Explain how two amino acid molecules react together to form a dipeptide, and describe the type of bond formed between them.

Edexcel A-Level Chemistry (9CH0) — 18.2 Amines, amides and amino acids · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Amino acids contain both an amine group (–NH₂) and a carboxylic acid group (–COOH). When two amino acids react together, a dipeptide is formed along with a small molecule as a by-product.

Model answer (5 marks)

The –COOH group of one amino acid reacts with the –NH₂ group of the other amino acid, forming a covalent bond and eliminating a molecule of water. This is a condensation reaction. The bond formed is a peptide bond, linking the carbonyl carbon of one residue to the nitrogen of the other.

Examiner tips

  • Use the exact terms – condensation reaction, peptide bond, –COOH, –NH₂, water eliminated.
  • Show the reaction sequence: carboxyl carbon + amine nitrogen → peptide bond + H₂O.
  • Keep the answer concise – 5 marks, no extra explanation needed.

Common mistakes

  • Saying the bond is a hydrogen bond instead of a peptide bond.
  • Forgetting to mention water as the by‑product.
  • Using vague terms like ‘linkage’ without specifying peptide bond.

Mark scheme (5 marks)

  1. The –COOH group of one amino acid reacts with the –NH₂ group of the other amino acid
  2. Water is eliminated / lost as a by-product during the reaction
  3. The reaction is described as a condensation reaction
  4. The bond formed between the two amino acids is called a peptide bond
  5. The peptide bond is formed between the carbon of the –CO– and the nitrogen of the –NH– group, linking the two amino acid residues

Key terms in this question

amino acid · dipeptide

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