Deduce polyamide repeat units from monomers
Connect amino nitrogens to carbonyl carbons, retain each residue and distinguish acid from acyl-chloride by-products.
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 atoms must appear in a polyamide link?
What you will learn
- Construct an amide-linked repeat
- Retain the correct monomer residues
- Check nitrogen, carbonyl and by-product accounting
Build the –CONH– link
A diamine and a dicarboxylic acid form a polyamide when an amino nitrogen connects to a carboxyl carbon. The repeated link is –CO–NH–, with the carbon skeletons of both monomers retained.
An aminocarboxylic acid has both ends on one molecule, so its repeat can be written [–NH–R–CO–]ₙ. Amino acids form peptide (amide) links in the same structural way.
Check each monomer pair
Use water as the by-product for a diamine with a dicarboxylic acid or for an aminocarboxylic acid. With a dioyl chloride, chlorine and amino hydrogen leave as HCl.
The carbonyl oxygen remains in the amide link and nitrogen remains in the chain. Do not convert an amide into an ester while simplifying the repeat.
Pause and explain
What is the by-product for a diamine reacting with a dicarboxylic acid?
Worked example
Deduce the repeat from H₂N(CH₂)₄NH₂ and ClCO(CH₂)₆COCl.
Show the worked solution
- Keep the diamine residue –NH(CH₂)₄NH–.
- Keep the diacyl residue –CO(CH₂)₆CO–.
- Join through –CO–NH– links and account for HCl.
Answer [–NH(CH₂)₄NHCO(CH₂)₆CO–]ₙ; HCl is eliminated at amide-link formation.
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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.
Write the repeat from H₂N(CH₂)₆NH₂ plus HOOC(CH₂)₄COOH.
Give me a hint
Place the diamine nitrogen beside the diacid carbonyl carbon.
Compare my reasoning
- Retain both N atoms in the diamine residue.
- Retain the four-carbon chain between carbonyls.
- Join as –NH–CO– links and show water as the by-product.
[–NH(CH₂)₆NHCO(CH₂)₄CO–]ₙ; water is eliminated as amide links form.
Look for these in your work
- Monomer residues correct
- Amide link correct
- By-product correct
Write a repeat unit for the aminocarboxylic acid H₂N(CH₂)₃COOH.
Give me a hint
The amine and acid groups are on the same monomer.
Compare my reasoning
- Keep the three CH₂ groups.
- Join the N of one residue to the carbonyl C of another.
- Represent the repeating residue as –NH(CH₂)₃CO–.
[–NH(CH₂)₃CO–]ₙ, with water eliminated during amide-link formation.
Look for these in your work
- Chain length retained
- Amide link shown
- Water identified
A proposed repeat has –COO– between a diamine residue and a diacid residue. Correct it and explain the atom clue.
Give me a hint
The diamine contributes nitrogen to the link.
Compare my reasoning
- The amino nitrogen joins the carboxyl carbon.
- The linking group is –CONH–.
- An ester –COO– link would require an alcohol oxygen instead.
Use –CONH–: the diamine supplies N, which bonds to the diacid's carbonyl carbon; –COO– is the polyester link.
Look for these in your work
- Monomer atom traced
- Amide corrected
- Ester contrast stated
Common mistakes
- Using –COO– instead of –CONH–
- Losing nitrogen from the diamine residue
- Changing the number of methylene groups
- Confusing HCl with water for an acyl-chloride monomer
What can you explain now?
Which atoms must appear in a polyamide link?
Compare with the explanation
Carbonyl carbon and nitrogen
The repeating link is –CO–NH–.
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Make it stick
How do you tell whether a proposed polyamide repeat has the correct nitrogen and carbonyl connectivity?
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.