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Polyunsaturated fatty acid

A polyunsaturated fatty acid is a fatty acid with two or more carbon-carbon double bonds. In Organic Chemistry, those double bonds are the sites where radicals can add and where oxidation can spread.

Last updated July 2026•Fiveable Content Team

What is polyunsaturated fatty acid?

A polyunsaturated fatty acid, or PUFA, is a fatty acid chain that contains two or more carbon-carbon double bonds, usually in the cis form. In Organic Chemistry, that means the molecule has multiple alkene-like regions that can react differently from a fully saturated chain. The double bonds change the shape of the chain, add reactivity, and create specific weak spots for radical chemistry.

The big idea is that PUFAs are not just “fats with more double bonds.” Their geometry matters. Each cis double bond bends the chain, so PUFAs pack less tightly than saturated fatty acids. That looser packing affects physical properties like melting point, but in this course the more useful point is chemical reactivity. A chain with several double bonds can undergo radical addition, hydrogen abstraction, and oxidation much more easily than a saturated chain.

That reactivity shows up clearly in biological radical chemistry. In the prostaglandin pathway, arachidonic acid is a 20-carbon PUFA with four cis double bonds. Those double bonds give enzymes a place to generate a carbon-centered radical, move it through the chain, and form new bonds by cyclization and oxygen addition. So when you see a PUFA in a mechanism, you should think “radical-sensitive alkene system,” not just “energy storage molecule.”

PUFAs are also prone to lipid peroxidation. A radical can remove a hydrogen next to a double bond, forming a pentadienyl radical that is stabilized by resonance. That stability makes the process easier to continue, which is why oxidation can spread across membrane lipids. In other words, the same conjugation-like electron distribution that makes the radical intermediate more stable also makes the molecule more vulnerable.

A common mistake is to treat every unsaturated fat the same. Monounsaturated fatty acids have one double bond, so they have fewer reactive sites and a different oxidation pattern. PUFAs have several double bonds, which makes them the main players in the biological radical addition chemistry you meet in this unit.

Why polyunsaturated fatty acid matters in Organic Chemistry

Polyunsaturated fatty acid is the starting point for the radical chemistry in this topic. If you can recognize a PUFA, you can predict where radical addition will happen, why the chain is easier to oxidize, and why certain biological products form from it instead of from a saturated fatty acid.

This term also connects structure to behavior. The number and placement of double bonds affect chain flexibility, membrane packing, and susceptibility to oxidation. That is exactly the kind of structure-to-reactivity thinking organic chemistry asks for, especially when a mechanism begins with a hydrogen abstraction and then moves through a resonance-stabilized carbon-centered radical.

It also helps you make sense of real biomolecules. Arachidonic acid is a classic PUFA in this unit because it feeds into prostaglandin synthesis. If you know what a PUFA is, the pathway stops looking random and starts looking like a controlled reaction sequence built from alkene radical chemistry.

How polyunsaturated fatty acid connects across the course

Arachidonic Acid

Arachidonic acid is a specific polyunsaturated fatty acid that shows up directly in the prostaglandin pathway. It is the example that makes the term concrete, because its four double bonds give the enzyme a chain that can undergo radical initiation, rearrangement, and cyclization. When you see arachidonic acid, think of a PUFA ready for radical chemistry.

Free Radical

PUFAs react the way they do because radicals can add to or abstract from their double bonds and adjacent hydrogens. In this unit, the radical is the active species that starts the chain of events. The fatty acid itself is not the radical, but its multiple alkenes make it a good target for radical-driven transformation.

Pentadienyl Radical

A pentadienyl radical often forms when a hydrogen is removed next to more than one double bond. That intermediate is resonance-stabilized, which is why PUFAs can undergo oxidation so readily. If you can spot a pentadienyl radical in a mechanism, you are usually looking at a PUFA-derived pathway or lipid peroxidation step.

Prostaglandin H2

Prostaglandin H2 is one of the main products formed when a PUFA like arachidonic acid goes through enzyme-mediated radical addition and cyclization. It is the payoff molecule in the pathway, so the PUFA is the substrate and PGH2 is the transformed product. That relationship is a good way to track what the reaction is doing overall.

Is polyunsaturated fatty acid on the Organic Chemistry exam?

A quiz question might give you a fatty acid structure and ask whether it is saturated, monounsaturated, or polyunsaturated. Your job is to count the carbon-carbon double bonds and explain why that matters for reactivity. In a mechanism question, you may need to point out the alkene positions where radical addition or hydrogen abstraction can happen.

For a reaction pathway, look for resonance-stabilized intermediates and the chain of steps that turns a PUFA into a signaling molecule such as prostaglandin H2. If the prompt asks why a lipid oxidizes easily, the answer usually starts with the multiple double bonds and the stability of the resulting radical. In short, you use this term to identify the reactive substrate and predict what radicals will do to it.

Polyunsaturated fatty acid vs Monounsaturated Fatty Acid

A monounsaturated fatty acid has one carbon-carbon double bond, while a polyunsaturated fatty acid has two or more. That difference changes both the shape of the chain and its chemistry. PUFAs have more sites for radical addition and oxidation, so they are generally more reactive in the kinds of mechanisms covered in this unit.

Key things to remember about polyunsaturated fatty acid

  • A polyunsaturated fatty acid is a fatty acid with two or more carbon-carbon double bonds.

  • In Organic Chemistry, those double bonds make the molecule a target for radical addition, hydrogen abstraction, and oxidation.

  • PUFAs are chemically more reactive than saturated fatty acids because their double bonds create weak spots in the chain.

  • Arachidonic acid is a major PUFA example in biological radical mechanisms.

  • When a PUFA oxidizes, resonance-stabilized intermediates such as pentadienyl radicals often explain why the reaction can keep going.

Frequently asked questions about polyunsaturated fatty acid

What is polyunsaturated fatty acid in Organic Chemistry?

It is a fatty acid that contains two or more carbon-carbon double bonds. In Organic Chemistry, that matters because those double bonds make the molecule reactive in radical addition and oxidation mechanisms.

How is a polyunsaturated fatty acid different from a monounsaturated fatty acid?

A monounsaturated fatty acid has one double bond, while a polyunsaturated fatty acid has two or more. The extra double bonds give PUFAs more reactive sites and make them more likely to participate in radical chemistry.

Why are polyunsaturated fatty acids easily oxidized?

Their double bonds create positions where a radical can remove a hydrogen or add to the chain. The resulting intermediates can be resonance-stabilized, which makes lipid peroxidation easier to start and continue.

What is a common example of a polyunsaturated fatty acid?

Arachidonic acid is a classic example. It has four cis double bonds and is the starting material for prostaglandin synthesis, so it shows up often in biological radical addition problems.