is a molecule
containing two ketone groups
. The simplest diketone is diacetyl
, also known as 2,3-butanedione. Diacetyl, acetylacetone
, and hexane-2,5-dione
are examples of 1,2-, 1,3-, and 1,4-diketones, respectively. Dimedone
is an example of a cyclic diketone.
An important member is biacetyl
, CH3C(O)C(O)CH3. These compounds are often generated by dehydrogenation of the diol
RCH(OH)CH(OH)R → RC(O)C(O)R + 2 H2
A distinctive feature of 1,2-diketones is the long C-C bond linking the carbonyl groups. This bond distance is about 1.54 Å, compared to 1.45 Å for the corresponding bond in 1,3-butadiene
. The effect is attributed to repulsion between the partial positive charges of the carbonyl carbon atoms.
1,2-Diketones condense with many bifunctional nucleophiles, such as urea
to give heterocycles. Upon condensation with aromatic amines, they convert to the diketimine
An important member is acetylacetone
, CH3C(O)CH2C(O)CH3. Acetylacetone is prepared industrially by the thermal rearrangement of isopropenylacetate.Hardo Siegel, Manfred Eggersdorfer "Ketones" in Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH, 2002, Wienheim.
CH2(CH3)COC(O)Me → MeC(O)CH2C(O)Me
Classically, 1,3-diketones are prepared by condensation of ketones with esters.
1,3-Diketones usually exist predominantly as enol
s because of conjugation
of the enol or enolate with the other carbonyl group, and the stability gained in forming a six-membered ring containing a hydrogen bond. The percent enol in acetylacetone
, and hexafluoroacetylacetone
are 85, 97, and 100%, respectively (neat, 33 °C).
Like other diketones, 1,3-diketones are versatile precursors to heterocycles. Hydrazine
, for example, condenses to give pyrazole
The conjugate base derived from 1,3-ketones form coordination complexes
. In the DeMayo reaction
1,3-diketones react with alkenes in a photochemical pericyclic reaction
to form (substituted) 1,5-diketones.
Diketones with two methylene groups separating the carbonyl groups typically coexist with their enol tautomer
s. 1,4-Diketones are useful precursors to heterocycles via the Paal-Knorr Synthesis
, which gives furan
s, and thiophene
The condensation of 1,4-diketones (and related substrates) with hydrazine
s afford dihydropyridazines, which can be converted to pyridazine
The reactions of diketones where the carbonyl centers are separated by three or more methylene groups are similar to those of simple ketones. These longer diketones are generally prone to intramolecular aldol condensation
In 2004, the EU conducted an examination of certain flavoring additives
grouped together as "Flavouring Group Evaluation FGE.11". This group included diketones. Opinion of the Scientific Panel on Food Additives, Flavourings, Processing Aids and Materials in contact with Food
, EFSA Journal
(2004) 166, 1-44 As part of that study, the EU found that acetylacetone
(pentane-2,4-dione) (FL 07.191) was genotoxic in vivo
and in vitro
, and deleted it from the register of permitted flavoring substances. COMMISSION DECISION of 18 May 2005 amending Decision 1999/217/EC as regards the register of flavouring substances used in or on foodstuffs
, Official Journal of the European Union
The register of permitted flavoring substances was adopted in 1999. Official Journal of the European Communities