Fats and oils are used in our diets to provide us with energy. They play an important role in the transport of vitamins which are soluble in fats around the human body.
Many fats and oils are obtained from plant sources (sunflower oil, palm oil, coconut oil) and animal sources (lard, cod liver oil).
All fats and oils are naturally occurring esters, formed from condensation reactions between the alcohol glycerol and different long chain carboxylic acids (fatty acids).
Glycerol is also known by its systematic name propane-1,2,3-triol. It is a triol, meaning that it has three hydroxyl functional groups.
Fatty acids are long chain carboxylic acids ranging from C4 to C28. Common fatty acids including stearic acid and oleic acid have eighteen carbon atoms in their chains.
The acid molecules can either be saturated or unsaturated. The fats and oils formed are also known as triglycerides.
Like other esters, fats and oils are formed by a reversible reaction.
Oils (liquids at room temperature) contain more carbon to carbon double bonds than fats (solid at room temperature).
The lower melting point of oils is related to the higher degree of unsaturation. The presence of carbon to carbon double bonds in the oil molecules distorts the long fatty acid chains and the molecule’s shape. As a result the molecules cannot pack closely together.
Fat molecules do not have the same degree of distortion and can pack closely together. This increases their melting point.
The poorer packing in oils makes London dispersion forces between the oil molecules weaker than between fat molecules. Less heat energy is needed to separate oil molecules, so oils have lower melting points than fats.
To increase the melting point of an oil, addition reactions between the double bonds and hydrogen can take place. This reduces unsaturation and causes hardening.
The bromine test can be used to determine the level of unsaturation in a fat or oil. Bromine solution will decolourise quickly when it reacts with unsaturated compounds. The more double bonds present in the fat or oil then the more bromine solution that can be decolourised.