Form a polyester from a diol and a dioyl chloride
A diol reacts at both ends with a dioyl chloride to form ester links and hydrogen chloride.
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
What by-product forms when a diol reacts with a dioyl chloride to make a polyester?
What you will learn
- Recognise a dioyl chloride as a bifunctional monomer
- Build ester links from –OH and –COCl
- Distinguish HCl from water as the by-product
The acyl chloride end reacts with the diol
A dioyl chloride contains two –COCl groups. Each reacts with a hydroxyl group from a diol to form an ester link, –COO–, while HCl is eliminated.
Both monomers have two reactive ends, so alternating residues can extend into a polyester. The diol's carbon skeleton and the dioyl chloride's carbonyl-containing skeleton remain in the chain.
Choose the correct by-product
The acyl chloride route releases HCl because chlorine leaves from –COCl and the hydroxyl hydrogen is removed during link formation. It is not the same by-product as the dicarboxylic-acid route, which eliminates water.
When drawing a repeat unit, put the ester link in the chain and list HCl separately as the small molecule eliminated.
Pause and explain
Which link forms between the diol and a dioyl chloride?
Worked example
Describe the polymerisation of a diol with a dioyl chloride.
Show the worked solution
- Pair one –OH group with one –COCl group.
- Form –COO– and eliminate HCl.
- Repeat at the two ends of both bifunctional monomers.
Answer The diol and dioyl chloride form an alternating polyester chain with ester links; HCl is eliminated.
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Test this topic →From guided practice to a new situation
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.
Complete the structural description for HO–R–OH reacting with ClCO–R′–COCl.
Give me a hint
Connect the two residues through ester links and account for both ends.
Compare my reasoning
- Form R–O–CO–R′ at one end.
- Repeat the ester link at the other end of the diol/dioyl chloride pair.
- Name the small molecule from –COCl + –OH.
The repeat contains alternating –O–R–O–CO–R′–CO– residues, and HCl is eliminated at each ester-forming link.
Look for these in your work
- Residues retained
- Ester links formed
- HCl stated
Why is HCl, rather than water, the by-product for this monomer pair?
Give me a hint
Compare the leaving group on –COCl with the –COOH group in the other route.
Compare my reasoning
- The acyl chloride contains chlorine.
- Chlorine and hydrogen are removed as HCl when –COO– forms.
- No carboxylic-acid –OH is used here.
The dioyl chloride loses Cl and the diol loses an O–H hydrogen during ester-link formation, giving HCl; no –COOH group is present to form water.
Look for these in your work
- Leaving group identified
- Hydrogen source identified
- Compared with acid route
A student draws Cl atoms in the polymer backbone. Correct the repeat-unit change.
Give me a hint
The chlorine belongs to the acyl chloride leaving group.
Compare my reasoning
- Replace each –COCl with –COO–R– at a link.
- The Cl leaves from the acyl chloride group.
- Keep the carbonyl carbon and diol residue.
The repeat contains ester links –COO–, not –COCl; chlorine leaves as HCl while the carbonyl carbon remains in the polyester.
Look for these in your work
- Link corrected
- By-product tracked
- Carbonyl carbon retained
Common mistakes
- Calling the product a polyamide
- Writing water as the by-product
- Leaving chlorine in the ester repeat
- Ignoring the second reactive end on each monomer
What can you explain now?
What by-product forms when a diol reacts with a dioyl chloride to make a polyester?
Compare with the explanation
HCl
The acyl chloride group loses chlorine as the ester link forms.
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Make it stick
How does the dioyl chloride route form an ester link, and which by-product distinguishes it from a diacid route?
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.