butyric acid or butanoic acid, CH₃CH₂CH₂CO₂H, viscous, foul-smelling, liquid carboxylic acid; m.p. about -5°C;; b.p. 163.5°C;. It is miscible with water, ethanol, and ether. It is a low molecular weight fatty acid that is present in butter as an ester of glycerol; the odor of rancid butter is due largely to the presence of free butyric acid. Butyric acid is used in the manufacture of plastics. Isobutyric acid, or 2-methylpropanoic acid, (CH₃)₂CHCO₂H, is a geometric isomer of the butyric acid described above; it has different physical properties but similar chemical properties.