<P> An aldehyde / ˈældɪhaɪd / or alkanal is an organic compound containing 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, also known as the formyl group . Aldehydes are common in organic chemistry . Many fragrances are aldehydes . </P> <P> Aldehydes feature an sp - 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 ordinarily 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 pK near 17, compared to the acidity of a typical alkane (pK about 50). 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 . </P>

Out of ketone and aldehyde groups which is the terminal functional group