Oleic Acid (Omega-9) (C₁₈H₃₄O₂) — Properties, Uses & Safety

Overview

Oleic acid (C18H34O2), systematically named (Z)-octadec-9-enoic acid, is a monounsaturated omega-9 fatty acid with a molar mass of 282.46 g/mol. It appears as a pale yellow oily liquid at room temperature, in contrast to the solid stearic acid from which it differs by just one double bond. This single cis double bond at the 9-position introduces a kink or bend in the otherwise straight hydrocarbon chain, preventing the molecules from packing tightly together — which is why oleic acid is a liquid while the fully saturated stearic acid is a solid. This structure-property relationship is one of the most elegant demonstrations of how a small molecular change can dramatically alter physical properties. Oleic acid is the most abundant fatty acid in human adipose tissue and the predominant fatty acid in olive oil (55–83%), avocado oil, and many nuts. The Mediterranean diet, rich in oleic acid from olive oil, has been associated with reduced cardiovascular disease risk, lower LDL cholesterol, and anti-inflammatory effects. Oleic acid is also used industrially in soap production, as an emulsifier in food processing, and as a lubricant. The cis configuration of the double bond is biologically significant — the trans isomer (elaidic acid) is associated with negative health effects, connecting oleic acid to discussions about trans fats in nutrition. Oleic acid has no significant GHS hazard classifications. For chemistry students, oleic acid beautifully illustrates cis-trans (geometric) isomerism, the effect of unsaturation on melting point and physical state, omega fatty acid nomenclature, the iodine number test for unsaturation, and the biochemistry of lipids. Comparing oleic acid with stearic acid side by side is one of the most effective ways to teach how molecular geometry influences intermolecular forces and macroscopic properties.

IUPAC Name

(Z)-octadec-9-enoic acid

CAS Number

112-80-1

PubChem CID

445639

Molecular Formula

C₁₈H₃₄O₂

Molar Mass

282.461 g/mol

State at STP

liquid

Appearance

Pale yellow oily liquid