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An alpha-hydroxy acid is a carboxylic acid with a hydroxyl group on the carbon immediately adjacent to the carboxyl group. That neighboring carbon is the alpha carbon; in simple systematic numbering it is carbon 2 because the carboxyl carbon is carbon 1. The hydroxyl oxygen within the carboxyl group itself is not the additional hydroxyl that defines this class.^[ref]

For a common saturated monocarboxylic example, the local structure can be writtenwith $R,R^{\prime }$ representing hydrogen or carbon-containing substituents. Glycolic acid, $\mathrm{HOC}{\mathrm{H}}_2\mathrm{COOH}$, has two hydrogens at that carbon. Lactic acid, $\mathrm{C}{\mathrm{H}}_3\mathrm{CH}(\mathrm{OH})\mathrm{COOH}$, has hydrogen and methyl groups there and is also called 2-hydroxypropanoic acid.^[ref][ref]

Position distinguishes the class. The structure $\mathrm{HOC}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{COOH}$ has its additional hydroxyl group at carbon 3 and is a beta-hydroxy acid, even though it has the same molecular formula as lactic acid. A molecular formula therefore does not determine whether a compound is an alpha-hydroxy acid; the connectivity is essential.

The alpha carbon can also be a stereogenic centre. In lactic acid it is bonded to four different groups: hydrogen, hydroxyl, methyl and carboxyl. Its two enantiomers share the alpha-hydroxy classification but differ in three-dimensional arrangement. Glycolic acid does not have that stereogenic centre because two substituents are identical hydrogens.

Both functional groups participate in the chemistry. Loss of the carboxyl proton gives a carboxylate, such as lactate from lactic acid, while the additional hydroxyl remains part of the structure. Distinguishing group identity, position, protonation and stereochemistry makes the class useful in organic and biological chemistry without reducing it to a list of commercial uses.

A beta-hydroxy acid is a carboxylic acid with a hydroxyl group on the beta carbon: the carbon two carbon–carbon bonds away from the carboxyl group carbon. For a simple chain numbered from the carboxyl carbon as carbon 1, the beta carbon is carbon 3. The hydroxyl within the carboxyl group itself does not establish the additional hydroxy substitution.

The small example 3-hydroxypropionic acid has structureStarting at the carboxyl carbon, the adjacent $\mathrm{C}{\mathrm{H}}_2$ is alpha and the terminal $\mathrm{C}{\mathrm{H}}_2\mathrm{OH}$ is beta. The primary ChEBI record gives the systematic name 3-hydroxypropanoic acid, formula ${\mathrm{C}}_3{\mathrm{H}}_6{\mathrm{O}}_3$ and its classification as a 3-hydroxy monocarboxylic acid.^[ref]

An alpha-hydroxy acid instead places the additional hydroxyl group one carbon closer to the carboxyl group. Thus lactic acid, $\mathrm{C}{\mathrm{H}}_3\mathrm{CH}(\mathrm{OH})\mathrm{COOH}$, and 3-hydroxypropionic acid, $\mathrm{HOC}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{COOH}$, have the same molecular formula but different atom connectivity. They are constitutional isomers; counting oxygen atoms or measuring molecular mass alone cannot distinguish them. The distinction is structural, not a statement about acidity or biological activity.^[ref]

The two functional groups also describe different possible transformations. Removing the carboxyl proton forms a carboxylate while preserving the beta-hydroxy carbon skeleton. Converting the hydroxyl group to a carbonyl changes the hydroxy-acid functionality. Which transformation occurs depends on reagents and conditions; the class name alone supplies no reaction rate or treatment claim. Tracking positions relative to the carboxyl group makes such structural changes easier to compare.

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.