= Beta-hydroxy acid
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 structure
$$
\mathrm{HO\! -\! CH_2\! -\! CH_2\! -\! C(=O)OH}.
$$
Starting at the carboxyl carbon, the adjacent $\mathrm{CH_2}$ is alpha and the terminal $\mathrm{CH_2OH}$ is beta. The primary ChEBI record gives the systematic name 3-hydroxypropanoic acid, formula $\mathrm{C_3H_6O_3}$ and its classification as a 3-hydroxy monocarboxylic acid.<3-hydroxypropionic acid in ChEBI>[CHEBI:33404, Definition, Formula, Names and Ontology]{ref}
An <alpha-hydroxy acid> instead places the additional <hydroxyl group> one carbon closer to the <carboxyl group>. Thus <lactic acid>, $\mathrm{CH_3CH(OH)COOH}$, and <3-hydroxypropionic acid>, $\mathrm{HOCH_2CH_2COOH}$, 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.<Lactic acid in the NIST Chemistry WebBook>[CAS 50-21-5, alpha-hydroxypropanoic-acid name]{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.
Back to article page
