Oils and Fats
- Oils/fats are important molecules for storing chemical energy in living systems.
- Oils and fats have the same basic structure; however fats are solid and oils are liquid at room temperature.
- Oils and fats are esters of glycerol (propane-1,2,3-triol) with long chained carboxylic acids, for example:
- Glycerol can react with three palmitic acid molecules to form a triester:
- The triesters found in natural oils are usually mixed triesters (the three fatty acid groups are different:
Fats and Fatty Acids
- The carboxylic acids in fats and oils are usually unbranched, containing an even number of carbon atoms ranging from C4 to C24.
- The fatty acids can either be fully saturated (all single bonds) or they can be unsaturated (contain one or more carbon-carbon double bond).
- Some common fatty acids are shown below:
Solid or Liquid?
- Natural oils and fats contain a mixture of many triesters.
- The nature of those fatty acids determines the properties of the substance and whether it is solid or liquid at room temperature.
- The following structure shows a saturated triglyceride:
- As the chains are unbranched, the triglycerides can pack closely together; this means that the intermolecular forces holding the molecules together are stronger and therefore more energy is required to separate the molecules; hence it is solid at room temperature.
- Many doctors believe that saturated fats cause blockages within blood vessels and can therefore lead to heart disease.
- The following diagram shows the structure of an unsaturated triglyceride:
- The unsaturated triglyceride molecules can not pack so closely together due to the kink caused by the presence of the cis double bonds; the intermolecular forces are therefore weaker and so less energy is needed to separate the molecules, so they have a lower melting point and are liquid at room temperature.
- Oils can be converted to fat in a hydrogenation reaction; this is how margarine is produced.
- The addition of hydrogen to unsaturated molecules reduces the number of double bonds; the triglycerides become more saturated.
- The conditions required for hydrogenation are a temperature of 180oC and a pressure 5 atmospheres using a nickel catalyst:
- This is an example of an addition reaction.
- It must be closely monitored, as if too much hydrogen is added, the fat can become too hard and brittle.
- The following flow diagram gives a simplified representation of how margarine is produced from oil:
Hydrolysis of Fats
- Like all esters, the ester linkage holding the molecules together can be hydrolysed.
- When this is done with concentrated sodium hydroxide solution, a soap is made.
- Soaps are the sodium or potassium salts of fatty acids and they are made by heating the oils and fats with sodium or potassium hydroxide.
- The free fatty acids are then released from the sodium salts by adding a dilute acid.
Different Types of Oil
- There are different types of oil that can be used as mediums within paints.
- There are drying oils; when exposed to air, these become rubbery solids.
- There are non-drying oils, which do not dry, but can become hardened by heating.
- There are semi-drying oils which thicken and form a skin when exposed to air and high temperatures.
- Paint layers dry from the outside inwards to give a tacky surface first and eventually they become hard throughout. The drying process is irreversible.
Natural oils can be classified according to their iodine number, which is based on how much iodine chloride (ICl) will react with the oil.
- The more ICl that reacts with the oil sample, the higher the iodine number.
- The higher the iodine number, the greater the drying potential the oil has.
- This can be related to the reaction that occurs between oils and iodine chloride; unsaturated bonds within the oil molecule can go through an electrophilic addition reaction with iodine chloride:
- This electrophilic addition reaction can only occur across unsaturated carbon carbon double bonds.
- The degree of saturation is also responsible for the drying potential of the oil; the more unsaturated the oil, the greater its drying potential.
- This is because in oxygen, a free radical polymerisation reaction occurs between the carbon carbon double bonds in adjacent molecules, cross-linking them together.
- This results in the hard protective coating being formed.
Useful books for revision
Revise A2 Chemistry for Salters (OCR A Level Chemistry B)
Salters (OCR) Revise A2 Chemistry