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, which is any generic alkyl or side chain. The group without R is called the aldehyde group or formyl group.ecause 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).