Acetylacetone

Acetylacetone is an organic compound with the formula CH3COCH2COCH3. It is a colorless liquid, classified as a 1,3-diketone. It exists in equilibrium with a tautomer CH3C(O)CH=C(OH)CH3. These tautomers interconvert so rapidly under most conditions that they are treated as a single compound in most applications.[2] It is a colorless liquid that is a precursor to acetylacetonate anion (commonly abbreviated acac−), a bidentate ligand. It is also a building block for the synthesis of heterocyclic compounds.

Acetylacetone
Ball-and-stick model of the enol tautomer
Ball-and-stick model of the keto tautomer
Space-filling model of the enol tautomer
Space-filling model of the keto tautomer
Names
Preferred IUPAC name
Pentane-2,4-dione
Other names
  • Hacac
  • 2,4-pentanedione
Identifiers
3D model (JSmol)
741937
ChEBI
ChEMBL
ChemSpider
ECHA InfoCard 100.004.214
EC Number
  • 204-634-0
2537
KEGG
RTECS number
  • SA1925000
UNII
UN number 2310
Properties
C5H8O2
Molar mass 100.117 g·mol−1
Density 0.975 g/mL[1]
Melting point −23 °C (−9 °F; 250 K)
Boiling point 140 °C (284 °F; 413 K)
16 g/100 mL
-54.88·10−6 cm3/mol
Hazards
GHS pictograms
GHS Signal word Danger
GHS hazard statements
 H226, H302, H311, H320, H331, H335, H341, H370, H402, H412
P201, P202, P210, P233, P240, P241, P242, P243, P260, P261, P264, P270, P271, P273, P280, P281, P301+312, P302+352, P303+361+353, P304+340, P305+351+338, P307+311, P308+313, P311, P312
NFPA 704 (fire diamond)
Flammability code 2: Must be moderately heated or exposed to relatively high ambient temperature before ignition can occur. Flash point between 38 and 93 °C (100 and 200 °F). E.g. diesel fuelHealth code 2: Intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury. E.g. chloroformReactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
2
2
0
Flash point 34 °C (93 °F; 307 K)
340 °C (644 °F; 613 K)
Explosive limits 2.4–11.6%
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Properties

Tautomerism

The keto and enol tautomers of acetylacetone coexist in solution. The enol form has C2v symmetry, meaning the hydrogen atom is shared equally between the two oxygen atoms.[3] In the gas phase, the equilibrium constant, Kketo→enol, is 11.7, favoring the enol form. The two tautomeric forms can be distinguished by NMR spectroscopy, IR spectroscopy and other methods.[4][5]

SolventKketo→enol
Gas phase11.7
Cyclohexane42
Toluene10
THF7.2
DMSO2
Water0.23

The equilibrium constant tends to be high in nonpolar solvents; the keto form becomes more favorable in polar, hydrogen-bonding solvents, such as water.[6] The enol form is a vinylogous analogue of a carboxylic acid.

Acid–base properties
solventT/°CpKa[7]
40% ethanol/water309.8
70% dioxane/water2812.5
80% DMSO/water2510.16
DMSO2513.41

Acetylacetone is a weak acid:

C5H8O2 ⇌ C
5H
7O−
2 + H+

IUPAC recommended pKa values for this equilibrium in aqueous solution at 25 °C are 8.99 ± 0.04 (I = 0), 8.83 ± 0.02 (I = 0.1 M NaClO4) and 9.00 ± 0.03 (I = 1.0 M NaClO4; I = Ionic strength).[8] Values for mixed solvents are available. Very strong bases, such as organolithium compounds, will deprotonate acetylacetone twice. The resulting dilithio species can then be alkylated at C-1.

Preparation

Acetylacetone is prepared industrially by the thermal rearrangement of isopropenyl acetate.[9]

CH2(CH3)COC(O)Me → MeC(O)CH2C(O)Me

Laboratory routes to acetylacetone begin also with acetone. Acetone and acetic anhydride upon the addition of boron trifluoride (BF3) catalyst:[10]

(CH3CO)2O + CH3C(O)CH3 → CH3C(O)CH2C(O)CH3

A second synthesis involves the base-catalyzed condensation of acetone and ethyl acetate, followed by acidification:[10]

NaOEt + EtO2CCH3 + CH3C(O)CH3 → NaCH3C(O)CHC(O)CH3 + 2 EtOH
NaCH3C(O)CHC(O)CH3 + HCl → CH3C(O)CH2C(O)CH3 + NaCl

Because of the ease of these syntheses, many analogues of acetylacetonates are known. Some examples include C6H5C(O)CH2C(O)C6H5 (dbaH) and (CH3)3CC(O)CH2C(O)CC(CH3)3. Hexafluoroacetylacetonate is also widely used to generate volatile metal complexes.

