BUTYRIC ACID, C4H8O2. Two acids are known corresponding to this formula, normal butyric acid, CH3·CH2·CH2·COOH, and isobutyric acid, (CH3)2·CH·COOH. Normal butyric acid or fermentation butyric acid is found in butter, as an hexyl ester in the oil of Heracleum giganteum and as an octyl ester in parsnip (Pastinaca sativa); it has also been noticed in the fluids of the flesh and in perspiration. It may be prepared by the hydrolysis of ethyl acetoacetate, or by passing carbon monoxide over a mixture of sodium acetate and sodium ethylate at 205° C. (A. Geuther, Ann., 1880, 202, p.306), C2H5ONa + CH3COONa + CO = H·CO2Na + CH3·CH2·CH2·COONa. It is ordinarily prepared by the fermentation of sugar or starch, brought about by the addition of putrefying cheese, calcium carbonate being added to neutralize the acids formed in the process. A. Fitz (Ber., 1878, 11 p. 52) found that the butyric fermentation of starch is aided by the direct addition of Bacillus subtilis. The acid is an oily liquid of unpleasant smell, and solidifies at -19° C.; it boils at 162.3° C., and has a specific gravity of 0.9746 (0° C.). It is easily soluble in water and alcohol, and is thrown out of its aqueous solution by the addition of calcium chloride. Potassium bichromate and sulphuric acid oxidize it to carbon dioxide and acetic acid, while alkaline potassium permanganate oxidizes it to carbon dioxide. The calcium salt, Ca(C4H7O2)2·H2O, is less soluble in hot water than in cold.
Isobutyric acid is found in the free state in carobs (Ceratonia siliqua) and in the root of Arnica dulcis, and as an ethyl ester in croton oil. It may be artificially prepared by the hydrolysis of isopropylcyanide with alkalies, by the oxidation of isopropyl alcohol with potassium bichromate and sulphuric acid (I. Pierre and E. Puchot, Ann. de chim. et de phys., 1873,  28, p. 366), or by the action of sodium amalgam on methacrylic acid, CH2·C(CH3)·COOH. It is a liquid of somewhat unpleasant smell, boiling at 155.5° C. Its specific gravity is 0.9697 (0°). Heated with chromic acid solution to 140° C., it gives carbon dioxide and acetone. Alkaline potassium permanganate oxidizes it to α-oxyisobutyric acid, (CH3)2·C(OH)·COOH, whilst concentrated nitric acid converts it into dinitroisopropane. Its salts are more soluble in water than those of the normal acid.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)