butyric acid [key], CH3CH2CH2CO2H, viscous, foul-smelling, liquid carboxylic acid; m.p. about −5℃; b.p. 163.5℃. 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.
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