

**Acetylacetone** (also called 2,4-pentanedione or pentane-2,4-dione, CH₃COCH₂COCH₃) is the simplest acyclic β-diketone, a colorless to yellow liquid used industrially as a solvent, intermediate, and above all as the parent compound of the acetylacetonate (acac⁻) chelating ligand.<sup>[1](https://webbook.nist.gov/cgi/cbook.cgi?ID=C123546&Mask=C)</sup><sup> • </sup><sup>[2](https://pubchem.ncbi.nlm.nih.gov/compound/acetylacetone)</sup> Its chemistry is dominated by a keto–enol tautomeric equilibrium that shifts dramatically with temperature, solvent, and complexation, and by the acidity of the central methylene group, which makes deprotonation to acetylacetonate easy.<sup>[3](https://pubmed.ncbi.nlm.nih.gov/24758690/)</sup><sup> • </sup><sup>[4](https://publications.iupac.org/pac-2007/1982/pdf/5412x2557.pdf)</sup>

| Key fact | Detail |
|---|---|
| Identity | C₅H₈O₂, molecular weight 100.12 g/mol, CAS Registry Number 123-54-6, IUPAC name pentane-2,4-dione<sup>[1](https://webbook.nist.gov/cgi/cbook.cgi?ID=C123546&Mask=C)</sup><sup> • </sup><sup>[2](https://pubchem.ncbi.nlm.nih.gov/compound/acetylacetone)</sup> |
| Physical state | Colorless liquid; boiling point 411 ± 2 K (about 140 °C), melting point 249.95 K (−23 °C), density 0.975 g/mL<sup>[1](https://webbook.nist.gov/cgi/cbook.cgi?ID=C123546&Mask=C)</sup><sup> • </sup><sup>[5](https://chem.libretexts.org/Courses/Southern_Illinois_University_Edwardsville/Inorganic_Chemistry_Laboratory_Manual_-_Version_1.0/04%3A_Experiments/4.03%3A_Synthesis_of_Metal_Acetyacetonates)</sup> |
| Tautomerism | Gas phase at 300 K is essentially all enol (100(3)%); 64(5)% enol at 671 K; in water only about 16% enol<sup>[3](https://pubmed.ncbi.nlm.nih.gov/24758690/)</sup><sup> • </sup><sup>[6](https://pubmed.ncbi.nlm.nih.gov/12197740/)</sup> |
| Acidity | IUPAC recommended pK<sub>a</sub> 8.99 ± 0.04 in water at 25 °C (I = 0)<sup>[4](https://publications.iupac.org/pac-2007/1982/pdf/5412x2557.pdf)</sup> |
| Ligand chemistry | Deprotonation gives acetylacetonate (acac⁻), a bidentate O,O-donor forming six-membered chelate rings with most metals<sup>[7](https://www.chem.purdue.edu/gchelp/cchem/bis.html)</sup> |
| Industrial route | Ketene–acetone condensation: acetic acid or acetone is cracked to ketene, absorbed by acetone to give isopropenyl acetate, then catalytically isomerized to acetylacetone<sup>[8](https://patents.google.com/patent/CN109675618B/en)</sup> |
| Hazards | Flammable liquid (flash point 34 °C, explosive limits 2.4–11.6 vol% in air); harmful if swallowed; occupational exposure limit 25 ppm TWA with skin notation<sup>[9](https://inchem.org/documents/icsc/icsc/eics0533.htm)</sup> |

## Structure and tautomerism

Acetylacetone is pentane in which the hydrogens at positions 2 and 4 are replaced by oxo groups, giving the 1,3-dicarbonyl (β-diketone) arrangement CH₃C(O)CH₂C(O)CH₃.<sup>[2](https://pubchem.ncbi.nlm.nih.gov/compound/acetylacetone)</sup> The molecule exists in equilibrium between this diketo form and the enol form, 4-hydroxy-3-penten-2-one, in which a proton migrates from the central methylene to one oxygen and a C=C bond forms.<sup>[3](https://pubmed.ncbi.nlm.nih.gov/24758690/)</sup>

The enol form is strongly stabilized by an intramolecular O–H···O hydrogen bond closing a six-membered ring, reinforced by π-electron delocalization across the O–C–C–C–O framework, a pattern described as resonance-assisted hydrogen bonding.<sup>[10](https://www.sciencedirect.com/science/article/abs/pii/S2210271X25004785)</sup> Gas-phase electron diffraction combined with quantum chemical calculations found the enol tautomer with C<sub>s</sub> symmetry, a planar ring, and a strongly asymmetric hydrogen bond, while the diketo form has C₂ symmetry.<sup>[3](https://pubmed.ncbi.nlm.nih.gov/24758690/)</sup>

