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A polyunsaturated fatty acid is a fatty acid with at least two carbon–carbon double bonds in its hydrocarbon chain. The carbon–oxygen double bond in the carboxyl group does not contribute to this count. Many biological examples have cis double bonds, whose geometry changes how chains pack.^[ref]

For example, arachidonic acid has twenty carbons and four carbon–carbon double bonds, while docosahexaenoic acid has twenty-two carbons and six. Chain length, bond positions and cis/trans geometry convey different information. A count such as “four double bonds” alone cannot uniquely identify the molecule or all of its physical properties.^[ref]

Terephthalic acid is benzene-1,4-dicarboxylic acid, an aromatic dicarboxylic acid with two carboxyl groups attached at opposite positions of a benzene ring. Its neutral molecular formula is ${\mathrm{C}}_8{\mathrm{H}}_6{\mathrm{O}}_4$. The positions of the groups are part of its identity, not information recoverable from the elemental formula alone.^[ref]

The two carboxyl groups make it a useful difunctional building block. Reaction with ethylene glycol, which has two alcohol groups, forms ester connections in polyethylene terephthalate, a polyester. This is step-growth polymerization: growing molecules react through their remaining functional groups.^[ref]

An original repeat-unit bookkeeping calculation isUsing rounded masses, $166.13+62.07-2(18.015)=192.17\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$ per repeat-unit contribution. The subtraction represents two esterification connections per repeat in the long-chain description; it is not an elementary reaction equation for one isolated pair of monomers forming a complete polymer.

For a finite linear chain assembled from $n$ acid molecules and $n$ glycol molecules, with no ring closure or side reaction, there are $2n-1$ connections. The chain therefore loses $2n-1$ water molecules, rather than $2n$. End groups supply the difference: its mass is approximately $n(192.17)+18.015\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$ under the same rounded-mass convention.

The molar mass and end-group balance consequently matter alongside conversion. Measuring consumption of acid groups and formation of ester connections tests the reaction model; identifying an aromatic diacid does not determine the final chain length or material properties.

A uronic acid is a monosaccharide derivative in which the terminal primary alcohol group opposite the potential aldehyde group has become a carboxyl group. The reference numbering of the original aldose is retained: carbon 1 remains the potential aldehyde carbon, rather than becoming the number assigned to the new carboxyl group.^[ref]

For glucose, the relevant terminal group is at carbon 6. Replacing its $\mathrm{C}{\mathrm{H}}_2\mathrm{OH}$ reference group by $\mathrm{COOH}$ gives glucuronic acid, while preserving the arrangement at the intervening stereocenters. This distinguishes the position of a modification from the mere presence of a carboxyl group. Glucuronic acid supplies one of the two alternating sugar residues in hyaluronic acid, connecting the chemical classification with a biological polysaccharide.