LACTIC

ACID (hydroxypropionic acid), C3H603  . Two lactic acids are known, differing from each other in the position occupied by the hydroxyl group in the molecule; they are known respectively as a-hydroxypropionic acid (fermentation or inactivelactic acid), CH3• CH(OH) . CO2H, and/3-hydroxypropionic acid (hydracrylic acid), (q.v.), CH2(OH)•CH2•CO2H . Although on structural grounds there should be only two hydroxypropionic acids, as a matter of fact four lactic acids are known . The third isomer (sarcolactic acid) is found in meat extract (J. v . Liebig), and may be prepared by the action of Penicillium glaucum on a solution of ordinary ammonium lactate . It is identical with a-hydroxypropionic acid in almost every respect, except with regard to its physical properties . The fourth isomer, formed by the action of Bacillus laevo-lacti on cane-sugar, resembles sarcolactic acid in every respect, except in its action on polarized light (see STEREOISOMERISM) . Fermentation, or ethylidene lactic acid, was isolated by K . W . Scheele (Trans . Stockholm Acad .

178o) from sour

milk (Lat. lac,lactis, milk, whence the name) . About twenty-four years later Bouillon Lag-range, and independently A . F. de Fourcroy and L . N . Vauquelin, maintained that Scheele's new acid was nothing but impure acetic acid . This notion was combated by J . Berzelius, and finally refuted (in 1832) by J. v . Liebig and E . Mitscherlich, who, by the elementary analyses of lactates, proved the existence of this acid as a distinct compound . It may be prepared by the lactic fermentation of starches, sugars, gums, &c., the sugar being dissolved in water and acidified by a small quantity of tartaric acid and then fermented by the addition of sour milk, with a little putrid cheese . Zinc carbonate is added to the mixture (to neutralize the acid formed), which is kept warm for some days and well stirred . On boiling and filtering the product, zinc lactate crystallizes out of the solution .

The acid may also be synthesized by the decomposition of alanine (a-aminopropionic acid) by nitrous acid (K . Strecker,

Ann., 185o, 75, p . 27); by the oxidation of propylene glycol (A . Wurtz) ; by boiling a-chlorpropionic acid with caustic alkalis, or with silver oxide and water; by the reduction of pyruvic acid with sodium amalgam; or from acetaldehyde by the cyanhydrin reaction (J . Wislicenus, Ann., 1863, I28, p . 13) CH3•CHO --i CH3.CH(OH).CN —~ CH3•CH(OH)•CO2H.decomposes on distillation under ordinary atmospheric pressure; but at very low pressures (about i mm.) it distils at about 85° C., and then sets to a crystalline solid, which melts at about 18° C . It possesses the properties both of an acid and of an alcohol . When heated with dilute sulphuric acid to 13o° C., under pressure, it is resolved into formic acid and acetaldehyde . Chromic acid oxidizes it to acetic acid and carbon dioxide; potassium permanganate oxidizes it to pyruvic acid; nitric acid to oxalic acid, and a mixture of manganese dioxide and sulphuric acid to acetaldehyde and carbon dioxide . Hydrobromic acid converts it into a-brompropionic acid, and hydriodic acid into propionic acid . Lactide, O< CH(CHs)•CO>O a crystalline solid, of melting-point CO•CH(CH3) 124° C., is one of the products obtained by the distillation of lactic acid .