= Alpha-hydroxy acid
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.<Organic Chemistry by John McMurry>[Chapter 20, carboxylic-acid structure and nomenclature]{ref}
For a common saturated monocarboxylic example, the local structure can be written
$$
\mathrm{R\! -\! C(OH)(R')\! -\! C(=O)OH},
$$
with $R,R'$ representing hydrogen or carbon-containing substituents. <Glycolic acid>, $\mathrm{HOCH_2COOH}$, has two hydrogens at that carbon. <Lactic acid>, $\mathrm{CH_3CH(OH)COOH}$, has hydrogen and methyl groups there and is also called 2-hydroxypropanoic acid.<Glycolic acid in the NIST Chemistry WebBook>[Identity and structural record]{ref}<Lactic acid in the NIST Chemistry WebBook>[Identity and Other Names]{ref}
Position distinguishes the class. The structure $\mathrm{HOCH_2CH_2COOH}$ 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.
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