BUTYRIC ACID is the first acid in the acetic acid series to show structural isomerism (q.v.). There are two acids of the formula Normal butyric acid or fermentation butyric acid is found in butter, as an hexyl ester in the oil of Heracleum giganteum, as an octyl ester in parsnip (Pastinaca sativa), and in the oil of Eucalyptus Perriniana as n-butyl butyrate ; it has also been noticed in meat juice, in perspiration and in excrementa. The acid is an oily liquid of unpleasant smell, solidifies at —19° C., and boils at 162.3° C. It is easily soluble in water and alcohol, and is thrown out of aqueous solution by calcium chloride. The calcium salt, is less soluble in hot water than in cold. It may be synthesised by hydrolysing ethylacetoacetic ester (see CHEMISTRY : Organic, [I] Aliphatic). 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 (b. 18i 8) found that the butyric fermentation of starch is aided by the direct addition of Bacillus subtilis (hay bacillus).
Isobutyric acid, (CH3)2•CH•COOH, 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 is a liquid of somewhat unpleasant smell, boiling at 155.5° C. It may be artificially prepared by the hydrolysis of isopropylcyanide with alkalis, by the oxidation of isobutyl alcohol with potassium bichromate and sulphuric acid (I. Pierre and E. Puchot, 18i3), or by hydrolysing dimethyla cetoacetic ester (cf. supra). Alkaline potassium permanganate oxidizes it to a-hydroxyisobutyric acid, whilst concentrated nitric acid converts it into dinitroisopropane. Its salts are more soluble in water than those of the normal acid, the calcium salt differing from its normal isomeride in being more soluble in hot than in cold water.