AQA A-Level Chemistry Paper 3, 2021: Question 23

1 mark · Medium difficulty · Multiple Choice

This question tests your understanding of stereoisomerism in compounds with C=C double bonds. Specifically, it focuses on E–Z isomerism, which occurs when each carbon in the double bond is bonded to two different groups. You are asked to identify the only compound that meets this condition.

Practise this question

Question

AQA A-Level Chemistry Paper 3, 2021: Question 23
Question text

23 Which compound has E–Z isomers?

[1 mark]

A CH2=CHBr

B CH2=CBr2

C CHBr=CHBr

D CBr2=CHBr

Mark scheme

Show the mark scheme Mark scheme for AQA A-Level Chemistry Paper 3, 2021: Question 23

23 C 1 CHBr=CHBr

How to answer it

Multiple Choice Explained: E–Z Isomerism

Correct Answer: C – CHBr=CHBr

Why? This molecule has two different groups on each carbon of the double bond. That’s the requirement for E–Z (geometric) isomerism: restricted rotation due to the double bond and two different substituents on each of the double-bonded carbons.

Why A is incorrect:

CH2=CHBr does not have two different groups on the left carbon (CH2). Both attached atoms are hydrogens, so E–Z isomerism is not possible.

Why B is incorrect:

CH2=CBr2 fails the test on both carbons. The left carbon is a CH2 (two identical H atoms), and the right has two Br atoms — again, no chance of having different groups on both carbons.

Why D is incorrect:

CBr2=CHBr has two identical Br atoms on the left-hand carbon of the double bond. Since there's no group difference there, E–Z isomerism isn't possible.

Topics

Organic Chemistry · 3.3.1 Introduction to Organic Chemistry · 3.3.4 Alkenes

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.