Oleic acid

C 18
H 34
O 2

Ball-and-stick Stick Space-filling

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54 atoms, 53 bonds.

IUPAC name: (Z)-octadec-9-enoic acid
Molar mass: 282.468(45) g/mol
Atoms: 54
SMILES: CCCCCCCCC=CCCCCCCCC(=O)O
InChIKey: ZQPPMHVWECSIRJ-KTKRTIGZSA-N
PubChem CID: 445639

Made of

C H O

Structural formula of oleic acid. Each unlabelled corner is a carbon atom, and the 30 hydrogens on carbon are not drawn. It shows which atoms are joined to which, not the shape. The model above shows the shape.

Oleic acid carries one carbon–carbon double bond, at the ninth of its 18 carbons. ChEBI (Chemical Entities of Biological Interest) gives that bond the Z, or cis, arrangement, and both details are written into the IUPAC name, (Z)-octadec-9-enoic acid. A carboxyl group, –COOH, closes one end of the chain, and the double bond makes it an unsaturated fatty acid. Eighteen carbon, 34 hydrogen and two oxygen atoms give a molar mass of 282.5 g/mol.

The acid is a colourless to pale yellow liquid with a mild smell, which HSDB, the Hazardous Substances Data Bank, likens to lard. HSDB puts its melting point at 16.3 °C, and the International Chemical Safety Card gives 13.4 °C. Lighter than water, it floats on it rather than mixing in. HSDB calls it practically insoluble in water, while alcohol, ether, chloroform and oils dissolve it. The safety card classes it as a weak acid.

Structure and bonding

Oleic acid is one unbranched chain of 54 atoms: 18 carbon, 34 hydrogen and two oxygen. Stick draws two bonds as doubled rods: a C=O at the acid end, beside an O–H, and a C=C between the ninth and tenth carbons. The model draws the neutral acid; ChEBI calls it the conjugate acid of oleate, the ion a base leaves when it strips the –COOH hydrogen.

At that C=C the chain changes direction, the carbons beside it leaving on the same side. In Ball-and-stick the four carbons around the C=C lie almost flat, with angles near 128° at the doubly bonded pair in PubChem’s computed conformer, a calculated value. This is the cis form of cis–trans isomerism: on Chem1’s account the two halves are locked on one side (the reason is under propylene). The chain’s other folds are one computed pose (propanol shows why).

In Space-filling, the outline keeps an elbow at the double bond. Chains kinked like this cannot pack closely enough to attract one another strongly, which on Chem1’s account is why unsaturated fatty acids tend to be liquids. Saturated ones tend to be waxy solids, a contrast taken up under palmitic acid.

Where it turns up

MeSH, the Medical Subject Headings vocabulary, ranks oleic acid as the fatty acid most widespread and most plentiful in nature. Olive oil is rich in it. So are high-oleic sunflower, safflower, canola and algal oils; a health-claim petition the US Food and Drug Administration (FDA) reviewed in 2018 named all five. ChEBI records it as a metabolite of mice, plants and the bacterium Escherichia coli, meaning their metabolism produces it. In higher animals it forms through an oxygen-using enzyme system in the endoplasmic reticulum, a membrane network inside their cells.

Commercial oleic acid is split with water from animal and vegetable fats and oils, tallow among them (the general reaction is under glycerol), and refined by distillation and crystallisation. Once filtered, the acid of commerce goes by the name red oil. It is a mixture: HSDB puts commercial grades at 67 to 80 per cent oleic acid.

Why it matters

Soaps and detergents, turkey red oil and fabric waterproofing all use oleic acid. So do polishing compounds, wool oiling, ore flotation and defoaming. It is also made into oleates and lotions and used as a pharmaceutical solvent. The US Environmental Protection Agency (EPA) classes it among surfactants. The oleate ion in its salts fits Chem1’s description of one: a hydrocarbon with an ionic group at one end, which water attracts strongly. At a surface such molecules gather with their hydrophobic, water-shunning ends pointing out.

Oleic acid in edible oils is the subject of a qualified health claim. The FDA completed its review of a petition for that claim on 19 November 2018. The agency found credible evidence that, used in place of fats and oils richer in saturated fat, it may lower the risk of coronary heart disease. The FDA’s wording calls the evidence supportive but not conclusive. A qualified claim rests on credible science without meeting the stricter standard of significant scientific agreement that an authorised FDA health claim needs.

Properties

Properties of oleic acid Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 445639.
Property	Value	Unit	Source
Molecular formula (Hill order)	C₁₈H₃₄O₂
Molar mass	282.468(45) g/mol	g/mol	IUPAC/CIAAW 2024
Carbon atoms	18
Hydrogen atoms	34
Oxygen atoms	2
Bonds	53
Single bonds	51
Double bonds	2
PubChem molecular weight	282.5 g/mol	g/mol	PubChem

Safety

Oleic acid is an ordinary part of food and a permitted food additive, and the EPA’s Safer Choice programme rates it of low concern with a green circle. In 65.4 per cent of reports under GHS, the Globally Harmonized System of classification, it meets no hazard criteria. Other reports warn that it causes skin irritation and serious eye irritation. The PubChem Laboratory Chemical Safety Summary for oleic acid also lists it as combustible.

Questions

Is oleic acid cis or trans?

