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 is alpha and the terminal is beta. The primary ChEBI record gives the systematic name 3-hydroxypropanoic acid, formula 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, , and 3-hydroxypropionic acid, , 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.
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