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aldehyde
An aldehyde or alkanal is an organic compound containing a formyl group. The formyl group is a functional group, with the structure R-CHO, consisting of a carbonyl center (a carbon double bonded to oxygen) bonded to hydrogen and an R group,〔IUPAC Gold Book (aldehydes )〕 which is any generic alkyl or side chain. The group without R is called the aldehyde group or formyl group. Aldehydes differ from ketones in that the carbonyl is placed at the end of a carbon skeleton rather than between two carbon atoms. Aldehydes are common in organic chemistry. Many fragrances are aldehydes.
==Structure and bonding==
Aldehydes feature an sp2-hybridized, planar carbon center that is connected by a double bond to oxygen and a single bond to hydrogen. The C-H bond is not acidic. Because of resonance stabilization of the conjugate base, an α-hydrogen in an aldehyde (not shown in the picture above) is far more acidic, with a pKa near 17, than a C-H bond in a typical alkane (pKa about 50).〔(Chemistry of Enols and Enolates - Acidity of alpha-hydrogens )〕 This acidification is attributed to (i) the electron-withdrawing quality of the formyl center and (ii) the fact that the conjugate base, an enolate anion, delocalizes its negative charge. Related to (i), the aldehyde group is somewhat polar.
Aldehydes (except those without an alpha carbon, or without protons on the alpha carbon, such as formaldehyde and benzaldehyde) can exist in either the keto or the enol tautomer. Keto-enol tautomerism is catalyzed by either acid or base. Usually the enol is the minority tautomer, but it is more reactive.
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