Form polyamides from amino acids
Amino acids link through amide (peptide) bonds when the carboxyl group of one reacts with the amino group of another.
Section 35 · Polymerisation
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 bond forms when amino acids condense?
What you will learn
- Identify amino and carboxyl groups in amino acids
- Describe peptide-link formation by condensation
- Distinguish a repeated polyamide chain from a single dipeptide
Amino-acid residues link through peptide bonds
An amino acid has both an amino group and a carboxyl group. The carboxyl group of one amino acid can react with the amino group of another, forming an amide link commonly called a peptide bond, –CO–NH–, and eliminating water.
Repeated condensation joins many amino-acid residues into a polypeptide chain. The side chains remain attached to their original alpha-carbon positions; the link joins the backbone rather than removing the side chains.
Use atom accounting at the link
For a dipeptide, one peptide bond forms and one water molecule is eliminated. A longer chain has additional peptide links between residues; do not count a water molecule for every atom or confuse the number of residues with the number of links.
Proteins and polypeptides contain amide links, so they fit the polyamide structural family. This lesson concerns the syllabus polymerisation pattern, not protein folding or biological synthesis mechanisms.
Pause and explain
How many water molecules are eliminated when two amino acids form one dipeptide bond?
Worked example
Describe the condensation between two amino-acid molecules.
Show the worked solution
- Use the carboxyl group of one residue and the amino group of the next.
- Form –CO–NH–.
- Eliminate H₂O and retain both side chains.
Answer Two amino acids form a dipeptide through one peptide (amide) bond, with one H₂O 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.
Explain how glycine and alanine can form a dipeptide.
Give me a hint
Join the carboxyl group of one amino acid to the amino group of the other.
Compare my reasoning
- Form a C–N bond at the peptide link.
- Write the link as –CO–NH–.
- Account for one molecule of water.
Glycine and alanine can condense to form a dipeptide with one peptide (amide) link and one water molecule eliminated.
Look for these in your work
- Groups joined
- Peptide link named
- Water counted
A short polypeptide contains four amino-acid residues in one chain. How many peptide links connect them in a simple unbranched chain?
Give me a hint
A chain of n residues has one fewer links than residues.
Compare my reasoning
- Start with four separate residues.
- Each link joins one pair of neighbours.
- A single chain has 4 − 1 links.
Three peptide links connect four residues in a simple unbranched chain.
Look for these in your work
- Residues counted
- Chain relation used
- Link count correct
A student removes a side chain while drawing a peptide repeat. Explain why this is not part of peptide-bond formation.
Give me a hint
The bond forms between backbone functional groups.
Compare my reasoning
- The carboxyl carbon and amino nitrogen form the peptide link.
- Side chains attach to the amino-acid carbon skeleton.
- Condensation removes atoms from the linking groups, not the whole side chain.
The side chain remains attached; only atoms from the carboxyl and amino groups are removed as water when the peptide link forms.
Look for these in your work
- Backbone link identified
- Side chain located
- Atom loss clarified
Common mistakes
- Calling a peptide bond an ester
- Removing the amino-acid side chains
- Forgetting water is eliminated
- Saying a dipeptide contains many residues
What can you explain now?
What bond forms when amino acids condense?
Compare with the explanation
A peptide (amide) bond
The link is –CO–NH– between residues.
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Make it stick
Which amino-acid groups form the peptide bond, and what small molecule is eliminated?
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.