AQA A-Level Chemistry Paper 3, 2017: Question 25

1 mark · Medium difficulty · Multiple Choice

Identify which compound can form a condensation polymer on its own without requiring another reagent.

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Question

Multiple choice question 25 asking: 'Which compound can form a polymer without needing another reagent?' with four options: A is HOCH2CH2OH, B is HOOCCH2CH2COOH, C is HOCH2CH2COCl, and D is ClCH2CH2COOH.
Question text

25 Which compound can form a polymer without needing another reagent?

[1 mark]

A HOCH2CH2OH

B HOOCCH2CH2COOH

C HOCH2CH2COCl

D ClCH2CH2COOH

Mark scheme

Show the mark scheme Mark scheme table showing question number 25 with correct answer C.

25 C

How to answer it

Condensation Polymerisation: Self-Polymerising Monomers

📋 What this question tests

This question assesses understanding of condensation polymerisation and the reactivity of functional groups:

  • The criteria for a single monomer species to polymerise independently (must possess two mutually reactive, complementary functional groups).
  • Distinguishing functional groups that react spontaneously without additional reagents (e.g. acyl chloride + alcohol vs chloroalkane + carboxylic acid).
  • Identifying polyester precursors formed from single bifunctional molecules.

Question 25 Analysis

Identify which compound self-polymerises without an additional reagent [1 Mark]

✅ Correct Answer

C: HOCH₂CH₂COCl

This molecule is 3-hydroxypropanoyl chloride. It possesses both an alcohol (-OH) group at one end and an acyl chloride (-COCl) group at the other.

Award 1 mark for selecting C.

💡 Key Knowledge

  • Polyester formation: Occurs by condensation between an alcohol group ( -OH ) and either a carboxylic acid ( -COOH ) or an acyl chloride ( -COCl ).
  • Single-monomer polymers: If a polymer is formed from only one monomer without any other reagent, the molecule must contain two different functional groups that react directly with each other.
  • Reactivity of Acyl Chlorides: Acyl chlorides are highly reactive towards alcohols via nucleophilic addition-elimination, reacting readily at room temperature to form an ester bond ( -COO- ) and eliminating HCl without requiring a catalyst or second reagent:
    n HO-CH₂CH₂-COCl → -[O-CH₂CH₂-CO]-ₙ + n HCl

🧠 Exam Technique: Systematic Elimination

Evaluate each option by checking if it contains two complementary groups that react together:

  • A (HOCH₂CH₂OH): Contains two identical alcohol groups (a diol). Alcohol groups do not react with other alcohol groups under standard conditions to form polymers; it requires a dicarboxylic acid or diacyl chloride partner. (Incorrect)
  • B (HOOCCH₂CH₂COOH): Contains two identical carboxylic acid groups (a dicarboxylic acid). Carboxylic acids cannot react with themselves to polymerise; it requires a diol or diamine partner. (Incorrect)
  • C (HOCH₂CH₂COCl): Has an alcohol group at one end and an acyl chloride at the other. Acyl chlorides readily react with alcohols to form ester linkages and eliminate HCl. (Correct)
  • D (ClCH₂CH₂COOH): Contains a primary chloroalkane group and a carboxylic acid group. Carboxylic acids and chloroalkanes do not react together to form a polymer chain without external activating reagents. (Incorrect)

❌ Common Errors & Pitfalls

  • Confusing C and D: Students often overlook the difference between an acyl chloride ( -COCl ) and a halogenoalkane ( Cl-CH₂- ). Acyl chlorides undergo rapid nucleophilic addition-elimination with alcohols, whereas primary chloroalkanes do not condense with carboxylic acids.
  • Assuming condensation always needs two monomers: Familiarity with poly(ethylene terephthalate) (PET) and Nylon-6,6 leads some students to assume polymers always require pairs like a diol + dicarboxylic acid. However, hydroxycarboxylic acids (or hydroxyacyl chlorides) can polymerise solo.
  • Selecting diols or dicarboxylic acids: Choosing A or B indicates a fundamental misunderstanding of the need for mutual reactivity between functional groups on the monomer chain.

📐 Polymer Structure & Reaction Details

Condensation Reaction:

n HO-CH₂-CH₂-COCl → -[-O-CH₂-CH₂-CO-]-ₙ + n HCl

  • Repeating Unit: -O-CH₂-CH₂-CO- (poly(3-hydroxypropanoate), a polyester).
  • Small Molecule Eliminated: Hydrogen chloride ( HCl gas).
  • Reaction Type: Nucleophilic addition-elimination forming an ester link.

Topics

Organic Chemistry · 3.3.9 Carboxylic Acids and Derivatives · 3.3.12 Polymers

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.