Formyl group

A formyl group is the acyl group $\mathrm{H}-\mathrm{C}(=\mathrm{O})-$, attached to the remainder of a molecule through its carbonyl carbon. It contains one carbon, one oxygen and one hydrogen; the terminal dash denotes the attachment bond, not a negative charge. IUPAC retains formyl as a preferred prefix and lists methanoyl as another name for the same substituent.^[ref]

The attachment atom matters. In acetaldehyde, $\mathrm{C}{\mathrm{H}}_3-\mathrm{CHO}$, the formyl group is attached to a carbon and gives an aldehyde. Attaching $\mathrm{HCO}-$ to a nitrogen instead gives an N-formyl derivative, with different chemical surroundings. Thus recognizing a formyl group does not make the entire compound an aldehyde. Nor is a bonded substituent automatically a free radical or an independently existing ion.

A useful molecular-formula calculation starts with methionine, ${\mathrm{C}}_5{\mathrm{H}}_{11}\mathrm{N}{\mathrm{O}}_2\mathrm{S}$. Formally replacing one hydrogen on its amino nitrogen by $\mathrm{CHO}$ adds $\mathrm{CO}$ overall: one hydrogen leaves and the attached group supplies one. The neutral free acid consequently becomes ${\mathrm{C}}_6{\mathrm{H}}_{11}\mathrm{N}{\mathrm{O}}_3\mathrm{S}$, the formula recorded for formylmethionine.^[ref] This is atom bookkeeping, not a preparative reaction or the formula of a residue already bound to transfer RNA.

The formyl group can be derived formally from formic acid, $\mathrm{HCOOH}$, by removing its hydroxyl group at the attachment site. This explains the nomenclature relationship while keeping the connectivity and the chemical species distinct.