OCR A-Level Chemistry Synthesis and analytical techniques (02), November 2020: Question 5

1 mark · Easy difficulty · Multiple Choice

Identify which chemical structure shows a section of poly(propene)

Practise this question

Question

Multiple-choice question asking to identify the structure of poly(propene) among four options labelled A, B, C, and D. Option A shows a conjugated diene-like carbon chain with alternating double bonds and dashed extension lines. Option B shows a similar chain with methyl side groups. Option C shows a saturated alkane chain without methyl groups. Option D shows a saturated carbon backbone with a methyl group attached to alternate carbon atoms, representing poly(propene).
Question text

5 Which structure shows a section of poly(propene)?

A

B

C

D

Your answer

[1]

Mark scheme

Show the mark scheme Mark scheme indicating the correct answer is D for question 5, worth 1 mark.

5 D 1 1.2

How to answer it

Identifying Poly(propene) Structure

What this question tests

This question assesses your understanding of addition polymerisation, specifically how to translate a monomer (propene, CH₂=CHCH₃) into its corresponding addition polymer repeating unit and polymer chain skeleton.

Question 5 (Multiple Choice)

Identifying a Section of Poly(propene)

✅ Correct Answer: D

Structure D represents poly(propene). During addition polymerisation, the double bond (C=C) of propene opens up to form a continuous carbon-carbon single bond backbone -C-C-C-C- , with a pendant methyl group -CH₃ attached to alternate carbon atoms along the chain.

💡 Key Knowledge

  • Monomer formula: Propene is CH₂=CHCH₃ .
  • Polymerisation mechanism: Alkenes undergo addition polymerisation where the pi (π) bond breaks.
  • Backbone structure: The main carbon chain consists entirely of single C-C bonds.
  • Branching: Every second carbon in the backbone carries a methyl ( -CH₃ ) group branch.

🧠 Exam Technique

  • Eliminate unsaturation: Polymers formed by addition polymerisation from alkenes have saturated carbon chains. Instantly rule out any options containing double bonds (A and B).
  • Count carbons in repeat unit: Propene has 3 carbons. Look for a repeat pattern that reflects a 3-carbon repeating unit along the backbone skeleton.

❌ Common Errors

  • Mistaking polyunsaturated backbones (A & B): Some students confuse addition polymers with conjugated diene structures or forget that the C=C double bond breaks during polymerisation.
  • Ignoring the methyl branches (C): Structure C shows poly(ethene) because it has a completely unbranched hydrocarbon chain with no methyl groups attached.
Mark allocation: 1 mark for selecting D. (Specification reference: 2.1.3 Alkenes & 5.3.1 Polymers).

Topics

Module 4: Core organic chemistry · 4.1 Basic concepts and hydrocarbons

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.