How are esters, fats and oils made and broken down?
The formation of esters by condensation of an alcohol and a carboxylic acid, the ester link, hydrolysis of esters, and the structure of fats and oils as esters of glycerol and fatty acids, including saturated and unsaturated chains.
An SQA Higher Chemistry answer on esters, fats and oils, covering the condensation reaction that forms an ester from an alcohol and carboxylic acid, the ester link, hydrolysis, and the structure of fats and oils as glycerol esters of saturated and unsaturated fatty acids.
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What this key area is asking
The SQA wants you to describe how esters form by condensation, recognise and name the ester link, explain hydrolysis, and describe fats and oils as esters of glycerol and fatty acids, including the difference between saturated and unsaturated chains. Ester naming and the condensation-versus-hydrolysis distinction are reliable exam earners.
Making esters
The ester link is the group. To name an ester, take the first part from the alcohol and the second part from the acid (for example, ethanol plus methanoic acid gives ethyl methanoate). In the lab the reaction is carried out by heating under reflux with a concentrated sulfuric acid catalyst.
Hydrolysis
Hydrolysis is the reverse of condensation: water breaks the ester link to give back the carboxylic acid and the alcohol. This is how the body and industry break esters down, and it is the basis of soap-making when fats are hydrolysed.
Fats and oils
Fats and oils are a concentrated source of energy in the diet and supply the body with fat-soluble vitamins.
Worked example: hydrolysis mole calculation
Saturated and unsaturated
- Saturated fatty acids have only carbon-to-carbon single bonds; the straight chains pack closely, giving stronger intermolecular forces (London dispersion forces) and higher melting points, so fats tend to be solid.
- Unsaturated fatty acids contain one or more carbon-to-carbon double bonds; the kinks stop close packing, giving weaker forces and lower melting points, so oils tend to be liquid.
You can test for unsaturation by adding bromine solution, which is decolourised by the carbon-to-carbon double bonds. Oils decolourise more bromine than fats.
Examples in context
Esterification and its reverse are central to the food and fragrance industries. Many fruity flavours and perfumes are esters: pentyl ethanoate smells of pears and octyl ethanoate of oranges, which is why food chemists synthesise esters as artificial flavourings. The hydrolysis of fats is exactly how the body digests dietary fat, with the enzyme lipase breaking the ester links to release glycerol and fatty acids for absorption. Industrially, the hydrolysis of vegetable oils makes the fatty acids used in soaps and the glycerol used in cosmetics and explosives, while hardening of liquid oils by adding hydrogen across the double bonds turns them into the semi-solid fats used in margarine.
Try this
Q1. Name the type of reaction used to make an ester from an alcohol and a carboxylic acid. [1 mark]
- Cue. Condensation, with the loss of a water molecule.
Q2. Explain why an oil is a liquid at room temperature but a fat is a solid. [2 marks]
- Cue. Oils are more unsaturated, so the chains cannot pack closely; the weaker intermolecular forces give a lower melting point.
Q3. Hydrolysis of an ester gives of ethanoic acid (). Calculate the moles of acid formed. [1 mark]
- Cue. .
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA Higher 20183 marksEthyl ethanoate is made by reacting ethanol with ethanoic acid. (a) Name the type of reaction and the small molecule released. (b) Draw or describe the ester link formed. (c) State the conditions used to carry out the reaction in the laboratory.Show worked answer →
Markers reward the reaction type and by-product, the ester link, and the laboratory conditions.
(a) The reaction is a condensation reaction, releasing a molecule of water.
(b) The ester link is the group, formed when the hydroxyl group of the alcohol combines with the carboxyl group of the acid (the of the acid and the of the alcohol leave as water).
(c) The mixture is heated under reflux with a concentrated sulfuric acid catalyst. Reflux prevents the volatile reactants and product escaping, and the concentrated acid catalyses the reaction and helps remove water.
A common loss is forgetting to mention reflux or quoting dilute rather than concentrated sulfuric acid.
SQA Higher 20214 marksAn ester is hydrolysed to give of propanoic acid () and methanol. (a) Name the ester. (b) Calculate the number of moles of propanoic acid formed. (c) Calculate the mass of methanol () produced, given a mole ratio.Show worked answer →
Part (a) tests ester naming; parts (b) and (c) are linked mole calculations.
(a) The ester is methyl propanoate (the methyl part from methanol, the propanoate part from propanoic acid).
(b) Moles of propanoic acid:
(c) The hydrolysis gives a ratio of acid to alcohol, so of methanol forms. Its mass is:
Markers reward the correct ester name and carrying the mole value through to the methanol mass.
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Sources & how we know this
- SQA Higher Chemistry Course Specification — SQA (2018)