Biochemistry Option B – London forces in fat and oils (Lipids topic)

Firstly, before I forget, a very Happy New Year to all of my readers, wherever in the world you may be!

Lipids are covered in the Biochemistry Option (Option B). They are large organic molecules that are not soluble in water. They are studies as they have a number of different uses in living systems, for example triglycerides (fats and oils), phospholipids (cell membranes) and cholesterol.

Fats and oils have essentially to same overall structure. The difference between them is that fats are solids and found in animals whereas oils are liquids and found in plants.

Both fats and oils are tri-esters. Remember, an ester is formed from a carboxylic acid and an alcohol, so this means fats and oils are formed from tri-alcohols (or tri-ols) and three carboxylic acids. The tri-ol in particular is propan-1,2,3-triol (can you draw this structure?)

The carboxylic acids are typically long chain acids (usually between 12 – 18 carbon atoms in length). These acids are often referred to as ‘fatty’ acids’ as they are fat soluble (but not water soluble).

The fatty acids do have IUPAC names but often the non IUPAC name is used (for example lauric acid, palmitic acid, stearic acid, oleic acid).

The nature of the fatty acid will determine if the triglyceride is a fat or oil. For example, stearic acid has 18 carbons and is saturated – it has no double bonds. Stearic acid melts at just under 70 oC. On the other hand, linoleic acid also has 18 carbons but it also has two double bonds (it has a Mr of four less than stearic acid). Linoleic acid however, melts at -5 oC – which is considerable less than stearic acid.

The reason behind this is due to London dispersion forces. They are weaker in linoleic acid. The reason for this is the double bond changes the shape of the fatty acid chain. Instead of zig zagging as found in the saturated molecule, the unsaturated molecule has ‘kinks’ in the hydrocarbon chain.

By I, Alex Ex, CC BY 2.5, https://commons.wikimedia.org/w/index.php?curid=2441070

The saturated chains can pack tightly together, hence the reason why they have greater London dispersion forces and therefore a higher melting point where as the unsaturated molecules with the kinks in the hydrocarbon structure cannot pack as tightly together which means weaker London dispersion forces and lower melting points.

I like this example of London dispersion forces. When we teach about these forces in the Bonding and Structure topic (Topic 4) it can seem a little abstract but this makes the abstract real. We are all familiar with fats and oils and the difference between them is just, essentially down to the presence or absence of a double bond and the effect that this has on the shape of the molecule.

Are you aware of any other ‘real world’ examples of London dispersion forces? If so, please post below as I’d love to hear of any other examples you may be familiar with.

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