Aldehyde

“Aldehydes and ketones are among the most important of all functional groups, both in the chemical industry and in biological pathways. ... Aldehydes are normally prepared in the laboratory by oxidation of primary alcohols or by partial reduction of esters.”

“In conclusion, the conjugate umpolung of α,β-unsaturated aldehydes represents a versatile and powerful method to synthesize different cyclic products such as β- and γ-lactones and cyclopentenes. More valuable applications based on the NHC-catalyzed umpolung are expected to be discovered in due course.”

“The carbonyl group in aldehydes and ketones is an oxygen analog of the carbon–carbon double bond. However, the electronegativity of oxygen polarizes the pbond, thereby rendering the acyl substituent electron withdrawing. The arrangement of bonds around the carbon and oxygen is planar, a consequence of sp hybridization.”