A dicarbonyl is a molecule containing two carbonyl (C=O) groups. Although this term could refer to any organic compound containing two carbonyl groups, it is used more specifically to describe molecules in which both carbonyls are in close enough proximity that their reactivity is changed, such as 1,2-, 1,3-, and 1,4-dicarbonyls. Their properties often differ from those of monocarbonyls, and so they are usually considered functional groups of their own. These compounds can have symmetrical or unsymmetrical substituents on each carbonyl, and may also be functionally symmetrical (dialdehydes, diketones, diesters, etc.) or unsymmetrical (keto-esters, keto-acids, etc.).
The only 1,2-dialdehyde is glyoxal, (CHO)2. Like many alkyldialdehydes, glyoxal is encountered almost exclusively as its hydrate and oligomers thereof. These derivatives often behave equivalently to the aldehydes since hydration is reversible. Glyoxal condenses readily with amines. Via such reactions, it is a precursor to many heterocycles, e.g. imidazoles.
- RCH(OH)CH(OH)R → RC(O)C(O)R + 2 H2
2,3-Butanedione, 2,3-pentanedione, and 2,3-hexanedione are found in small amounts in various foods. They are used as aroma components in alcohol-free beverages and in baked goods. Benzil, (PhCO)2, is the corresponding diphenyl derivative.
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.
ortho-Quinone, C4H4(CO)2, is the parent of a large family of 1,2-diketones.
1,2-Diesters and diacids
Oxalic acid and its esters define this family of compounds. The diacid is produced industrially by oxidation of waste sugars. It occurs naturally (as the conjugate base), notably in members of the plant species Oxalis. Condensation of the diesters with diamines gives cyclic diamides.
α-Keto- and formylcarboxylic acids
α-Keto-acids and -esters are well known. Pyruvic acid (CH3C(O)CO2H) is the parent α-ketoacid. Its conjugate base, pyruvate (CH3C(O)CO2−), is a component of the citric acid cycle and product of glucose metabolism (glycolysis). The corresponding aldehyde-acid is glyoxalic acid (HC(O)CO2H).
The parent 1,3-dialdehyde is malondialdehyde (CH2(CHO)2). Like most dialdehydes, it is rarely encountered as such. Instead it is handled almost exclusively as its hydrate, methyl acetal, and oligomers thereof. These derivatives often behave like the parent. Many 2-substituted derivatives are known. They are often prepared by alkylation of the enolate of malondialdehyde.
An important member is acetylacetone, CH3C(O)CH2C(O)CH3. Dimedone is a cyclic 1,3-diketone. Acetylacetone is prepared industrially by the thermal rearrangement of isopropenylacetate. Another cyclic 1,3-diketone is 2,2,4,4-tetramethylcyclobutanedione, which is a precursor to a useful diol.
- CH2(CH3)COC(O)Me → MeC(O)CH2C(O)Me
1,3-Diketones that can tautomerize to an enol that is conjugated to the other carbonyl usually exist predominantly in the enol form, and especially when the product can be further stabilized by a six-membered ring containing a hydrogen bond. For example, the percent enol in acetylacetone, trifluoroacetyacetone, and hexafluoroacetylacetone are 85, 97, and 100%, respectively (neat, 33 °C).
The conjugate base derived from 1,3-ketones can serve as ligand s to form metal acetylacetonate coordination complexes. In the DeMayo reaction 1,3-diketones react with alkenes in a photochemical pericyclic reaction to form (substituted) 1,5-diketones.
Classically, 1,3-diketones are prepared by the Claisen condensation of a ketone with an ester.
1,3-Diesters and diacids
Malonic acid and its esters are the parent members of this class of dicarbonyls. Also common are the 2-substituted derivatives with the formula RCH(CO2R)2, which arise by C-alkylation of the conjugate base (the enolate) NaCH(CO2R)2.
Succinaldehyde (CH2CHO)2 is the simplest and parent 1,4-dialdehyde.
Diketones with two methylene groups separating the carbonyl groups, also called γ-diketones, typically coexist with their enol tautomers. The preeminent member is acetonylacetone. 1,4-Diketones are useful precursors to heterocycles via the Paal–Knorr synthesis, which gives pyrroles:
This reactivity is the basis of the neurotoxicity of γ-diketones. 1,4-Diketones are also precursor to furans and thiophenes. The condensation of 1,4-diketones (and related substrates) with hydrazines afford dihydropyridazines, which can be converted to pyridazines.
para-quinone, C4H4(CO)2, is the parent of a large family of 1,4-diketones.
1,4-Diesters and diacids
Succinic acid and its esters are the parent members of this family of 1,4-dicarbonyls. Succinic acid is notable as a component in the citric acid cycle. It forms a cyclic acid anhydride, succinic anhydride. Unsaturated members include maleic and fumaric acids and esters.
These diketones have three methylene groups separating the carbonyl groups.
1,5-Diesters and diacids
Glutaric acid (CH2)3(CO2H)2 is the parent 1,5-diacid.
Hydration and cyclization
Small aldehydes tend to hydrate. Hydration is prevalent for dialdehydes. Glyoxal forms a series of cyclic hydrates. Succinaldehyde hydrates readily to give 2,5-dihydroxytetrahydrofuran. The aromatic phthalaldehyde also forms hydrated.
A number of dicarbonyl compounds are bioactive. Diacetyl is known to cause the lung disease bronchiolitis obliterans in those individuals exposed to it in an occupational setting. Dialdehydes, e.g. glutaraldehyde and malonaldehyde, are fixatives or sterilizers.
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