oleic acid

oleic acid, the most widely distributed of all the fatty acids, apparently occurring to some extent in all oils and fats. In oils such as olive, palm, peanut, and sunflower, it is the principal acid obtained by saponification. Oleic acid, CH₃(CH₂)₇CH=CH(CH₂)₇CO₂H, like other fatty acids, does not occur in the free state but is normally found as an ester of glycerol—i.e., as a glyceride or as an ester of a long-chain alcohol.

Oleic acid is a solid with a low melting point; two crystalline forms (α-form, melting point 13.4 °C [56 °F], and β-form, melting point 16.3 °C [61 °F]) are known. It is a long-chain carboxylic acid; its molecule contains one double bond between C₉ and C₁₀ with the cis configuration. The structure of oleic acid may be deduced from the following reactions. Treatment with selenium or oxides of nitrogen partially transforms it into the trans isomer elaidic acid. The arrangement of its 18 carbon atoms and the position of the double bond are demonstrated by its catalytic hydrogenation to stearic acid, CH₃(CH₂)₁₆CO₂H, and its oxidative cleavage to nonanoic (pelargonic) acid, CH₃(CH₂)₇CO₂H, and azelaic acid, HO₂C(CH₂)₇CO₂H. When oleic acid is heated with alkali, migration of the double bond occurs, giving the α,β-unsaturated acid CH₃(CH₂)₁₄CH=CHCO₂H, and this, on further heating with alkali, gives palmitic acid, CH₃(CH₂)₁₄CO₂H.

Oleic acid shows the standard reactions of a carboxylic acid and a disubstituted ethylene, including the formation of a dibromide with bromine and a glycol with dilute aqueous potassium permanganate.