OCR A-Level Chemistry AS Breadth in chemistry (01), November 2021: Question 13

1 mark · Medium difficulty · Multiple Choice

Identify the total number of unsaturated isomers (structural and stereoisomers) with the molecular formula C4H8.

Practise this question

Question

Multiple choice question 13 asks for the number of unsaturated isomers, both structural and stereoisomers, with the molecular formula C4H8. Four options are provided: A (3), B (4), C (5), and D (6), alongside an answer box and a 1-mark allocation.
Question text

13 What is the number of unsaturated isomers (structural and stereoisomers) that have the molecular

formula C4H8?

A 3

B 4

C 5

D 6

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme for question 13 indicates that the correct answer is B, awarding 1 mark.

13 B 1 AO2.5 ALLOW 4

How to answer it

Counting Isomers of C₄H₈

What this question tests

This question assesses your ability to identify and count structural isomers and stereoisomers (E-Z isomers) for a given molecular formula ( C₄H₈ ). It tests your knowledge of unsaturation (alkenes and cycloalkanes) and spatial arrangement.

Question 13: Unsaturated Isomers of C₄H₈

Full Worked Solution & Examiner Guidance

✅ Correct Answer

Option B (4)

There are 4 unsaturated isomers with the molecular formula C₄H₈ .

💡 Key Knowledge

  • Degree of Unsaturation: C₄H₈ corresponds to a general formula of CₙH₂ₙ , indicating 1 degree of unsaturation (either 1 double bond or 1 ring).
  • The Question Constraint: The question specifically asks for unsaturated isomers, meaning we only consider alkenes (double bonds), not cyclic alkanes (cyclobutane/methylcyclopropane).

🧠 Exam Technique

Systematically list the carbon skeletons and move the functional group (the double bond) before checking for stereoisomerism (E-Z isomerism):

  1. But-1-ene ( CH₂=CHCH₂CH₃ )
  2. E-but-2-ene
  3. Z-but-2-ene
  4. 2-methylpropene ( CH₂=C(CH₃)₂ )

❌ Common Errors

  • Including cyclic structures: Cyclobutane and methylcyclopropane have formula C₄H₈ , but they are saturated (single bonds only). Including them leads to an incorrect total of 6 (Option D).
  • Missing E-Z isomers: Forgetting that but-2-ene exhibits geometric stereoisomerism will lead students to choose Option A (3).
Mark Scheme Note: Correct option is B (1 mark). Examiners noted that top-tier students carefully separated structural options from stereoisomers without accidentally counting saturated ring compounds.

Topics

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

Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.