Linoleic acid

Key takeaways

Linoleic acid belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Linoleic acid to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Linoleic acid from memory before moving on to harder problems.

Reference excerpt

Linoleic acid (LA) is an organic compound with the formula CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH. Both alkene groups (−CH=CH−) are cis. It is a fatty acid sometimes denoted 18:2 (n−6) or 18:2 cis-9,12. A linoleate is a salt or ester of this acid. Linoleic acid is a polyunsaturated, omega−6 fatty acid. It is a colorless liquid that is virtually insoluble in water but soluble in many organic solvents. It typically occurs in nature as a triglyceride (ester of glycerin) rather than as a free fatty acid. It is one of two essential fatty acids for humans, who must obtain it through their diet, and the most essential, because the body uses it as a base to make the others. The word "linoleic" derives from Latin linum 'flax' and oleum 'oil', reflecting the fact that it was first isolated from linseed oil.

History In 1844, F. Sacc, working at the laboratory of Justus von Liebig, isolated linoleic acid from linseed oil. In 1886, K. Peters determined the existence of two double bonds. Its essential role in human diet was discovered by G. O. Burr and others in 1930. Its chemical structure was determined by T. P. Hilditch and others in 1939, and it was synthesized by R. A. Raphael and F. Sondheimer in 1950.

In physiology

The consumption of linoleic acid is vital to proper health, as it is an essential fatty acid.

Metabolism and eicosanoids

Linoleic acid (LA: C18H32O2; 18:2,n−6) is a precursor to arachidonic acid (AA: C20H32O2; 20:4,n−6) with elongation and unsaturation. AA is the precursor to some prostaglandins, leukotrienes (LTA, LTB, LTC), thromboxane (TXA) and the N-acylethanolamine (NAE) arachidonoylethanolamine (AEA: C22H37NO2; 20:4,n−6), and other endocannabinoids and eicosanoids. The metabolism of LA to AA begins with the conversion of LA into gamma-linolenic acid (GLA), effected by Δ6 desaturase. GLA is converted to dihomo-γ-linolenic acid (DGLA), the immediate precursor to AA. LA is also converted by various lipoxygenases, cyclooxygenases, cytochrome P450 enzymes (the CYP monooxygenases), and non-enzymatic autoxidation mechanisms to mono-hydroxyl products viz., 13-Hydroxyoctadecadienoic acid, and 9-Hydroxyoctadecadienoic acid; these two hydroxy metabolites are enzymatically oxidized to their keto metabolites, 13-oxo-octadecadienoic acid and 9-oxo-octadecdienoic acid. Certain cytochrome P450 enzymes, the CYP epoxygenases, catalyze oxidation of LA to epoxide products viz., its 12,13-epoxide, vernolic acid, and its 9,10-epoxide, coronaric acid. These linoleic acid products are implicated in human physiology and pathology. Hydroperoxides derived from the metabolism of anandamide (AEA: C22H37NO2; 20:4,n−6), or its linoleoyl analogues, are by a lipoxygenase action found to be competitive inhibitors of brain and immune cell FAAH, the enzyme that breaks down AEA and other endocannabinoids, and the compound linoleoyl-ethanol-amide (C20H37NO2; 18:2,n−6), an N-acylethanolamine, - the ethanolamide of linoleic acid (LA: C18H32O2; 18:2,n−6) and its metabolized incorporated ethanolamine (MEA: C2H7NO), is the first natural inhibitor of FAAH, discovered.

Uses and reactions Linoleic acid is a component of quick-drying oils, which are useful in oil paints and varnishes. These applications exploit the lability of the doubly allylic C−H groups (−CH=CH−CH2−CH=CH−) toward oxygen in air (autoxidation). Addition of oxygen leads to crosslinking and formation of a stable film. Reduction of the carboxylic acid group of linoleic acid yields linoleyl alcohol. Linoleic acid is a surfactant with a critical micelle concentration of 1.5×10−4 M at pH 7.5. Linoleic acid has become increasingly popular in the beauty products industry because of its beneficial properties on the skin. Research points to linoleic acid's anti-inflammatory, acne reductive, skin-lightening and moisture retentive properties when applied topically on the skin. Linoleic acid is also used in some bar of soap products.

Dietary sources

It is abundant in safflower, and corn oil, and comprises over half their composition by weight. It is present in medium quantities in soybean oils, sesame, and almonds.

Other occurrences Cockroaches release oleic and linoleic acid upon death, which discourages other roaches from entering the area. This is similar to the mechanism found in ants and bees, which release oleic acid upon death.

Related compounds While polyunsaturated fatty acids are unusual in plant cuticles, a diunsaturated dicarboxylic acid has been reported as a component of the surface waxes or polyesters of some plant species. Thus, octadeca-c6,c9-diene-1,18-dioate, a derivative of linoleic acid, is present in Arabidopsis and Brassica napus cuticle. Taxoleic acid is isomeric to linoleic acid.

Health effects Consumption of linoleic acid has been associated with lowering the risk of cardiovascular disease, diabetes and premature death. There is high-quality evidence that increased intake of linoleic acid decreases total blood cholesterol and low-density lipoprotein. Higher in vivo circulating and tissue levels of linoleic acid are associated with a lower risk of major cardiovascular events. Clinical trials have shown that increased linoleic acid intake does not increase markers of inflammation or oxidative stress. The American Heart Association advises people to replace saturated fat with linoleic acid to reduce CVD risk.

See also Conjugated linoleic acid Essential fatty acid interactions Essential nutrients Fulgidic acid Laballenic acid Linolein

References

Further reading "Compound Summary: Linoleic acid". PubChem. U.S. National Library of Medicine.

External links Linoleic acid MS Spectrum Fatty Acids: Methylene-Interrupted Double Bonds, AOCS Lipid Library

Illustrations

Worked examples

Example 1 — a first encounter with Linoleic acid

Start with the simplest possible case. Write down what Linoleic acid claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Linoleic acid before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Linoleic acid ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Linoleic acid

In research: Linoleic acid appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Linoleic acid in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Linoleic acid is common in secondary-school and first-year university syllabi. It links to neighbouring topics 5α-Reductase inhibitors, Alkenoic acids, Aromatase inhibitors, so understanding it makes those chapters shorter.
In everyday life: Look for Linoleic acid outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Linoleic acid in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Linoleic acid means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Linoleic acid out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Linoleic acid in simple terms?

Linoleic acid (LA) is an organic compound with the formula CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH. Both alkene groups (−CH=CH−) are cis.

Why does Linoleic acid matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Linoleic acid?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Linoleic acid.