Edexcel A-Level Chemistry AS Paper 2, November 2021: Question 1

1 mark · Easy difficulty · Multiple Choice

Identify the type of isomerism shown by two given alkene structures.

Practise this question

Question

Multiple-choice question 1 showing the skeletal structures of two alkene isomers. Below are four options: A carbon chain, B positional, C E/Z, and D cis/trans. The total for question 1 is 1 mark.
Question text

1 The structures of two isomers are shown.

What kind of isomerism is shown by these molecules?

A carbon chain

B positional

C E/Z

D cis/trans

(Total for Question 1 = 1 mark)

Mark scheme

Show the mark scheme Mark scheme for question 1 showing that the only correct answer is C (E/Z) for 1 mark, with explanations for why options A, B, and D are incorrect.

Question Answer Mark

Number

1 The only correct answer is C (E/Z) (1)

A is not correct because there are no differences in the carbon chain

B is not correct because the functional group (C=C) is in the same position in both molecules

D is not correct because cis/trans isomerism does not assign groups in order of priority

(Total for Question 1 = 1 mark)

How to answer it

Identifying Types of Stereoisomerism in Alkenes

What this question tests

This question assesses your ability to distinguish between different types of structural and stereoisomerism in organic molecules, specifically focusing on why certain alkene stereoisomers are classified using the E/Z priority system rather than older nomenclature.

Question 1 Breakdown

Multiple-Choice Analysis

✅ Correct Answer: C (E/Z)

The two skeletal structures show restricted rotation around a carbon-carbon double bond (C=C) with different spatial arrangements of groups on each carbon atom, requiring Cahn-Ingold-Prelog (CIP) priority rules to classify them.

💡 Key Knowledge

  • Stereoisomerism: Compounds with the same structural formula but with a different arrangement of atoms in space.
  • Restricted Rotation: Caused by the pi (π) bond in the C=C double bond, preventing free rotation.

🧠 Exam Technique

When looking at isomers containing a C=C double bond, immediately check whether the groups attached to each carbon of the double bond are different. If they are, it is a stereoisomer. Next, evaluate whether cis/trans or E/Z is more rigorous.

❌ Common Errors

  • Choosing D ( cis/trans ): Students often default to cis/trans . However, cis/trans requires at least one hydrogen atom on each carbon of the double bond, which is not always the case for complex substituted alkenes. Edexcel expects the more rigorous E/Z IUPAC system.
  • Confusing with Structural Isomers: Selecting A or B shows a failure to recognize that the carbon skeleton and functional group positions are identical in both drawings.
Total for Question 1 = 1 mark

Topics

Organic Chemistry · Topic 6: Organic Chemistry I

Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 2, November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.