Alpha hydroxy carboxylic acids, or α-hydroxy carboxylic acids (AHAs), are a group of carboxylic acids featuring a hydroxy group located one carbon atom away from the acid group. This structural aspect distinguishes them from beta hydroxy acids, where the functional groups are separated by two carbon atoms. Notable AHAs include glycolic acid, lactic acid, mandelic acid, and citric acid.

α-Hydroxy acids are stronger acids compared to their non-alpha hydroxy counterparts, a property enhanced by internal hydrogen bonding. AHAs serve a dual purpose: industrially, they are utilized as additives in animal feed and as precursors for polymer synthesis. In cosmetics, they are commonly used for their ability to chemically exfoliate the skin.

Aldonic acids, a type of sugar acid, are a class of naturally occurring hydroxycarboxylic acids. They have the general chemical formula, HO₂C(CHOH)_nCH₂OH. Gluconic acid, a particularly common aldonic acid, the oxidized derivative of glucose.

2-Hydroxy-4-(methylthio)butyric acid is an intermediate in the biosynthesis of 3-dimethylsulfoniopropionate, precursor to natural dimethyl sulfide.

One common synthesis route involves the hydrolysis of α-halocarboxylic acids, readily available precursors, to produce 2-hydroxycarboxylic acids. For instance, the production of glycolic acid typically follows this method, utilizing a base-induced reaction, followed by acid workup. Similarly, unsaturated acids and fumarate and maleate esters undergo hydration to yield malic acid derivatives from esters, and 3-hydroxypropionic acid from acrylic acid.

Another synthetic pathway for α-hydroxy acids involves the addition of hydrogen cyanide to ketones or aldehydes, followed by the acidic hydrolysis of the cyanohydrin intermediate.

Furthermore, specialized synthetic routes include the reaction of dilithiated carboxylic acids with oxygen, followed by aqueous workup.

Additionally, α-keto aldehydes can be transformed into α-hydroxy acids through the Cannizzaro reaction.