| Acetylacetone | |
|---|---|
|
Pentane-2,4-dione |
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|
Other names
Hacac |
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| Identifiers | |
| CAS number | 123-54-6 |
| ChemSpider | 29001 |
| UNII | 46R950BP4J |
| KEGG | C15499 |
| ChEBI | CHEBI:14750 |
| ChEMBL | CHEMBL191625 |
| Jmol-3D images | Image 1 Image 2 |
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| 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% |
| (verify) (what is: /?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) |
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| 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 |
| 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.
Acetylacetone is a weak acid:
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 |
Acetylacetone is prepared industrially by the thermal rearrangement of isopropenylacetate.[7]
Laboratory routes to acetylacetone begin also with acetone. Acetone and acetic anhydride upon the addition of BF3 catalyst:[8]
A second synthesis involves the base-catalyzed condensation of acetone and ethyl acetate, followed by acidification:[8]
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.
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).
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):
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.
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]