OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2022: Question 13

1 mark · Medium difficulty · Multiple Choice

Identify which given equations represent propagation steps in the radical substitution of pentane to form 1-chloropentane.

Practise this question

Question

Multiple choice question asking which of three numbered equations could be part of the propagation step in the radical substitution of C5H12 to form C5H11Cl. Equation 1: C5H11 radical + Cl2 -> C5H11Cl + Cl radical. Equation 2: C5H12 + Cl radical -> C5H11Cl + H radical. Equation 3: C5H11 radical + Cl radical -> C5H11Cl. Four options A, B, C, D are provided.
Question text

13 Which equation(s) could be part of the propagation step in the radical substitution of C5H12 to

form C5H11Cl?

1 C5H11• + Cl2 C5H11Cl + Cl•

2 C5H12 + Cl• C5H11Cl + H•

3 C5H11• + Cl• C5H11Cl

A 1, 2 and 3

B Only 1 and 2

C Only 2 and 3

D Only 1

Your answer

[1]

Mark scheme

Show the mark scheme Mark scheme indicating the correct answer is D (Only 1), worth 1 mark.

13 D 1 AO1.1

How to answer it

Radical Substitution Propagation Steps

What this question tests

This question assesses your knowledge of free radical substitution mechanisms of alkanes (specifically initiation, propagation, and termination steps), core definitions of reaction steps, and your ability to balance radical equations by ensuring radicals appear on both sides of propagation steps.

Question 13: Multiple Choice Analysis

Exam Part: Single-Best-Answer Multiple Choice (Option D)

✅ Correct Answer

Option D (Only 1) is the correct choice because propagation steps must always feature a radical on both the reactant side and the product side.

💡 Key Knowledge

  • Propagation definition: Two steps that form a chain reaction where a radical reacts with a non-radical molecule to form a new molecule and a new radical.
  • Golden Rule: Every propagation equation must consume one free radical and produce one free radical.

🧠 Exam Technique

Evaluate each numbered statement individually by checking the nature of the species on both sides of the reaction arrow:

  • Equation 1: C₅H₁₁• + Cl₂ → C₅H₁₁Cl + Cl• → Consumes 1 radical ( C₅H₁₁• ) and produces 1 radical ( Cl• ). Valid propagation step.
  • Equation 2: C₅H₁₂ + Cl• → C₅H₁₁Cl + H• → Produces a hydrogen radical ( H• ). Hydrogen radicals are virtually never formed in alkane chlorination because H-Cl bonds are much stronger and C-H homolytic fission is energetically unviable.
  • Equation 3: C₅H₁₁• + Cl• → C₅H₁₁Cl → Two radicals combine to form a single non-radical product. This is a termination step, not propagation.

❌ Common Errors

  • Confusing termination steps (two radicals combining to make a stable molecule) with propagation steps.
  • Failing to count the radical dots ( • ), leading students to mistakenly accept Equation 3 because it results in the correct haloalkane formula.
  • Inventing chemically unreasonable radical species like hydrogen radicals ( H• ) in standard OCR mechanisms.
Mark Allocation: [1] mark awarded for selecting D.

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), June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.