Acetylacetone

Acac redirects here. For other uses, see ACAC.
Acetylacetone
Identifiers
CAS number 123-54-6 Y
ChemSpider 29001 Y
UNII 46R950BP4J Y
KEGG C15499 Y
ChEBI CHEBI:14750 Y
ChEMBL CHEMBL191625 Y
Jmol-3D images Image 1
Image 2
Properties
Molecular formula C5H8O2
Molar mass 100.13 g/mol
Density 0.98 g/mL
Melting point

−23 °C

Boiling point

140 °C

Solubility in water 16 g/100 mL
Hazards
EU Index 606-029-00-0
EU classification Harmful (Xn)
R-phrases R10, R22
S-phrases (S2), S21, S23, S24/25
NFPA 704
2
2
0
Flash point 34 °C
Autoignition
temperature
340 °C
Explosive limits 2.4–11.6%
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Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Acetylacetone is an organic compound that famously exists in two tautomeric forms that rapidly interconvert. The less stable tautomer is a diketone formally named pentane-2,4-dione. The more common tautomer is the enol form. The pair of tautomers rapidly interconvert and are treated as a single compound in most applications. It is a colourless liquid that is a precursor to acetylacetonate (acac), a common bidentate ligand. It is also a building block for the synthesis of heterocyclic compounds.

Contents

Properties

Solvent Kketo-enol
Gas Phase 11.7
Cyclohexane 42
Toluene 10
THF 7.2
DMSO 2
Water 0.23

The keto and enol forms of acetylacetone coexist in solution; these forms are tautomers. The C2v symmetry for the enol form displayed on the left in Scheme 1 has been verified by many methods, most prominently being NMR spectroscopy and IR spectroscopy.[2] In the gas phase, the equilibrium constant, Kketo-enol is 11.7, favoring the enol form.[3] The equilibrium constant tends to remain high in nonpolar solvents; the keto form becomes more favorable in polar, hydrogen-bonding solvents, such as water.[4] The enol form is a vinylogous analogue of a carboxylic acid.

Acid-base properties

Acetylacetone is a weak acid:

C5H8O2 C5H7O2 + 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).[5] Values for mixed solvents are avalable. Very strong bases, such as organolithium compounds, will deprotonate acetylacetone twice. The resulting dilithio species can then be alkylated at C-1.

solvent T/°C pKa[6]
40% ethanol/water 30 9.8
70% dioxane/water 28 12.5
80% DMSO/water 25 10.16
DMSO 25 13.41

Preparation

Acetylacetone is prepared industrially by the thermal rearrangement of isopropenylacetate.[7]

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 BF3 catalyst:[8]

(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:[8]

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 aryl- and alkylamines to gives the mono- and then the didiketimines wherein the O atoms in acetylacetone are replaced by NR (R = aryl, alkyl).

Coordination chemistry

The acetylacetonate anion, acac-, forms complexes with many transition metal ions. A general method of synthesis is to react the metal ion with acetylacetone in the presence of a base (B):

Mz+ + z (acacH) M(acac)z +z BH+

which assists the removal of a proton from acetylacetone and shifts the equilibrium in favour of the complex. 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. Since the metal complex carries no electrical charge, it is soluble in non-polar organic solvents.

Biodegradation

Enzymatic breakdown: The enzyme acetylacetone dioxygenase cleaves the carbon-carbon bond of acetyacetone, producing acetate and 2-oxopropanal. The enzyme is Fe(II)-dependent, but it has been proven to bind to zinc as well. Acetylacetone degradation has been characterized in the bacterium Acinetobacter johnsonii.[9]

C5H8O2 + O2 → C2H4O2 + C3H4O2

References

  1. ^ a b Evan's pKa table
  2. ^ Kimberly A. Manbeck, Nicholas C. Boaz, Nathaniel C. Bair, Allix M. S. Sanders, and Anderson L. Marsh (2011). "Substituent Effects on Keto-Enol Equilibria Using NMR Spectroscopy". Journal of Chemical Education 88 (10): 1444–1445. doi:10.1021/ed1010932. . Z. Yoshida, H. Ogoshi, T. Tokumitsu "Intramolecular hydrogen bond in enol form of 3-substituted-2,4-pentanedione" Tetrahedron 1970, volume 26, pp. 5691-5697. doi:10.1016/0040-4020(70)80005-9
  3. ^ W. Caminati, J.-U. Grabow (2006). "The C2v Structure of Enolic Acetylacetone". Journal of the American Chemical Society 128 (3): 854–857. doi:10.1021/ja055333g. PMID 16417375. 
  4. ^ Solvents and Solvent Effects in Organic Chemistry, Christian Reichardt Wiley-VCH; 3 edition 2003 ISBN 3-527-30618-8
  5. ^ Stary, J.; Liljenzin, J.O. (1982). "Critical evaluation of equilibrium constants involving acetylacetone and its metal chelates". Pure and Applied Chemistry 54 (12): 2557–2592. doi:10.1351/pac198254122557. http://www.iupac.org/publications/pac/pdf/1982/pdf/5412x2557.pdf. 
  6. ^ IUPAC SC-Database A comprehensive database of published data on equilibrium constants of metal complexes and ligands
  7. ^ Hardo Siegel, Manfred Eggersdorfer “Ketones” in Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH, 2002, Wienheim. doi:10.1002/14356007.a15_077
  8. ^ a b C. E. Denoon, Jr., "Acetylacetone", Org. Synth., http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=cv3p0016 ; Coll. Vol. 3: 16 
  9. ^ 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 (Pt 3): 573–581. doi:10.1042/BJ20021047. PMC 1223103. PMID 12379146. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=1223103. 

External links