​MANDELIC ACID (Phenylglycollic Acid), C₈H₈O₃ or C₆H₅·CH(OH)·COOH, an isomer of the cresotinic and the oxymethylbenzoic acids. Since the molecule contains an asymmetric carbon atom, the acid exists in three forms, one being an inactive “ racemic ” mixture, and the other two being optically active forms. The inactive variety is known as paramandelic acid. It may be prepared by the action of hydrochloric acid on the addition compound of benzaldehyde and hydrocyanic acid:—
C₆H₅CHO + HCN + HCl + 2H₂O = C₆H₅·CHOH·COOH + NH₄Cl,
(F. L. Winckler, Ann., 1836, 18, 310), by boiling phenylchloracetic acid with alkalis (A. Spiegel, Ber., 1881, 14, 239), by heating benzoylformaldehyde with alkalis (H. v. Pechmann, Ber., 1887, zo, 2905), and by the action of dilute alkalies on w-dibromacetophenone (C. Engler, Ber., 1887, 20, 2202):—
C₆H₅COCHBr₂ + 3KHO = 2KBr + H₂O + C₆H₅·CHOH·CO₂K.

It crystallizes from water in large rhombic crystals, which melt at 116º C. Oxidizing agents convert it into benzaldehyde. When heated with hydriodic acid and phosphorus it forms phenylacetic acid; whilst concentrated hydrobromic acid and hydrochloric acid at moderate temperatures convert it into phenylbrom- and phenylchlor-acetic acids. The inactive mixture may be resolved into its active components by fractional crystallization of the cinchonine salt, when the salt of the dextro modification separates first; or the ammonium salt may be fermented by Penicillium glaucum, when the laevo form is destroyed and the dextro form remains untouched; on the other hand, Saccharomyces ellipsoïdeus destroys the dextro form, but does not touch the laevo form. A mixture of the two forms in equivalent quantities produces the inactive variety, which is also obtained when either form is heated for some hours to 160° C.