Normal butyric acid or fermentation butyric acid is found in butter, as an hexyl ester in the oil of Heracleum giganteum (cow parsnip) 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.
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, II, p. 52) found that the butyric fermentation of starch is aided by the direct addition of Bacillus subtilis. The acid is an oily colorless liquid that solidifies at -5° C; it boils at 164° C. It is easily soluble in water, ethanol and ether, and is thrown out of its aqueous solution by the addition of calcium chloride. Potassium bichromate and sulfuric 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, strucural formula (CH3)2-CH-COOH and also known as 2-methylpropanoic 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. Isobutyric acid, compared to n-butyric acid, has different physical properties but similar chemical properties. It may be artificially prepared by the hydrolysis of isopropylcyanide with alkalies, by the oxidation of isopropyl alcohol with potassium bichromate and sulfuric acid (I. Pierre and E. Puchot, Ann. de chim. et de pliys., 1873, 28, p. 366), or by the action of sodium amalgam on methacrylic acid. It is a liquid of somewhat unpleasant smell, boiling at ‘55.5° 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 a-oxyisobutyric acid, (CH3)2-C(OH)-COOH, whilst concentrated nitric acid converts it into dinitroisopropanc. Its salts are more soluble in water than those of the normal acid.
This article incorporates information from the 1911 encyclopedia.