= Arachidonic acid
{title2=$20{:}4\,n\! -\!6$}
<Arachidonic acid> is a twenty-<carbon> <polyunsaturated fatty acid> with four cis carbon–carbon <double bonds>, at positions 5, 8, 11 and 14 when counted from the <carboxyl group>. Its molecular formula is $\mathrm{C_{20}H_{32}O_2}$. The shorthand $20{:}4\,n\! -\!6$ records twenty carbons, four double bonds and the first double bond six carbons from the methyl end.<PubChem Compound database by National Center for Biotechnology Information>[Arachidonic acid, CID 444899, formula and (5Z,8Z,11Z,14Z) stereochemistry]{ref}
The indexing conventions describe the same structure from opposite ends: the bond between carbons 14 and 15 from the acid end is the first encountered from the methyl end. The cis geometry also distinguishes this molecule from <stereoisomers> with the same formula. Composition, double-bond positions and geometry all belong to its chemical identity.
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