Acetylacetone

Acetylacetone is an organic compound with the chemical formula Template:Chem2. It is classified as a 1,3-diketone. It exists in equilibrium with a tautomer Template:Chem2. 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.<ref>Template:Cite encyclopedia</ref> Acetylacetone is a building block for the synthesis of many coordination complexes as well as heterocyclic compounds.

Properties

Tautomerism

Solvent	Kketo→enol
Gas phase	11.7
Cyclohexane	42
Toluene	10
THF	7.2
[[Deuterated chloroform|Template:Chem2]]<ref>Template:Cite journal</ref>	5.7
DMSO	2
Water	0.23

File:Acetylaceton-Tautomerie.svg

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.<ref>Template:Cite journal</ref> 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.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>

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.<ref>Template:Cite book</ref> The enol form is a vinylogous analogue of a carboxylic acid.Template:Citation needed

Acid–base properties

Solvent	T/°C	pKa<ref name=scdb>IUPAC SC-Database Template:Webarchive A comprehensive database of published data on equilibrium constants of metal complexes and ligands</ref>
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 Template:Chem2 (commonly abbreviated Template:Chem2): Template:Block indent

File:Acetylacetonate anion.png

The structure of the acetylacetonate anion (Template:Chem2)

In the acetylacetonate anion, both Template:Chem2 bonds are equivalent. Both Template:Chem2 central bonds are equivalent as well, with one hydrogen atom bonded to the central carbon atom (the atom numbered C3 according to the IUPAC nomenclature of organic chemistry). These equivalencies are because there is a resonance between the four bonds in the O−C2−C3−C4−O linkage in the acetylacetonate anion. Each of the four bonds in the linkage has a bond order of about 1.5, and the two 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 [[Sodium perchlorate|Template:Chem2]]) and 9.00 ± 0.03 (I = 1.0 M Template:Chem2; I = Ionic strength).<ref>Template:Cite journal</ref> 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

Acetylacetone is prepared industrially by the thermal rearrangement of isopropenyl acetate.<ref>Template:Cite encyclopedia</ref>

CH3C(O)OC(CH2)CH3 becomes CH3C(O)CH2C(O)CH3

Laboratory routes to acetylacetone also begin with acetone. Acetone and acetic anhydride (Template:Chem2) upon the addition of boron trifluoride (Template:Chem2) catalyst:<ref name = denoon>Template:OrgSynth</ref> Template:Block indent

A second synthesis involves the base-catalyzed condensation (e.g., by sodium ethoxide Template:Chem2) of acetone and ethyl acetate, followed by acidification of the sodium acetylacetonate (e.g., by hydrogen chloride HCl):<ref name = denoon/> Template:Block indent Template:Block indent

Because of the ease of these syntheses, many analogues of acetylacetonates are known. Some examples are benzoylacetone, dibenzoylmethane and tert-butyl analogue 2,2,6,6-tetramethyl-3,5-heptanedione. Trifluoroacetylacetone and hexafluoroacetylacetonate are also used to generate volatile metal complexes.

Reactions

Condensations

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

File:Vanadyl-acetylacetonate-from-xtal-3D-balls.png

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:<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Template:Chem2

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

The enzyme acetylacetone dioxygenase cleaves a central 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.<ref>Template:Cite journal</ref>

Template:Chem2

External links

• Template:ICSC