The enol fraction depends strongly on conditions. In the gas phase at 300 K the sample is essentially entirely enol, falling to 64(5)% enol at 671 K.<sup>[3](https://pubmed.ncbi.nlm.nih.gov/24758690/)</sup> Proton NMR gives about 79% enol in the neat liquid and 93–95% in carbon tetrachloride.<sup>[11](https://www.sciencedirect.com/science/article/abs/pii/S0022286012003018)</sup> Polar, hydrogen-bonding solvents shift the equilibrium toward the keto form: in water only about 16% of the molecules are enol.<sup>[6](https://pubmed.ncbi.nlm.nih.gov/12197740/)</sup> Concentrated salts amplify this effect; in a solution of 6.39 M lithium perchlorate in acetylacetone the enol content drops from 81% to 7.4%, and kinetic analysis shows preferential coordination of the keto tautomer to Li⁺.<sup>[6](https://pubmed.ncbi.nlm.nih.gov/12197740/)</sup> Because the two tautomers give well-separated NMR signals and interconvert slowly on the NMR timescale, this equilibrium is a standard physical chemistry laboratory experiment.<sup>[12](https://pubs.acs.org/jceda8/article/91/5/739/842133/Expansion-of-the-Classic-Acetylacetone-Physical)</sup>

Thermochemical data reflect the same balance. The enthalpy of enolization in the liquid phase is reported as −11.3 ± 0.4 kJ/mol, and standard enthalpies of vaporization cluster around 41.8 to 43.2 kJ/mol.<sup>[1](https://webbook.nist.gov/cgi/cbook.cgi?ID=C123546&Mask=C)</sup>

## Acidity and the acetylacetonate anion

The central methylene hydrogens sit between two carbonyl groups, so acetylacetone is a far stronger carbon acid than simple ketones. The IUPAC-recommended pK<sub>a</sub> in aqueous solution at 25 °C is 8.99 ± 0.04 at zero ionic strength, with values of 8.83 ± 0.02 at I = 0.1 M and 9.00 ± 0.03 at I = 1.0 M NaClO₄.<sup>[4](https://publications.iupac.org/pac-2007/1982/pdf/5412x2557.pdf)</sup> The resulting anion, acetylacetonate (acac⁻), delocalizes its negative charge over both oxygens and the central carbon.<sup>[13](https://old.goldbook.iupac.org/html/E/E02123.html)</sup>

This anion is one of the most widely used chelating ligands in coordination chemistry. It binds metal ions through both oxygen atoms, forming a six-membered chelate ring; with trivalent metals such as chromium it gives neutral octahedral complexes of formula M(acac)₃, such as Cr(acac)₃.<sup>[7](https://www.chem.purdue.edu/gchelp/cchem/bis.html)</sup> A 1966 Nature note described acetylacetone as one of the most versatile bidentate chelates known.<sup>[14](https://www.nature.com/articles/210522a0)</sup> Coordination is not limited to the conventional O,O mode: in K[Pt(acac)₂Cl] one acetylacetone group bonds through the γ-carbon atom instead, and added pyridine can convert an oxygen-bonded ligand to the carbon-bonded form.<sup>[15](https://www.nature.com/articles/202589b0)</sup><sup> • </sup><sup>[16](https://www.sciencedirect.com/topics/chemistry/acetylacetone)</sup> Undergraduate inorganic laboratories routinely prepare Al(acac)₃, Cr(acac)₃, Fe(acac)₃, Co(acac)₃, and Cu(acac)₂ from metal salts and acetylacetone in ammonia or with sodium acetate.<sup>[5](https://chem.libretexts.org/Courses/Southern_Illinois_University_Edwardsville/Inorganic_Chemistry_Laboratory_Manual_-_Version_1.0/04%3A_Experiments/4.03%3A_Synthesis_of_Metal_Acetyacetonates)</sup>

