= Formyl group
A \b[formyl group] is the <acyl group> $\mathrm{H{-}C(=O){-}}$, 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 \i[formyl] as a preferred prefix and lists \i[methanoyl] as another name for the same substituent.<Nomenclature of Organic Chemistry: IUPAC Recommendations and Preferred Names 2013 by IUPAC>[P-65.1.7.2.1 and Appendix 2: HCO–, formyl/methanoyl. Primary locators https://iupac.qmul.ac.uk/BlueBook/P6.html and https://iupac.qmul.ac.uk/BlueBook/Papp2.html .]{ref}
The attachment atom matters. In <acetaldehyde>, $\mathrm{CH_3{-}CHO}$, 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>, $\mathrm{C_5H_{11}NO_2S}$. 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 $\mathrm{C_6H_{11}NO_3S}$, the formula recorded for <formylmethionine>.<N-formyl-L-methionine in ChEBI>[CHEBI:16552, structure and neutral-acid formula.]{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.
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