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Acetylacetone

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Acetylacetone
Skeletal structures of both tautomers

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IUPAC name
Pentane-2,4-dione
Other names
Hacac
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This page is a soft redirect. Properties

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EU Index 606-029-00-0 EU classification Harmful (Xn) R-phrases R10, R22 S-phrases (S2), S21, S23, S24/25 NFPA 704

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Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Acetylacetone is an organic compound that exists in two tautomeric forms that interconvert rapidly. The pair of tautomers rapidly interconvert and are treated as a single compound in most applications. Although the compound is formally named as the diketone form, pentane-2,4-dione, the enol form forms a substantial component of the material[2] and is actually the favored form in many solvents. 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.

Properties

Tautomerism

The keto and enol forms of acetylacetone coexist in solution; these forms are tautomers. The enol form has C2v symmetry, meaning the hydrogen 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 easily be distinguished by NMR spectroscopy, IR spectroscopy, and other methods[4][5]

The equilibrium constant tends to remain 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.

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


Acid-base properties

solvent T/°C pKa[7]
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:

C5H8O2 15px 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).[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 isopropenylacetate.[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 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 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):

MBz + z (acacH) 15px 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.[11]

C5H8O2 + O2 → C2H4O2 + C3H4O2

References

  1. http://www.sigmaaldrich.com/catalog/product/sial/p7754?lang=en&region=GB
  2. Co(tfa)3 & Co(acac)3 handout. Brian O'Brien, Gustavus Adolphus College.
  3. W. Caminati, J.-U. Grabow (2006). "The C2v Structure of Enolic Acetylacetone". Journal of the American Chemical Society 128 (3): 854–857. PMID 16417375. doi:10.1021/ja055333g. 
  4. 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. .
  5. Z. Yoshida, H. Ogoshi, T. Tokumitsu (1970). "Intramolecular hydrogen bond in enol form of 3-substituted-2,4-pentanedione". Tetrahedron 26: 5691–5697. doi:10.1016/0040-4020(70)80005-9. 
  6. Solvents and Solvent Effects in Organic Chemistry, Christian Reichardt Wiley-VCH; 3 edition 2003 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. Hardo Siegel, Manfred Eggersdorfer “Ketones” in Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH, 2002, Wienheim. doi:10.1002/14356007.a15_077
  10. 10.0 10.1 C. E. Denoon, Jr. "Acetylacetone". Org. Synth. ; Coll. Vol. 3, p. 16 
  11. 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. PMC 1223103. PMID 12379146. doi:10.1042/BJ20021047. 

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