Isobutyric acid is a colorless liquid with a light odor of rancid butter. Corrosive to metals and tissue. Isobutyric acid, also known as 2-methylpropanoic acid, is a carboxylic acid. It is found in the free state in carobs (Ceratonia siliqua), in vanilla and in the root of Arnica dulcis, and as an ethyl ester in croton oil. Isobutyric acid is an isomer of n-butyric acid; they have the same chemical formula but a different structure.