AQA A-Level Chemistry Paper 2, 2021: Question 6

7 marks · Hard difficulty · State/Explain/Describe

Isomerism and Spectroscopy in C₅H₁₀O This question explores structural and stereoisomerism in molecules with molecular formula C₅H₁₀O, including identification of optically active and geometric (E/Z) isomers. Students are also tested on their ability to distinguish enantiomers experimentally using polarimetry, and apply NMR spectral data to deduce structures of cyclic ether isomers.

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Question

AQA A-Level Chemistry Paper 2, 2021: Question 6
Question text

06 This question is about isomers with the molecular formula C5H10O

06.1 Draw the skeletal formula of a branched chain aldehyde with molecular formula

C5H10O that is optically active.

[1 mark]

06.2 Describe how you distinguish between separate samples of the two enantiomers of

the branched chain aldehyde C5H10O

[2 marks]

06.3 Draw the E and Z forms of a structural isomer of C5H10O that shows both

optical and geometric isomerism.

[2 marks]

E isomer Z isomer

06.4 Isomer J is cyclic and has an ether functional group (C–O–C)

Isomer J has only three peaks in its 13C NMR spectrum.

Isomer J

Draw two other cyclic isomers of C5H10O that have an ether functional group and only

three peaks in their 13C NMR spectra.

[2 marks]

Mark scheme

Show the mark scheme Mark scheme for AQA A-Level Chemistry Paper 2, 2021: Question 6

Question Answers Additional Comments/Guidelines Mark

06.1

Use Plane polarised light M1

06.2

rotates (the plane of) in opposite directions M2

Must be E isomer M1

06.3

Must be Z isomer

M2

Allow 1 mark out of 2 for 2 correct structures but

shown in the wrong boxes

– A-LEVEL CHEMISTRY – –

M1

06.4

M2

How to answer it

Study Guide: Optical, Geometric, and Structural Isomerism

Q1. Drawing an Optically Active Aldehyde

Structure needed: A branched aldehyde with a chiral centre.

Correct answer:
CH₃CH(CH₃)CH₂CHO

To be optically active, the molecule must have a carbon bonded to four different groups.

Q2. Distinguishing Enantiomers

Method: Use plane polarised light .

  • Pass light through each sample
  • The two enantiomers rotate the plane of polarised light in opposite directions
Always mention the light is plane polarised and specify direction of rotation differs.

Q3. Drawing E and Z Isomers That Also Show Optical Isomerism

Target compound: Must have a C=C double bond with two different groups on each carbon (for E/Z) and a chiral centre (for optical).

Correct structures:

  • E isomer: CH₃CH=C(CH₃)CH(OH)CH₃
  • Z isomer: CH₃CH=C(CH₃)CH(OH)CH₃ (with swapped groups around the C=C)
Over 70% of students scored zero here — many missed the need for both geometric and optical isomerism.
Make sure one carbon in the double bond connects to CH₃ and H, the other to CH₃ and a chiral C (with OH and other groups).

Q4. Cyclic Ethers with Three 13C NMR Peaks

Requirements:

  • Molecular formula C₅H₁₀O
  • Ether group (C–O–C)
  • Only 3 peaks in 13C NMR → symmetry!

Correct structures include:

  • Isomer 1: Tetrahydropyran (6-membered ring with O)
  • Isomer 2: 1,2-dimethyltetrahydrofuran (5-ring with O and symmetrical methyls)
Count unique carbon environments when answering any NMR question — symmetry reduces the number!

Topics

Organic Chemistry · 3.3.15 Nuclear Magnetic Resonance Spectroscopy · 3.3.7 Optical Isomerism

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