Form a polyester from a hydroxycarboxylic acid
A hydroxycarboxylic acid links to itself through ester groups, eliminating water to form a polyester.
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 35. Track the functional groups, repeat-unit connectivity and small-molecule by-products. Written practice is self-checked
Before you begin
- Recognise alkenes, carboxylic acids, acyl chlorides, alcohols, amines, amides and ester linkages.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which two groups allow a hydroxycarboxylic acid to self-condense?
What you will learn
- Recognise both functional groups on one monomer
- Describe self-condensation into an ester-linked chain
- Deduce the residue retained in the repeat
One monomer carries both ends
A hydroxycarboxylic acid contains an alcohol –OH group and a carboxylic acid –COOH group. The –OH of one molecule can react with the –COOH of another to form an ester link.
Because each molecule has one group of each type, the reaction can continue in both directions through repeated condensation. Water is eliminated as ester links form, and the carbon skeleton between –OH and –COOH remains in the chain.
Keep the monomer residue intact
For a monomer HO–R–COOH, the polyester repeat can be represented as [–O–R–CO–]ₙ. The oxygen from the hydroxyl group and the carbonyl from the carboxyl group appear in the ester linkage.
A hydroxycarboxylic acid is one monomer type with two different functional groups; it is not a mixture of a separate diol and diacid.
Pause and explain
What linkage forms when a hydroxycarboxylic acid self-condenses?
Worked example
Deduce the repeat from 2-hydroxyethanoic acid, HOCH₂COOH.
Show the worked solution
- Identify –OH and –COOH in the same molecule.
- Join the hydroxyl oxygen to a neighbouring carbonyl carbon.
- Retain the CH₂ residue and show ester connectivity.
Answer The repeat is [–OCH₂CO–]ₙ; water is eliminated as the ester links form.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Write a repeat unit for HOCH₂CH₂COOH polymerising by condensation.
Give me a hint
Keep the two-carbon residue between hydroxyl oxygen and carbonyl carbon.
Compare my reasoning
- The monomer has both –OH and –COOH.
- Join molecules through –O–CO–.
- Bracket the repeated –OCH₂CH₂CO– residue.
[–OCH₂CH₂CO–]ₙ, with water eliminated as ester links form.
Look for these in your work
- Correct monomer residue
- Ester linkage
- Water by-product
Explain why one type of hydroxycarboxylic-acid monomer can form a chain without a separate diol.
Give me a hint
Count the different functional groups on each molecule.
Compare my reasoning
- Each molecule has one hydroxyl group.
- Each molecule also has one carboxyl group.
- Those two ends link successive molecules in both directions.
Each monomer is bifunctional: its –OH and –COOH groups can form ester links to neighbouring monomers, so one monomer type can make a chain.
Look for these in your work
- Both groups named
- Bifunctionality explained
- Chain growth explained
A proposed repeat for HOCH₂COOH is [–CH₂CH₂CO–]ₙ. Identify the error.
Give me a hint
Trace the monomer's atoms and the oxygen in the ester link.
Compare my reasoning
- The monomer has one CH₂ group, not two.
- Its hydroxyl oxygen remains in the ester linkage.
- The repeat should be –OCH₂CO–.
The correct repeat is [–OCH₂CO–]ₙ: the proposed structure adds a carbon and loses the hydroxyl oxygen.
Look for these in your work
- Carbon count checked
- Oxygen tracked
- Repeat corrected
Common mistakes
- Treating the molecule as two separate monomers
- Removing the CH₂ carbon skeleton
- Drawing an amide link
- Forgetting that condensation eliminates water
What can you explain now?
Which two groups allow a hydroxycarboxylic acid to self-condense?
Compare with the explanation
–OH and –COOH
One molecule carries both groups needed to form an ester link.
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Make it stick
How can one hydroxycarboxylic-acid monomer form a polyester chain?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.