= 4-Methylpentanal
<4-Methylpentanal> is a branched six-carbon <aldehyde> with <molecular formula> $\mathrm{C_6H_{12}O}$. Its condensed structure is $\mathrm{CHO{-}CH_2{-}CH_2{-}CH(CH_3){-}CH_3}$. The aldehyde carbon is numbered 1, so the branch is a <methyl group> at carbon 4 of the five-carbon parent chain.<4-Methylpentanal in ChEBI>[CHEBI:17998, Formula, SMILES and IUPAC name]{ref}
The name makes a useful atom-counting example. A pentanal parent chain supplies five carbons, and the methyl substituent supplies a sixth. The hydrogen count is $1+2+2+1+3+3=12$, including the hydrogen attached to the aldehyde carbon. Five in the parent-chain name does not mean that the entire molecule contains only five carbons.
<4-Methylpentanal> also provides a chemically identifiable coproduct of <cholesterol> side-chain cleavage. The <cholesterol side-chain cleavage enzyme> makes <pregnenolone> and <4-Methylpentanal>, separating the original 27-carbon skeleton into 21- and six-carbon products.<Cholesterol side-chain cleavage enzyme in IUBMB Enzyme Nomenclature>[EC 1.14.15.6, overall reaction]{ref} In an ideal fully coupled reaction, one mole of pregnenolone corresponds to one mole of this aldehyde. Detecting both products can therefore test the proposed material balance more specifically than measuring total steroid-related signal alone.
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