Cis. ChEBI gives the double bond at C-9 the Z, or cis, configuration. That Z is the letter in brackets at the front of the IUPAC name, (Z)-octadec-9-enoic acid. In the 3D model, the carbons on either side of that bond point the same way and the chain bends there. On Chem1's account, the same arrangement holds for most of the unsaturated fatty acids that occur in nature.

Which foods contain oleic acid?

Oleic acid occurs naturally in olives, avocados, nuts and sunflower seeds, according to the US Food and Drug Administration (FDA). Edible oils, milk, cheese, eggs and pasta contain it too, as do meats such as beef, chicken and pork. It is also a permitted direct food additive in the United States, serving as a flavouring agent and as a lubricant or release agent.

Does oleic acid go rancid?

It can. Exposed to air, oleic acid oxidises, and more readily when impure, according to the Hazardous Substances Data Bank. It darkens through yellow to brown and picks up a rancid smell. CAMEO Chemicals, a US hazardous-materials database, files it as a peroxidisable compound, one that may build up peroxides once air reaches it.

Is oleic acid used in soap?

Yes: soaps and detergents are among its uses, and with an alkali the acid forms soap. Many carboxylic acids that do not dissolve in water go into a basic solution because neutralisation turns them into a soluble salt, as CAMEO Chemicals describes. For oleic acid that salt is an oleate, built on the oleate ion left once the –COOH hydrogen has gone. Calcium and heavy-metal oleates are the exception: they do not dissolve.

Made of the same elements

C₁₆H₃₂O₂ Palmitic acid 50 atoms, 49 bonds.
C₂₁H₃₀O₂ Progesterone 53 atoms, 56 bonds.
C₁₉H₂₈O₂ Testosterone 49 atoms, 52 bonds.
C₂₁H₃₀O₅ Cortisol 56 atoms, 59 bonds.
C₁₈H₂₄O₂ Estradiol 44 atoms, 47 bonds.
C₂₇H₄₄O Cholecalciferol 72 atoms, 74 bonds.
C₁₅H₁₆O₂ Bisphenol A 33 atoms, 34 bonds.
C₁₃H₁₈O₂ Ibuprofen 33 atoms, 33 bonds.

All 51 compounds made of carbon, hydrogen and oxygen

C₂₇H₄₆O Cholesterol 74 atoms, 77 bonds.
C₁₀H₂₀O Menthol 31 atoms, 31 single bonds.
C₂₀H₁₄O₄ Phenolphthalein 38 atoms, 41 bonds.
C₁₄H₁₄O₃ Naproxen 31 atoms, 32 bonds.
C₁₂H₂₂O₁₁ Lactose 45 atoms, 46 single bonds.
C₁₂H₂₂O₁₁ Sucrose 45 atoms, 46 single bonds.
C₉H₈O₄ Aspirin 21 atoms, 21 bonds.
C₈H₈O₃ Methyl salicylate 19 atoms, 19 bonds.
C₈H₈O₃ Vanillin 19 atoms, 19 bonds.
C₆H₁₂O₆ Fructose 24 atoms, 24 single bonds.
C₆H₁₂O₆ Glucose 24 atoms, 24 single bonds.
C₄H₉OH 1-Butanol 15 atoms, 14 single bonds.
C₆H₅COOH Benzoic acid 15 atoms, 15 bonds.
C₂H₅OC₂H₅ Diethyl ether 15 atoms, 14 single bonds.
C₆H₅CHO Benzaldehyde 14 atoms, 14 bonds.
C₃H₇COOH Butyric acid 14 atoms, 13 bonds.
CH₃COOC₂H₅ Ethyl acetate 14 atoms, 13 bonds.
C₇H₆O₃ Salicylic acid 16 atoms, 16 bonds.
C₄H₈O Butanone 13 atoms, 12 bonds.
C₆H₅OH Phenol 13 atoms, 13 bonds.
C₃H₈O₂ Propylene glycol 13 atoms, 12 single bonds.
C₃H₇OH 1-Propanol 12 atoms, 11 single bonds.
C₆H₈O₆ Ascorbic acid 20 atoms, 20 bonds.
C₃H₈O₃ Glycerol 14 atoms, 13 single bonds.
C₃H₈O Isopropyl alcohol 12 atoms, 11 single bonds.
C₆H₈O₇ Citric acid 21 atoms, 20 bonds.
C₂H₅COOH Propionic acid 11 atoms, 10 bonds.
C₃H₆O Acetone 10 atoms, 9 bonds.
C₂H₆O₂ Ethylene glycol 10 atoms, 9 single bonds.
C₃H₆O₃ Lactic acid 12 atoms, 11 bonds.
CH₃OCH₃ Dimethyl ether 9 atoms, 8 single bonds.
C₂H₅OH Ethanol 9 atoms, 8 single bonds.
C₄H₆O₅ Malic acid 15 atoms, 14 bonds.
CH₃COOH Acetic acid 8 atoms, 7 bonds.
C₄H₆O₆ Tartaric acid 16 atoms, 15 bonds.
CH₃CHO Acetaldehyde 7 atoms, 6 bonds.
C₂H₄O Ethylene oxide 7 atoms, 7 single bonds.
C₂H₄O₃ Glycolic acid 9 atoms, 8 bonds.
CH₃OH Methanol 6 atoms, 5 single bonds.
HCOOH Formic acid 5 atoms, 4 bonds.
H₂CO₃ Carbonic acid 6 atoms, 5 bonds.
CH₂O Formaldehyde 4 atoms, 3 bonds.
H₂C₂O₄ Oxalic acid 8 atoms, 7 bonds.