How are esters formed and broken down?
Describe the formation of esters by esterification and their breakdown by hydrolysis, writing equations and naming products.
How carboxylic acids and alcohols form esters in a reversible, acid-catalysed esterification, and how esters are split by acidic and basic hydrolysis (saponification).
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What this dot point is asking
You must write equations for esterification and hydrolysis and name the products.
Esterification
A carboxylic acid reacts with an alcohol to make an ester plus water:
For example:
(ethanoic acid + ethanol → ethyl ethanoate + water).
Naming the ester
The ester name has two parts: the alkyl group from the alcohol comes first, then the -oate from the acid.
- ethanoic acid + methanol → methyl ethanoate
- propanoic acid + ethanol → ethyl propanoate
Hydrolysis
Hydrolysis reverses esterification, splitting the ester with water.
Acidic hydrolysis (dilute acid catalyst) is the reverse of esterification and gives the carboxylic acid and the alcohol:
Basic hydrolysis (saponification) uses sodium hydroxide and gives the carboxylate salt plus the alcohol:
Basic hydrolysis goes essentially to completion (it is not reversible) because the carboxylate ion is removed from the equilibrium.
Exam-style practice questions
Practice questions written in the style of SACE Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2024 SACE Stage 21 marksTriglycerides are broken down in the intestine by alkaline hydrolysis. Name the functional group in triglycerides that is hydrolysed.Show worked answer →
The functional group that is hydrolysed is the ester group (ester linkage).
A triglyceride is a triester formed from glycerol and three fatty acids; each of the three ester linkages is broken during hydrolysis. One mark for "ester".
2022 SACE Stage 21 marksBiodiesel is a mixture of esters. Write the formula of one biodiesel molecule made from stearic acid, C17H35COOH.Show worked answer →
Biodiesel molecules are the methyl esters of fatty acids (made by reacting the fatty acid, or its triglyceride, with methanol).
The methyl ester of stearic acid is C17H35COOCH3 (methyl stearate). One mark for a correct methyl ester formula of stearic acid. (Accept the structural form C17H35COOCH3.)
2023 SACE Stage 22 marksGlycerol is one product of the alkaline hydrolysis of triglycerides. Complete and balance the equation for the production of glycerol from the triglyceride formed from glycerol and three C15H31COO chains (using NaOH).Show worked answer →
Alkaline hydrolysis (saponification) splits each of the three ester linkages, releasing glycerol and the sodium salts of the fatty acids.
Balanced equation: triglyceride (the C15H31 triester of glycerol) + 3NaOH -> glycerol (C3H5(OH)3) + 3C15H31COO-Na+.
One mark for producing glycerol plus the three carboxylate (soap) products, one mark for the correct balancing coefficient of 3 in front of NaOH and the carboxylate salt.