What Is Mandelic Acid?
Mandelic acid, also known as alpha-hydroxyphenylacetic acid, phenylglycolic acid, or apricot acid, is a hydroxycarboxylic acid. It is a colorless crystal with a plate-like shape and is soluble in water, ethanol, ether, and isopropanol. Its aqueous solution exhibits acidic properties.
Mandelic acid can be produced synthetically by the reaction of benzaldehyde with hydrogen cyanide, followed by hydrolysis. It has an asymmetric carbon atom and exists as both D- and L-isomers, commonly known in the racemic form. The L-isomer is obtained by optical resolution with brucine, and the D-isomer with l-menthol. Mandelic acid is also a urinary metabolite of styrene and serves as an exposure indicator for workers handling styrene-containing organic solvents. Its name derives from the German word “Mandel,” meaning almond.
Uses of Mandelic Acid
Mandelic acid and its derivatives are used as antibacterial agents in treating urinary tract infections. It also serves as an analytical reagent, such as in the determination of zirconium, and as a precursor in the dye industry. Furthermore, it is utilized as an optical resolution agent for amines and in chiral preparations for the pharmaceutical, agrochemical, and cosmetic industries. As an α-hydroxy acid, it is also employed in skin care for keratin treatment.
Properties of Mandelic Acid
Mandelic acid is a colorless, plate-like crystal. It is naturally found in seeds, fruits, and bark of almonds and other trees. The compound contains both carboxyl and phenolic hydroxyl groups, exhibiting acid and phenolic properties. It is a chiral molecule, often produced industrially as a racemic mixture.
As a chiral intermediate, it is used in pharmaceuticals, agrochemicals, and cosmetics. Additionally, it is a urinary metabolite of both styrene and ethylbenzene. It is used as a health indicator for workers exposed to these compounds and related organic solvents.
Structure of Mandelic Acid
Mandelic acid, an aromatic α-hydroxycarboxylic acid with the chemical formula C8H8O3, consists of a benzene ring, a hydroxyl group, and an aldehyde group. It has a chiral carbon atom at the α-position and exists as two optical isomers. The racemic mixture is known as para-mandelic acid, with specific isomers obtainable through optical resolution with brucine and l-menthol.
Other Information on Mandelic Acid
How Mandelic Acid Is Produced
Naturally found in almond and apricot seeds, mandelic acid is also industrially produced through organic synthesis. Mandelonitrile is synthesized by reacting benzaldehyde with sodium cyanide and sodium hydrogen sulfite. Hydrolysis of mandelonitrile with concentrated hydrochloric acid yields mandelic acid and ammonium chloride. The separation of these compounds is typically achieved using organic solvent extraction.
Due to environmental and toxicity concerns associated with metal cyanides, alternative synthesis methods have been developed, such as using acetone cyanohydrin instead of hydrogen cyanide and oxidizing phenylglycine under an acid catalyst.