A-Level Chemistry May/June 2025 Q7(c)(ii): Ethanedioic acid, HOOCCOOH, can react with propane-1,3-diamine, H2NCH2CH2CH2NH2, to for…

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  This is a condensation polymerisation reaction between a dicarboxylic acid (ethanedioic acid) and a diamine (propane-1,3-diamine), which forms a polyamide.

  The hydroxyl group (–OH) from a carboxylic acid group reacts with a hydrogen atom (–H) from an amine group to form a water molecule (H₂O) and an amide linkage.

  To find the repeat unit, we draw one molecule of each monomer and remove a water molecule from the reacting ends.

  Monomer 1: HOOC–COOH Monomer 2: H₂N–CH₂–CH₂–CH₂–NH₂

  The resulting repeat unit consists of the residues of these two monomers joined by an amide link. The question requires the new functional group (the amide link) to be displayed, and for continuation bonds to be shown.

  Structure of one repeat unit of Polymer W:

     H           H   O   O
     |           |  ||  ||
  --(N - CH₂ - CH₂ - CH₂ - N - C - C)--
  

  This structure shows one complete repeat unit derived from one molecule of propane-1,3-diamine and one molecule of ethanedioic acid. The amide group –NH–CO– is correctly displayed, and the continuation bonds indicate that this unit repeats to form the polymer chain.

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  How the marks are awarded

  • M1 — The first mark is for correctly drawing the displayed amide group, –N(H)–C(=O)–, and showing it is correctly bonded to the adjacent –CH₂– group from the diamine and the adjacent –C(=O)– group from the ethanedioic acid residue.
  • M2 — The second mark is for drawing the complete and correct remainder of the single repeat unit, which includes the –(CH₂)₃– chain and the second carbonyl group, and showing continuation bonds at both ends of the unit to indicate it is part of a polymer chain.

  Common mistakes

  • Drawing an incorrect repeat unit, for example by joining two diamine units or two diacid units instead of one of each.
  • Failing to display the amide linkage as requested (e.g., writing –CONH– instead of showing the C=O and N–H bonds separately).
  • Forgetting to include continuation bonds at both ends of the structure, which is essential to show it is a repeating unit in a polymer.
  • Using the incorrect number of atoms in the monomer backbones, e.g., drawing a –(CH₂)₂– chain instead of the –(CH₂)₃– chain from propane-1,3-diamine.

  Examiner tip: For polymerisation questions, always draw out the two monomers side-by-side, circle the small molecule that is eliminated (like H₂O), and then connect the remaining fragments to build the repeat unit.

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