An aldehyde /ˈældᵻhaɪd/ or alkanal is an organic compound containing a formyl group, a functional group with the structure -CHO, consisting of a carbonyl center (a carbon double bonded to oxygen) with the carbon atom also bonded to hydrogen and to an R group, which is any generic alkyl or side chain. The group without R is 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.