AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Vegetable oils and animal fats are esters of propane-1,2,3-triol (glycerol).
Full specification wording
Carboxylic acids and esters (A-level only): Vegetable oils and animal fats are esters of propane-1,2,3-triol (glycerol).
Glycerol is propane-1,2,3-triol, a three-carbon chain carrying an -OH on every carbon.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 4 of 8
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Glycerol is propane-1,2,3-triol, a three-carbon chain carrying an -OH on every carbon. Each -OH can be esterified by a long-chain carboxylic acid, so one glycerol plus three acid molecules gives a triglyceride with three ester linkages and three molecules of water. The three chains need not be the same. In a fat the chains are saturated, so they are straight, pack closely and have large contact areas for van der Waals forces, which makes fats solids at room temperature. In an oil the chains carry Z (cis) C=C bonds that kink them, so they pack badly, the van der Waals forces are weaker and the oil is a liquid. Bromine adds across each C=C, so the mole ratio of bromine to oil counts the double bonds per molecule.
What earns the marks
- Glycerol identified as propane-1,2,3-triol with three -OH groups.
- Three ester linkages formed and three molecules of water released per triglyceride.
- The difference between a fat and an oil described in terms of C=C bonds in the chains.
- Explaining the melting point difference by chain packing and the strength of van der Waals forces.
Common mistakes and how to put them right
Drawing a triglyceride with only one ester link, so two -OH groups are left on the glycerol.
Stating that oils contain no C=C bonds and fats do.
Assuming the three chains in a triglyceride must be identical.
Explaining the melting point by the strength of the covalent bonds rather than the forces between molecules.
Exam tips
- If bromine is used to find the degree of unsaturation, remember one mole of bromine reacts with one C=C, so a 5:1 ratio means five double bonds per molecule.
- When you draw a triglyceride, draw the glycerol backbone first and then hang three identical -COO- links off it.
Try the lesson
The start of the Carboxylic acids and esters (A-level only): Vegetable oils and animal fats are esters of propane-1,2,3-triol (glycerol). lesson, made from checked teaching notes for this specification point.
Have a go first
What is the alcohol component of vegetable oils and animal fats?
Listen
Glycerol is propane-1,2,3-triol, a three-carbon chain carrying an -OH on every carbon. Each -OH can be esterified by a long-chain carboxylic acid, so one glycerol plus three acid molecules gives a triglyceride with three ester linkages and three molecules of water. The three chains need not be the same. In a fat the chains are saturated, so they are straight, pack closely and have large contact areas for van der Waals forces, which makes fats solids at room temperature. In an oil the chains carry Z (cis) C=C bonds that kink them, so they pack badly, the van der Waals forces are weaker and the oil is a liquid. Bromine adds across each C=C, so the mole ratio of bromine to oil counts the double bonds per molecule. A strong answer earns marks for these points. glycerol identified as propane-1,2,3-triol with three -OH groups. three ester linkages formed and three molecules of water released per triglyceride. the difference between a fat and an oil described in terms of C=C bonds in the chains. explaining the melting point difference by chain packing and the strength of van der Waals forces. Exam tip: If bromine is used to find the degree of unsaturation, remember one mole of bromine reacts with one C=C, so a 5:1 ratio means five double bonds per molecule.
Key words
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triglycerides
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glycerol backbone
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saturated and unsaturated chains
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van der Waals forces
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See every definition
- triglycerides
- Formed when one glycerol plus three acid molecules gives three ester linkages and three molecules of water.
- glycerol backbone
- Propane-1,2,3-triol, a three-carbon chain carrying an -OH on every carbon, each of which can be esterified.
- saturated and unsaturated chains
- In a fat the chains are saturated, while in an oil the chains carry Z (cis) C=C bonds that kink them.
- van der Waals forces
- Large contact areas between closely packed straight chains give strong forces, but kinked chains pack badly so the forces are weaker.
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