## Physical properties

NIST's compiled phase-change data give a boiling point of 411 ± 2 K, an average of nine measurements, and a melting point of 249.95 K with an assigned uncertainty of 0.3 K.<sup>[1](https://webbook.nist.gov/cgi/cbook.cgi?ID=C123546&Mask=C)</sup> The International Chemical Safety Card lists a relative density of 0.98, water solubility of 16 g/100 mL, vapor pressure of 0.93 kPa at 20 °C, relative vapor density 3.45 (air = 1), flash point 34 °C, and auto-ignition temperature 340 °C.<sup>[9](https://inchem.org/documents/icsc/icsc/eics0533.htm)</sup> The vapor is heavier than air and can travel to an ignition source and flash back.<sup>[9](https://inchem.org/documents/icsc/icsc/eics0533.htm)</sup>

## Industrial production

The dominant industrial synthesis is the ketene–acetone route.<sup>[8](https://patents.google.com/patent/CN109675618B/en)</sup> [Acetic acid](https://www.edgepedia.org/acetic-acid) or acetone is thermally cracked to ketene; the ketene is absorbed by acetone under acid catalysis at low temperature to form isopropenyl acetate; and the isopropenyl acetate is then rearranged at high temperature over a catalyst to acetylacetone, followed by rectification.<sup>[8](https://patents.google.com/patent/CN109675618B/en)</sup><sup> • </sup><sup>[17](https://eureka.patsnap.com/patent/CN121266162A)</sup> Chinese patents describe refinements of each step, including mixed antimony, cobalt, and manganese esterification catalysts, continuous ketene absorption, and by-product recovery, aimed at raising single-pass conversion and reducing energy use and carbonization losses.<sup>[18](https://patents.google.com/patent/CN1850763A/en)</sup><sup> • </sup><sup>[8](https://patents.google.com/patent/CN109675618B/en)</sup>

## Uses

Acetylacetone serves as a chemical intermediate, solvent, metal chelating agent, lubricant additive, corrosion inhibitor, polyolefin modifier, and pesticide intermediate, according to Hawley's and ACGIH compilations reproduced by PubChem.<sup>[2](https://pubchem.ncbi.nlm.nih.gov/compound/acetylacetone)</sup> An industrial chemicals reference lists it as an extraction solvent in mineral processing, a metal deactivator in lubricants and fuels, a paint drier, and a solvent for cellulose acetate.<sup>[19](https://www.industrialchemistry.org/PDF/AD05.pdf)</sup> Its metal complexes are used as gasoline additives, lubricant additives, and driers for varnishes and printer's inks.<sup>[2](https://pubchem.ncbi.nlm.nih.gov/compound/acetylacetone)</sup>

Metal acetylacetonates are also classic precursors for metal–organic chemical vapor deposition (MOCVD) of oxide, fluoride, sulfide, and metal films; tris(acetylacetonato) complexes of Al, Sc, Cr, Mn, Fe, Co, Ru, Rh, In, and Ir have been studied for their volatilization thermodynamics.<sup>[20](https://www.mdpi.com/2079-6412/13/8/1458)</sup> The chelate structure formed by the two oxygen donors stabilizes these β-diketonate complexes, though for large cations oligomeric structures can limit volatility, and simple acac derivatives of alkaline-earth metals are often too involatile for electronic materials, prompting the use of bulkier or fluorinated ligand variants.<sup>[21](https://www.sciencedirect.com/topics/engineering/metal-precursor)</sup>

## Safety

Acetylacetone is classified as a flammable liquid (GHS H226) and as harmful if swallowed (H302); above 34 °C explosive vapor/air mixtures can form, and the substance may polymerize under the influence of light.<sup>[9](https://inchem.org/documents/icsc/icsc/eics0533.htm)</sup> It is transported as UN 2310, hazard class 3 with subsidiary risk 6.1 (toxic), packing group III.<sup>[9](https://inchem.org/documents/icsc/icsc/eics0533.htm)</sup> The occupational exposure limit is 25 ppm as an 8-hour time-weighted average with a skin absorption notation, and the German MAK value is 20 ppm (83 mg/m³).<sup>[9](https://inchem.org/documents/icsc/icsc/eics0533.htm)</sup> Acute overexposure by inhalation causes incoordination, dizziness, drowsiness, headache, labored breathing, nausea, and vomiting; the liquid can be absorbed through the skin and irritates eyes and skin.<sup>[9](https://inchem.org/documents/icsc/icsc/eics0533.htm)</sup> Rat oral LD₅₀ values fall in the range of 575 to 1000 mg/kg, and rats inhaling 650 ppm six hours five times weekly for 14 weeks showed degenerative changes in the brain and thymus, with no neurotoxicity at 307 ppm.<sup>[22](https://pubchem.ncbi.nlm.nih.gov/compound/123-54-6)</sup>

