Acetylacetone is an organic compound with the chemical formula CH3−C(=O)−CH2−C(=O)−CH3. It is classified as a 1,3-diketone. It exists in equilibrium with a tautomer CH3−C(=O)−CH=C(−OH)−CH3. The mixture is a colorless liquid. These tautomers interconvert so rapidly under most conditions that they are treated as a single compound in most applications.[2] Acetylacetone is a building block for the synthesis of many coordination complexes as well as heterocyclic compounds.

Acetylacetone
Skeletal structures of both tautomers
Ball-and-stick model of the enol tautomer
Ball-and-stick model of the enol tautomer
Ball-and-stick model of the keto tautomer
Ball-and-stick model of the keto tautomer
Space-filling model of the enol tautomer
Space-filling model of the enol tautomer
Space-filling model of the keto tautomer
Space-filling model of the keto tautomer
Names
IUPAC names
(3Z)-4-Hydroxy-3-penten-2-one (enol form)
Pentane-2,4-dione (keto form)
Other names
  • Hacac
  • 2,4-Pentanedione
Identifiers
3D model (JSmol)
741937
ChEBI
ChEMBL
ChemSpider
ECHA InfoCard 100.004.214 Edit this at Wikidata
EC Number
  • 204-634-0
2537
KEGG
RTECS number
  • SA1925000
UNII
UN number 2310
  • InChI=1S/C5H8O2/c1-4(6)3-5(2)7/h3H2,1-2H3 checkY
    Key: YRKCREAYFQTBPV-UHFFFAOYSA-N checkY
  • InChI=1/C5H8O2/c1-4(6)3-5(2)7/h3H2,1-2H3
    Key: YRKCREAYFQTBPV-UHFFFAOYAO
  • O=C(C)CC(=O)C
  • CC(=O)CC(=O)C
  • Enol form: CC(O)=CC(=O)C
Properties
C5H8O2
Molar mass 100.117 g·mol−1
Appearance Colorless liquid
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 labelling:
GHS02: FlammableGHS06: ToxicGHS07: Exclamation markGHS08: Health hazard
Danger
H226, H302, H311, H320, H331, H335, H341, H370, H412
P201, P202, P210, P233, P240, P241, P242, P243, P260, P261, P264, P270, P271, P273, P280, P281, P301+P312, P302+P352, P303+P361+P353, P304+P340, P305+P351+P338, P307+P311, P308+P313, P311, P312, P321, P322, P330, P337+P313, P361, P363, P370+P378, P403+P233, P403+P235, P405, P501
NFPA 704 (fire diamond)
NFPA 704 four-colored diamondHealth 2: Intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury. E.g. chloroformFlammability 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 fuelInstability 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).
checkY verify (what is checkY☒N ?)

Properties edit

Tautomerism edit

Solvent Kketo→enol
Gas phase 11.7
Cyclohexane 42
Toluene 10
THF 7.2
CDCl3[3] 5.7
DMSO 2
Water 0.23
 

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.[4] 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.[5][6]

The equilibrium constant tends to be high in nonpolar solvents; when Kketo→enol is equal or greater than 1, the enol form is favoured. The keto form becomes more favourable in polar, hydrogen-bonding solvents, such as water.[7] The enol form is a vinylogous analogue of a carboxylic acid.

Acid–base properties edit

Solvent T/°C pKa[8]
40% ethanol/water 30 9.8
70% dioxane/water 28 12.5
80% DMSO/water 25 10.16
DMSO 25 13.41

Acetylacetone is a weak acid. It forms the acetylacetonate anion C5H7O2 (commonly abbreviated acac):

C5H8O2 ⇌ C5H7O2 + H+
 
The structure of the acetylacetonate anion (acac)

In the acetylacetonate anion, both C-O bonds are equivalent. Both C-C central bonds are equivalent as well, with one hydrogen atom bonded to the central carbon atom (the C3 atom). Those two equivalencies are because there is a resonance between the four bonds in the O-C2-C3-C4-O linkage in the acetylacetonate anion, where the bond order of those four bonds is about 1.5. Both oxygen atoms equally share the negative charge. The acetylacetonate anion is a bidentate ligand.

