An
aldehyde is an organic compound containing a terminal
carbonyl group. This
functional group, which consists of a
carbon atom bonded to a
hydrogen atom and
double-bonded to an
oxygen atom (chemical formula O=CH-), is called the
aldehyde group. The aldehyde group is also called the
formyl or
methanoyl group.
The word aldehyde seems to have arisen from alcohol dehydrogenated. In the past, aldehydes were sometimes named after the corresponding alcohols, for example, vinous aldehyde for acetaldehyde. (Vinous is from Latin vinum = wine, the traditional source of ethanol; compare vinyl.)
The aldehyde group is polar. Oxygen, more electronegative than carbon, pulls the electrons in the carbon-oxygen bond towards itself, creating an electron deficiency at the carbon atom.
Owing to resonance stabilization of the conjugate base, an a-hydrogen in an aldehyde is more acidic than a hydrogen atom in an alkane, with a typical pKa of 17.
