Phosphate triester

A phosphate triester is a phosphoric ester in which three organic groups are attached through oxygen to one phosphorus center. A general connectivity formula isUnlike a neutral-acid phosphate monoester or phosphate diester, this representation contains no remaining P–OH group. The organic groups may nevertheless contain their own ionizable sites; “triester” does not guarantee that every molecule in the class is electrically neutral.

Trimethyl phosphate supplies a concrete example: $\mathrm{O}=\mathrm{P}(\mathrm{OC}{\mathrm{H}}_3{)}_3$ has formula ${\mathrm{C}}_3{\mathrm{H}}_9{\mathrm{O}}_4\mathrm{P}$. Its three methyl groups each contribute one carbon and three hydrogens, while the central unit and linking oxygens contribute one phosphorus and four oxygens.^[ref]

An original connectivity comparison replaces one $-\mathrm{OC}{\mathrm{H}}_3$ group by $-\mathrm{OH}$. The resulting fragment, $\mathrm{O}=\mathrm{P}(\mathrm{OC}{\mathrm{H}}_3{)}_2(\mathrm{OH})$, is a diester because only two organic groups remain attached through oxygen. Merely removing a proton from that O–H group changes charge without restoring a third ester linkage.

The suffix counts esterification at one phosphorus center. Three separate phosphate monoesters elsewhere on a large molecule do not constitute one phosphate triester. This local counting rule helps distinguish a change in connectivity from a change in pH, protonation or total phosphorus content.