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Blida 1 University Faculty of Science Department of Chemistry UEF 3.1.1: Chemistry of Polyfunctional Compounds 3rd Year, BSc in Organic Chemistry 2025 - 2026
Chapter IV : Dicarbonyl compounds
I. Introduction
Dicarbonyl compounds are organic molecules that contain two carbonyl groups within their molecular framework. These compounds play a fundamental role in various areas of organic chemistry, particularly in the synthesis of complex molecules and in reaction mechanisms involving both nucleophilic and electrophilic processes.
II. Structure of Dicarbonyl Compounds
Dicarbonyl compounds are characterized by the presence of two carbonyl functions, which can take the form of two ketone groups , two aldehyde groups , two ester groups , or a combination of these groups. Their general structure can be represented by R–CO–(CH₂)ₙ– CO–R′ , where R and R′ can be hydrogen atoms, alkyl or aryl groups, and n denotes the number of carbon atoms separating the two carbonyl groups. 1,2-Dialdehydes : Both carbonyl groups are adjacent. Example: Ethanedial (glyoxal), with the general molecular formula C₂H₂O₂ and the structural formula O=CH–CH=O. 1,2-Diketones (α-diketones): The two carbonyl groups are located on adjacent carbon atoms.
Example: 1,3-Diketones (β-diketones): The carbonyl groups are separated by one carbon atom Example: Pentane-2,4-dione (acetylacetone) or ethyl acetoacetate as a related β-dicarbonyl compound. 1,4-Diketones: The carbonyl groups are separated by two carbon atoms. Example: III. Properties of Dicarbonyl Compounds Dicarbonyl compounds exhibit distinctive chemical properties due to the presence of two carbonyl (C=O) groups, which significantly influence their reactivity. Polarity: The carbonyl groups are polar, imparting a degree of solubility in polar solvents such as water and ethanol. This polarity also contributes to their ability to participate in dipole–dipole interactions and hydrogen bonding with protic solvents. Acidity: The hydrogen atoms located in the α-position relative to a carbonyl group are acidic (labile) because the resulting negative charge can be delocalized through both carbonyl groups, a phenomenon known as enolization****. Butane-2,3-dione (diacetyl), C₄H₆O₂
Mechanism IV.b Oxidation of a secondary ketol The controlled oxidation of a secondary ketol leads to the formation of a dicarbonyl compound. Example IV.c Mixed Claisen Condensation This is a reaction that allows the formation of a carbon-carbon bond between two esters or between an ester and a carbonyl compound, in the presence of a strong base. This reaction leads to the formation of a β-ketoester or a β-diketone. Example 1
Example 2
V. Reactivity of α-Dicarbonyl Compounds
Dicarbonyl compounds exhibit higher reactivity than isolated carbonyl compounds, mainly due to the proximity of two carbonyl functions (C=O) acting as electrophilic centers. Their reactions are generally similar to those of classical carbonyl compounds. However, under basic conditions and upon heating, an α-diketone undergoes a specific rearrangement called a benzilic rearrangement. Example Mechanism Note : This reaction is similar to the disproportionation of benzaldehyde in basic medium, also known as the Cannizzaro reaction, where a hydride ion is transferred from benzaldehyde hydrate to another benzaldehyde molecule. Example 2
Reduction of Quinones Quinones can be reduced to hydroquinones through one- or two-electron transfer processes, a transformation of great biological and chemical significance. Example