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).[9] Values for mixed solvents are available. Very strong bases, such as organolithium compounds, will deprotonate acetylacetone twice. The resulting dilithium species can then be alkylated at the carbon atom at the position 1.

Preparation edit

Acetylacetone is prepared industrially by the thermal rearrangement of isopropenyl acetate.[10]

 

Laboratory routes to acetylacetone also begin with acetone. Acetone and acetic anhydride ((CH3C(O))2O) upon the addition of boron trifluoride (BF3) catalyst:[11]

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

A second synthesis involves the base-catalyzed condensation (e.g., by sodium ethoxide CH3CH2ONa+) of acetone and ethyl acetate, followed by acidification of the sodium acetylacetonate (e.g., by hydrogen chloride HCl):[11]

CH3CH2ONa+ + CH3C(O)OCH2CH3 + CH3C(O)CH3 → Na+[CH3C(O)CHC(O)CH3] + 2 CH3CH2OH
Na+[CH3C(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 are benzoylacetone, dibenzoylmethane (dbaH)[clarification needed] and tert-butyl analogue 2,2,6,6-tetramethyl-3,5-heptanedione. Trifluoroacetylacetone and hexafluoroacetylacetonate are also used to generate volatile metal complexes.

Reactions edit

Condensations edit

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

Coordination chemistry edit

 
A ball-and-stick model of VO(acac)2

Sodium acetylacetonate, Na(acac), is the precursor to many acetylacetonate complexes. A general method of synthesis is to treat a metal salt with acetylacetone in the presence of a base:[12]

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 edit

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.[13]

C5H8O2 + O2C2H4O2 + C3H4O2

References edit

  1. ^ "05581: Acetylacetone". Sigma-Aldrich.
  2. ^ Thomas M. Harris (2001). "2,4-Pentanedione". e-EROS Encyclopedia of Reagents for Organic Synthesis. doi:10.1002/047084289X.rp030. ISBN 0471936235.
  3. ^ Smith, Kyle T.; Young, Sherri C.; DeBlasio, James W.; Hamann, Christian S. (12 April 2016). "Measuring Structural and Electronic Effects on Keto–Enol Equilibrium in 1,3-Dicarbonyl Compounds". Journal of Chemical Education. 93 (4): 790–794. doi:10.1021/acs.jchemed.5b00170.
  4. ^ Caminati, W.; Grabow, J.-U. (2006). "The C2v Structure of Enolic Acetylacetone". Journal of the American Chemical Society. 128 (3): 854–857. doi:10.1021/ja055333g. PMID 16417375.
  5. ^ 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". Journal of Chemical Education. 88 (10): 1444–1445. Bibcode:2011JChEd..88.1444M. doi:10.1021/ed1010932.
  6. ^ 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.
  7. ^ Reichardt, Christian (2003). Solvents and Solvent Effects in Organic Chemistry (3rd ed.). Wiley-VCH. ISBN 3-527-30618-8.
  8. ^ IUPAC SC-Database Archived 2017-06-19 at the Wayback Machine A comprehensive database of published data on equilibrium constants of metal complexes and ligands
  9. ^ 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. S2CID 96848983.
  10. ^ Siegel, Hardo; Eggersdorfer, Manfred (2002). "Ketones". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a15_077. ISBN 9783527306732.
  11. ^ a b Denoon, C. E., Jr.; Adkins, Homer; Rainey, James L. (1940). "Acetylacetone". Organic Syntheses. 20: 6. doi:10.15227/orgsyn.020.0006.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  12. ^ O'Brien, Brian. "Co(tfa)3 & Co(acac)3 handout" (PDF). Gustavus Adolphus College.
  13. ^ 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". Biochemical Journal. 369 (3): 573–581. doi:10.1042/BJ20021047. PMC 1223103. PMID 12379146.

External links edit