OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2017: Question 14

1 mark · Medium difficulty · Multiple Choice

Identify which radicals are present in the mechanism of the radical substitution reaction between ethane and chlorine to form chloroethane.

Practise this question

Question

Multiple choice question 14 stating: Ethane reacts with chlorine by radical substitution to form chloroethane. Which radical(s) is/are present in the mechanism? Listed are 1: H dot, 2: Cl dot, 3: C2H5 dot. Options are A: 1, 2 and 3; B: Only 1 and 2; C: Only 2 and 3; D: Only 1. An empty answer box and mark allocation [1] are at the bottom.
Question text

14 Ethane reacts with chlorine by radical substitution to form chloroethane.

Which radical(s) is/are present in the mechanism?

1 H

2 Cl

3 C2H5

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 The mark scheme table shows question number 14 with the correct answer C and 1 mark.

14 C 1

How to answer it

Radical Substitution Intermediates in Ethane Halogenation

What this question tests

This question assesses your understanding of the mechanism of free radical substitution involving alkanes and halogens (specifically ethane and chlorine). You need to recall the propagation and termination steps of the mechanism to identify which specific radical species are generated and present during the reaction path.

Question 14

Identifying Radicals in the Mechanism of Ethane Chlorination

Correct Answer: C (Only 2 and 3)

✅ Correct Answer Breakdown

The correct option is C because chlorine radicals ( Cl• ) and ethyl radicals ( C₂H₅• ) are both active intermediates produced and consumed during the propagation steps of the radical substitution mechanism.

💡 Key Knowledge

  • Initiation: Cl₂ -> 2Cl• (creates chlorine radicals).
  • Propagation Step 1: C₂H₆ + Cl• -> C₂H₅• + HCl (creates ethyl radicals).
  • Propagation Step 2: C₂H₅• + Cl₂ -> C₂H₅Cl + Cl• (consumes and regenerates radicals).
  • Hydrogen radicals ( H• ) are never formed in standard alkane halogenation mechanisms because the C-H bond undergoes heterolytic or homolytic attack yielding HCl , not free hydrogen atoms.

🧠 Exam Technique

Write out the propagation steps mentally or on your rough paper if you are unsure. Look at each proposed species and ask: "Does this species appear in the propagation cycle?" Cl• and C₂H₅• appear repeatedly, whereas H• does not.

❌ Common Errors

Students often incorrectly assume that substitution of a hydrogen atom means free H• atoms must be floating around in the mixture. Remember that the hydrogen atom is abstracted by a chlorine radical to form stable HCl molecules, not hydrogen radicals.

Mark Allocation: 1 mark for selecting option C.

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 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.