Acetylacetone

The keto and enol tautomers of acetylacetone coexist in solution. The enol form has C_2v symmetry, meaning the hydrogen atom is shared equally between the two oxygen atoms.^[4] In the gas phase, the equilibrium constant, K_keto→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 K_keto→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.^{[citation needed]}

Acetylacetone is a weak acid. It forms the acetylacetonate anion C₅H₇O−2 (commonly abbreviated acac⁻):

C₅H₈O₂ ⇌ C₅H₇O−2 + H⁺

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 pK_a 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.

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 CH₃CH₂O⁻Na⁺) of acetone and ethyl acetate, followed by acidification of the sodium acetylacetonate (e.g., by hydrogen chloride HCl):^[11]

CH₃CH₂O⁻Na⁺ + 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.

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).

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]

MB_z + z Hacac ⇌ M(acac)_z + zBH

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.

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 + O₂ → C₂H₄O₂ + C₃H₄O₂

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