Glycolic acid

Glycolic acid, also called hydroxyacetic acid, is the carboxylic acid $\mathrm{HOC}{\mathrm{H}}_2\mathrm{COOH}$. It has a hydroxy group on the carbon immediately adjacent to its carboxyl carbon, making it an alpha-hydroxy acid. NIST records its neutral molecular formula as ${\mathrm{C}}_2{\mathrm{H}}_4{\mathrm{O}}_3$ and CAS number as 79-14-1.^[ref]

The two oxygen-containing groups have different connectivity. One is the alcohol-like $\mathrm{HO}-\mathrm{C}{\mathrm{H}}_2-$ group; the other is the $-\mathrm{C}(=\mathrm{O})\mathrm{OH}$ acid group. Ethylene glycol, $\mathrm{HOC}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{OH}$, instead has two alcohol groups and formula ${\mathrm{C}}_2{\mathrm{H}}_6{\mathrm{O}}_2$. The related names do not make them the same compound.

Formal loss of the carboxyl proton gives glycolate:This changes charge and hydrogen count. A glycolate salt also includes a charge-balancing counterion; its formula and molar mass should not be substituted for those of the neutral acid.

Using the recorded neutral-acid molar mass about $76.0514\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$, an original $1.00\mathrm{mmol}$ amount corresponds to about $76.05\mathrm{mg}$. Atom bookkeeping also clarifies its relation to polyglycolic acid. Forming one ester link between two glycolic-acid units with formal loss of one water molecule gives compositionFor a linear chain made from $n$ such units with $n-1$ formal water losses, the composition is ${\mathrm{C}}_{2n}{\mathrm{H}}_{2n+2}{\mathrm{O}}_{2n+1}$. This expresses ideal chain/end-group bookkeeping, not an industrial synthesis procedure.

The manufacturer's polyglycolic acid brochure gives the polymer repeat structure and identifies glycolic acid as its hydrolysis product.^[ref] A polymer repeat unit and the isolated acid molecule differ by the atoms associated with linkage formation and end groups.

Glycolate, also called hydroxyacetate, is the conjugate base of glycolic acid. Its connectivity is $\mathrm{HOC}{\mathrm{H}}_2\mathrm{CO}{\mathrm{O}}^{-}$, with molecular formula ${\mathrm{C}}_2{\mathrm{H}}_3{\mathrm{O}}_3^{-}$ and net charge $-1$. ChEBI explicitly distinguishes this anion from its neutral acid.^[ref]

Removing the carboxyl proton changes both hydrogen count and charge:The alcohol-like hydroxy group remains in the specified singly charged species. Formula bookkeeping must therefore distinguish the acid, the anion and a complete salt containing the anion.

For example, one monovalent positive counterion balances one glycolate, while a divalent positive counterion requires two. A nominal calcium glycolate formula is $\mathrm{Ca}({\mathrm{C}}_2{\mathrm{H}}_3{\mathrm{O}}_3{)}_2$. An original amount example with $1\mathrm{mmol}$ of that formula unit contains $2\mathrm{mmol}$ of glycolate units. Hydration or another salt form changes formula mass and sample composition, so the mass of the isolated anion is not a universal salt molar mass.

Acid–base equilibrium determines how much is present as glycolate. For a suitable dilute-solution model,At $\mathrm{pH}=\mathrm{p}{K}_a$ the two forms have equal modeled concentrations; one pH unit above gives a $10:1$ ratio and base fraction $10/11\simeq 0.909$. This relative example does not assign a numerical acid dissociation constant. Activities, ionic strength and other equilibria matter when the dilute approximation is inadequate.

The chemical identity is independent of whether the ion was prepared as a salt or produced by deprotonating the acid in solution. An analysis that measures total glycolic-acid-derived material must state whether its reported quantity includes both protonation states or specifically the glycolate anion.