## References

1. [Acetylacetone, NIST Chemistry WebBook, SRD 69](https://webbook.nist.gov/cgi/cbook.cgi?ID=C123546&Mask=C)
2. [Acetylacetone, CID 31261, PubChem](https://pubchem.ncbi.nlm.nih.gov/compound/acetylacetone)
3. [Tautomeric properties and gas-phase structure of acetylacetone, PubMed (J. Org. Chem.)](https://pubmed.ncbi.nlm.nih.gov/24758690/)
4. [Equilibrium constants: acetylacetone and its metal chelates, Pure and Applied Chemistry 54(12), 2557–2592 (1982)](https://publications.iupac.org/pac-2007/1982/pdf/5412x2557.pdf)
5. [4.3: Synthesis of Metal Acetylacetonates, Chemistry LibreTexts](https://chem.libretexts.org/Courses/Southern_Illinois_University_Edwardsville/Inorganic_Chemistry_Laboratory_Manual_-_Version_1.0/04%3A_Experiments/4.03%3A_Synthesis_of_Metal_Acetyacetonates)
6. [Modulation of tautomeric equilibria by ionic clusters. Acetylacetone in solutions of lithium perchlorate–diethyl ether, PubMed](https://pubmed.ncbi.nlm.nih.gov/12197740/)
7. [Structures With Bidentate Ligands, Purdue University](https://www.chem.purdue.edu/gchelp/cchem/bis.html)
8. [CN109675618B – Acetylacetone synthesis catalyst and preparation method thereof, Google Patents](https://patents.google.com/patent/CN109675618B/en)
9. [ICSC 0533 – 2,4-PENTADIONE, International Chemical Safety Cards](https://inchem.org/documents/icsc/icsc/eics0533.htm)
10. [Insights into intramolecular hydrogen bonding and π-electron delocalization in β-diketone compounds, ScienceDirect](https://www.sciencedirect.com/science/article/abs/pii/S2210271X25004785)
11. [Tautomeric properties and gas-phase structure of 3-methyl-2,4-pentanedione, ScienceDirect](https://www.sciencedirect.com/science/article/abs/pii/S0022286012003018)
12. [Expansion of the Classic Acetylacetone Physical Chemistry Laboratory NMR Experiment, Journal of Chemical Education](https://pubs.acs.org/jceda8/article/91/5/739/842133/Expansion-of-the-Classic-Acetylacetone-Physical)
13. [IUPAC Gold Book – enolates](https://old.goldbook.iupac.org/html/E/E02123.html)
14. [Synthesis of Transition Metal Complexes of the Unknown Chelate, Dithioacetylacetone, Nature](https://www.nature.com/articles/210522a0)
15. [A Novel Form of Co-ordination of Acetylacetone to Platinum(II), Nature](https://www.nature.com/articles/202589b0)
16. [Acetylacetone – an overview, ScienceDirect Topics](https://www.sciencedirect.com/topics/chemistry/acetylacetone)
17. [CN121266162A – System and method for recovering by-products in acetylacetone production process, Patsnap Eureka](https://eureka.patsnap.com/patent/CN121266162A)
18. [CN1850763A – Process for preparing high-purity acetylacetone, Google Patents](https://patents.google.com/patent/CN1850763A/en)
19. [Acetylacetone, Ashford's Dictionary of Industrial Chemicals extract](https://www.industrialchemistry.org/PDF/AD05.pdf)
20. [Breaking through the Thermodynamics "Wilds" of Metal–Organic Chemical Vapor Deposition Precursors: Metal tris-Acetylacetonates, Coatings (MDPI)](https://www.mdpi.com/2079-6412/13/8/1458)
21. [Metal Precursor – an overview, ScienceDirect Topics](https://www.sciencedirect.com/topics/engineering/metal-precursor)
22. [Acetylacetone, CID 31261 (toxicity sections), PubChem](https://pubchem.ncbi.nlm.nih.gov/compound/123-54-6)

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*Topic: Encyclopedia › Physical world and mathematics › Chemistry › Organic substances › Carbonyl and carboxyl chemistry › Aldehydes and ketones › Dicarbonyls and poly-carbonyl compounds › Beta-dicarbonyl compounds (1,3-dicarbonyls)*

*Initially written Sep 17, 2026 · Reviewed: Oct 15, 2026 · Edited: Oct 15, 2026 · Last review: Oct 15, 2026*

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