n-Butyric acid, IUPAC name n-Butanoic acid, or normal butyric acid, is a carboxylic acid with structural formula CH₃CH₂CH₂-COOH. It is notably found in rancid butter, parmesan cheese, or vomit and has an unpleasant odor and acrid taste, with a sweetish aftertaste (similar to ether).

Butyric acid can be detected by mammals with good scent detection abilities (e.g. dogs) at 10 ppb, while humans can detect it in concentrations above 10 ppm.

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. 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.

There is an isomer, isobutyric acid, which has the same chemical formula C₄H₈ O₂ but a different structure. It has similar chemical properties but different physical properties.

This article incorporates information from the 1911 encyclopedia.