Everything about Acetylacetone totally explained
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Acetylacetone is an
organic compound with molecular formula C
5H
8O
2. This
diketone is formally named 2,4-pentanedione. It is a precursor to acetylacetonate (abbreviated
acac), a common bidentate
ligand. It is also a building block for the synthesis of
heterocyclic compounds.
Properties
The keto and enol forms of acetylacetone coexist in solution; these forms are
tautomers. The C
2v symmetry for the enol form displayed on the right in
scheme 1 has been verified by many methods including
microwave spectroscopy. Hydrogen bonding in the enol reduces the steric repulsion between the carbonyl groups. In the gas phase K is 11.7. The equilibrium constant tends to be high in nonpolar solvents:
cyclohexane is 42,
toluene is 10,
THF 7.2,
dimethyl sulfoxide (K=2), and water (K=0.23).
Preparation
Acetylacetone is prepared industrially by the thermal rearrangement of isopropenylacetate.
» Me
2CHOC(O)Me → MeC(O)CH
2C(O)Me
Laboratory routes to acetylacetone begin also with
acetone. Acetone and
acetic anhydride upon the addition of
BF3 catalyst:
» (CH
3CO)
2O + CH
3C(O)CH
3 → CH
3C(O)CH
2C(O)CH
3
A second synthesis involves the base-catalyzed condensation of acetone and
ethyl acetate, followed by acidification:
Coordination chemistry
The acetylacetonate
anion forms complexes with many
transition metal ions wherein both oxygen atoms bind to the metal to form a six-membered chelate ring. Some examples include: Mn(acac)
3,
VO(acac)2, Fe(acac)
3, and Co(acac)
3. Any complex of the form M(acac)
3 is
chiral (has a non-superimposable mirror image). Additionally, M(acac)
3 complexes can be reduced electrochemically, with the reduction rate being dependent on the solvent and the metal center. Bis- and tris complexes of the type M(acac)
2 and M(acac)
3 are typically soluble in organic solvents, in contrast to the related metal halides. Because of thess properties, these complexes are widely used as
catalyst precursors and reagents. Important applications include their use as NMR "shift reagents" and as catalysts for
organic synthesis, and precursors to industrial hydroformylation catalysts.
C
5H
7O
2− in some cases also binds to metals through the central carbon atom; this bonding mode is more common for the third-row transition metals such as platinum(II) and iridium(III).
Metal acetylacetonates
Chromium(III) acetylacetonate
Cr(acac)3 is used as a spin relaxation agent to improve the sensitivity in quantitative
Carbon-13 NMR spectroscopy.
Copper(II) acetylacetonate
Cu(acac)
2, prepared by treating acetylacetone with aqueous Cu(NH
3)
42+ and is available commercially, catalyzes coupling and carbene transfer reactions.
Copper(I) acetylacetonate
Unlike the copper(II) chelate, copper(I) acetylacetonate is an air sensitive oligomeric species. It is employed to catalyze
Michael additions.
Manganese(III) acetylacetonate
Mn(acac)
3, a one-electron oxidant, is used for coupling phenols.
Nickel(II) acetylacetonate
"Nickel acac" isn't Ni(acac)
2 but the trimer [Ni(acac)
2]
3. This emerald green solid, which is benzene soluble, is widely employed in the preparation of Ni(O) complexes. Upon exposure to the atmosphere, [Ni(acac)
2]
3 converts to the chalky green monomeric hydrate.
Vanadyl acetylacetonate
Vanadyl acetylacetonate is a blue complex with the formula V(O)(acac)
2. It is useful in epoxidation of allylic alcohols.
Zinc acetylacetonate
The monoaquo complex Zn(acac)
2H
2O (m.p. 138-140 °C) is pentacoordinate, adopting a square pyramidal structure. Dehydration of this species gives the hygroscopic anhydrous derivative (m.p. 127 °C). This more volatile derivative has been used as a precursor to films of
ZnO.
Iridium acetylacetonates
Both iridium(I) and Ir(III) form stable acetylacetonato complexes. The Ir(III) derivatives include
trans-Ir(acac)
2(
CH(COMe)
2)(H
2O) and the more conventional
D3-symmetric Ir(acac)
3. The C-bonded derivative is a precursor to homogeneous catalysts for
C-H activation and related chemistries. Iridium(I) derivatives include square-planar Ir(acac)(CO)
2 (
C2v-symmetry).
Aluminium(III) acetylacetonate
Al(C
5H
7O
2)
n, or shortened to Al(acac)
C-bonded acetylacetonates
C
5H
7O
2− in some cases also binds to metals through the central carbon atom (C3); this bonding mode is more common for the third-row transition metals such as platinum(II) and iridium(III). The complexes Ir(acac)
3 and corresponding Lewis-base adducts Ir(acac)
3L (L = an
amine) contain one carbon-bonded acac ligand. The IR spectra of O-bonded acetylacetonates are characterized by relatively low-energy νCO bands of 1535 cm
−1, whereas in carbon-bonded acetylacetonates, the carbonyl vibration occurs closer to the normal range for ketonic C=O, for example 1655 cm
−1.
Other reactions of acetylacetone
- Deprotonations: Very strong bases will doubly deprotonate acetylacetone, starting at C3 but also at C1. The resulting species can then be alkylated at C-1.
- Precursor to heterocycles: Acetylacetone is a versatile precursor to heterocycles. Hydrazine reacts to produce pyrazoles. Urea gives pyrimidines.
- Precursor to related imino ligands: Acetylacetone condenses with amines to give, successively, the mono- and the di-diketimines wherein the O atoms in acetylacetone are replaced by NR (R = aryl, alkyl).
- 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. » C5H8O2 + O2 → C2H4O2 + C3H4O2
- Arylation: Acetylacetonate displaced halides from certain halo-substituted benzoic acid. This reaction is copper-catalyzed. » 2-BrC6H4CO2H + NaC5H7O2 → 2-(CH3CO)2HC)-C6H4CO2H + NaBr
References
Further Information
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