AQA A-Level Chemistry Paper 3, June 2018: Question 21

1 mark · Easy difficulty · Multiple Choice

Identify the monomer or pair of monomers used to form the specified condensation polymer repeating unit.

Practise this question

Question

Question 21 shows the repeating unit of a polymer: a carbonyl group bonded to a chain of four CH2 groups, followed by an amide group (CONH), then six CH2 groups, and finally an NH group with open bonds at each end. Four multiple-choice options (A to D) list possible monomer combinations to make this polymer: A: ClOC(CH2)4NH2 only; B: ClOC(CH2)4COCl only; C: ClOC(CH2)4COCl and H2N(CH2)6NH2; D: ClOC(CH2)6COCl and H2N(CH2)4NH2.
Question text

21 The repeating unit of a polymer is shown.

Which monomer or pair of monomers could be used to make this polymer?

[1 mark]

A ClOC(CH2)4NH2 only

B ClOC(CH2)4COCl only

C ClOC(CH2)4COCl and H2N(CH2)6NH2

D ClOC(CH2)6COCl and H2N(CH2)4NH2

Mark scheme

Show the mark scheme Mark scheme table indicating question number 21 has the correct answer C.

21 C

How to answer it

Condensation Polymers: Identifying Monomers of Nylon-6,6

📌 What this question tests

Core syllabus focus: Condensation polymers, specifically polyamides (Nylon) and nucleophilic addition-elimination reactions of acyl chlorides.

  • Recognising the repeating unit of a polyamide and identifying the amide link: -CO-NH- .
  • Deducing monomer structures by "breaking" condensation links back to their functional group precursors (acyl chloride vs amine).
  • Careful counting of carbon atoms in aliphatic chains, particularly inside brackets like -(CH₂)₄- versus -(CH₂)₆- .

Question 21 Analysis

Multiple Choice: Monomers for Polyamide Repeat Unit [1 Mark]

✅ Correct Answer

C: ClOC(CH₂)₄COCl and H₂N(CH₂)₆NH₂

Award 1 mark for selecting option C.

💡 Key Knowledge

  • Polyamide linkage: Formed when a carbonyl carbon reacts with a nitrogen, eliminating a small molecule (here, HCl ).
  • Acyl chlorides vs Carboxylic acids: Diacyl chlorides react much faster with diamines at room temperature than dicarboxylic acids.
  • Nylon-6,6 structure: Hexanedioyl dichloride (6 carbons total: 2 carbonyl carbons + 4 in chain) + Hexane-1,6-diamine (6 carbons total).

📐 Step-by-Step Structural Breakdown ("Unzipping" the Repeat Unit)

  1. Locate the amide bond: Find the -CO-NH- connection in the repeating unit:
    -CO-(CH₂)₄-CO- attached to -NH-(CH₂)₆-NH- .
  2. Cleave the amide bond: Cut between the carbonyl carbon ( C=O ) and the amine nitrogen ( N-H ).
  3. Restore monomer functional groups:
    • Add -Cl back to both carbonyl groups: gives ClOC(CH₂)₄COCl (hexanedioyl dichloride).
    • Add -H back to both amine groups: gives H₂N(CH₂)₆NH₂ (hexane-1,6-diamine).
  4. Verify total carbon counts:
    • Diacyl chloride: 1 + 4 + 1 = 6 carbons.
    • Diamine: 6 carbons.

🧠 Exam Technique: Elimination Strategy

  • Rule out B immediately: A diacyl chloride cannot polymerise alone; it lacks nucleophilic groups to attack the acyl carbon.
  • Rule out A: Option A has an amine and an acyl chloride on the same molecule with only 4 CH₂ groups. Its polymer would repeat every 5 carbons, not alternate between 4 and 6 methylene units.
  • Differentiate C and D: Look closely at which block has 4 methylenes and which has 6. The C=O groups flank the (CH₂)₄ group, so the diacyl chloride must contain (CH₂)₄ , not (CH₂)₆ . This eliminates D.

❌ Common Student Pitfalls

  • Swapping chain lengths (Picking D): Glancing too quickly and matching the 6-carbon chain to the acyl chloride and 4-carbon chain to the diamine. Always follow the connectivity of the C=O group.
  • Miscounting chain length in the monomer: Forgetting that ClOC(CH₂)₄COCl has 6 carbons in total (hexanedioic acid derivative), not 4.
  • Overlooking eliminated molecule: Forgetting that reaction of an acyl chloride with an amine produces HCl (condensation), not water.

Topics

Organic Chemistry · 3.3.12 Polymers · 3.3.9 Carboxylic Acids and Derivatives

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.