Reactions

Condensations

Acetylacetone is a versatile bifunctional precursor to heterocycles because both keto groups undergo condensation. Hydrazine reacts to produce pyrazoles. Urea gives pyrimidines. Condensation with two aryl- and alkylamines to gives NacNacs, wherein the oxygen atoms in acetylacetone are replaced by NR (R = aryl, alkyl).

Coordination chemistry
Main article: Metal acetylacetonates
A ball-and-stick model of VO(acac)2

The acetylacetonate anion, acac−, forms complexes with many transition metal ions. A general method of synthesis is to treat a metal salt with acetylacetone in the presence of a base:[11]

MBz + z Hacac ⇌ M(acac)z + z BH

Both oxygen atoms bind to the metal to form a six-membered chelate ring. In some cases the chelate effect is so strong that no added base is needed to form the complex.

Biodegradation

Enzymatic breakdown: The enzyme acetylacetone dioxygenase cleaves the carbon-carbon bond of acetylacetone, producing acetate and 2-oxopropanal. The enzyme is iron(II)-dependent, but it has been proven to bind to zinc as well. Acetylacetone degradation has been characterized in the bacterium Acinetobacter johnsonii.[12]

C5H8O2 + O2 → C2H4O2 + C3H4O2
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References
  1. "05581: Acetylacetone". Sigma-Aldrich.
  2. Thomas M. Harris (2001). "2,4-Pentanedione". 2,4‐Pentanedione. e-EROS Encyclopedia of Reagents for Organic Synthesis. doi:10.1002/047084289X.rp030. ISBN 0471936235.
  3. Caminati, W.; Grabow, J.-U. (2006). "The C2v Structure of Enolic Acetylacetone". J. Am. Chem. Soc. 128 (3): 854–857. doi:10.1021/ja055333g. PMID 16417375.
  4. Manbeck, Kimberly A.; Boaz, Nicholas C.; Bair, Nathaniel C.; Sanders, Allix M. S.; Marsh, Anderson L. (2011). "Substituent Effects on Keto–Enol Equilibria Using NMR Spectroscopy". J. Chem. Educ. 88 (10): 1444–1445. Bibcode:2011JChEd..88.1444M. doi:10.1021/ed1010932.
  5. Yoshida, Z.; Ogoshi, H.; Tokumitsu, T. (1970). "Intramolecular hydrogen bond in enol form of 3-substituted-2,4-pentanedione". Tetrahedron. 26 (24): 5691–5697. doi:10.1016/0040-4020(70)80005-9.
  6. Reichardt, Christian (2003). Solvents and Solvent Effects in Organic Chemistry (3rd ed.). Wiley-VCH. ISBN 3-527-30618-8.
  7. IUPAC SC-Database A comprehensive database of published data on equilibrium constants of metal complexes and ligands
  8. Stary, J.; Liljenzin, J. O. (1982). "Critical evaluation of equilibrium constants involving acetylacetone and its metal chelates" (PDF). Pure and Applied Chemistry. 54 (12): 2557–2592. doi:10.1351/pac198254122557.
  9. Siegel, Hardo; Eggersdorfer, Manfred (2002). "Ketones". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a15_077. ISBN 9783527306732.
  10. C. E. Denoon, Jr. "Acetylacetone". Organic Syntheses.; Collective Volume, 3, p. 16
  11. O'Brien, Brian. "Co(tfa)3 & Co(acac)3 handout" (PDF). Gustavus Adolphus College.
  12. Straganz, G.D.; Glieder, A.; Brecker, L.; Ribbons, D.W.; Steiner, W. (2003). "Acetylacetone-cleaving enzyme Dke1: a novel C–C-bond-cleaving enzyme from Acinetobacter johnsonii". Biochem. J. 369 (3): 573–581. doi:10.1042/BJ20021047. PMC 1223103. PMID 12379146.
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