AQA A-Level Chemistry Paper 2, 2019: Question 7
7 marks · Medium difficulty · Long Answer
Name cyclopentanone and explain how and why it can be distinguished from cyclopent-3-en-1-ol by comparing their boiling points, 13C NMR spectra, and infrared spectra.
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Question text
07 Isomers X and Y have the molecular formula C5H8O
07.1 Give the IUPAC name for isomer X.
[1 mark]
07.2 Explain how and why isomers X and Y can be distinguished by comparing each of
their
• boiling points
• 13C NMR spectra
• infrared spectra.
Use data from Tables A and C in the Data Booklet in your answer.
[6 marks]
Mark scheme
Show the mark scheme
Question Answers Additional Comments/Guidelines
rk
Cyclopentanone Allow cyclopentan -1-one but no other numbers
G 07.1 1
Ignore spaces, commas and hyphens
This question is marked using Levels of Response. Refer to the Mark Indicative Chemistry content
Scheme Instructions for Examiners for guidance. Stage 1: boiling points
Level 3 All stages are covered and each stage is generally 1a) Y has a higher bp
correct and virtually complete. 1b) Y has H-bonds between molecules
5-6 marks
Answer is well structured with no repetition or irrelevant and X has dip-dip imf
points. Accurate and clear expression of ideas with no errors 1c) More energy required to overcome H-bonds
in use of technical terms. Mention of covalent bond breaking loses 1c
07.2
Level 2 All stages are covered but stage(s) may be incomplete or 13
may contain inaccuracies OR two stages are covered and Stage 2: C NMR
3-4 marks 13
are generally correct and virtually complete. 2a) Both have 3 peaks/absorptions in their C NMR
2b) X has peaks at 20-50 OR 190-220ppm
Answer shows some attempt at structure 2c) Y has peaks at 50-90 OR 90-150ppm 6
Ideas are expressed with reasonable clarity with, perhaps,
(Ignore peaks at 5-40ppm - present in both)
some repetition or some irrelevant points.
Some minor errors in use of technical terms Stage 3: ir
3a) X has a peak (for C=O) at 1680-1750 cm-1
Level 1 Two stages are covered but stage(s) may be incomplete
3b) Y has peak (for O H) at 3230-3550 cm-1
or may contain inaccuracies OR only one stage is
1-2 marks -1
covered but is generally correct and virtually complete. OR peak (for C=C) at 1620-1680 cm
3c) They would have different fingerprint regions
Answer includes isolated statements and these are presented -1
in a logical order. (below 1500 cm )
Answer may contain valid points which are not clearly linked.
Errors in the use of technical terms.
0 mark Insufficient correct chemistry to gain a mark.
.
How to answer it
Distinguishing Isomers: Nomenclature, Intermolecular Forces & Spectroscopy
This question assesses your mastery of IUPAC nomenclature for cyclic ketones, comparing physical properties (boiling points) based on intermolecular forces (hydrogen bonding vs permanent dipole-dipole forces), predicting ¹³C NMR spectra using molecular symmetry and chemical shift ranges, and identifying distinctive functional group absorptions in infrared (IR) spectroscopy.
IUPAC Naming of Isomer X
Give the IUPAC name for isomer X.
✅ Correct Answer
Cyclopentanone
Also acceptable: cyclopentan-1-one (spaces, hyphens, and commas are ignored).
💡 Key Knowledge
- A saturated five-membered ring of carbons = cyclopentan-
- A ketone functional group (C=O on a secondary carbon) = suffix -one
- Since all five ring positions are equivalent prior to substitution, the locant number 1 is unnecessary, but allowed.
❌ Common Errors
- Omitting the cyclo- prefix (e.g., writing pentan-2-one or pentanone).
- Writing numbers other than 1 (e.g., cyclopentan-2-one), which is incorrect and rejected.
Distinguishing Isomers X and Y (Extended Response)
Explain how and why isomers X and Y can be distinguished by comparing each of their:
- boiling points
- ¹³C NMR spectra
- infrared spectra
Use data from Tables A and C in the Data Booklet in your answer.
✅ Model Level 3 Answer (Structured by Stages)
Stage 1: Boiling Points
- Difference: Isomer Y has a higher boiling point than isomer X.
- Intermolecular forces: Isomer Y contains an –OH group and therefore forms hydrogen bonds between molecules. Isomer X has a carbonyl group (C=O) and forms permanent dipole–dipole forces (and van der Waals forces) between molecules.
- Energy explanation: Hydrogen bonds are stronger than dipole–dipole forces, so more energy is required to overcome the intermolecular forces in Y.
Stage 2: ¹³C NMR Spectra
- Number of peaks: Both X and Y have a plane of symmetry, so both show 3 peaks in their ¹³C NMR spectra. (Therefore, the number of peaks alone cannot distinguish them; chemical shift values must be compared).
- Shift for X: X shows a characteristic carbonyl peak at 190–220 ppm (C=O in ketones) [and a peak at 20–50 ppm].
- Shift for Y: Y shows peaks at 50–90 ppm (C–OH) and 90–150 ppm (alkene C=C).
Stage 3: Infrared (IR) Spectra
- Distinguishing peaks: X displays an absorption at 1680–1750 cm⁻¹ due to the C=O bond. Y displays a broad absorption at 3230–3550 cm⁻¹ due to the alcohol O–H bond (or an absorption at 1620–1680 cm⁻¹ due to C=C).
- Fingerprint region: X and Y will also have different absorption patterns in the fingerprint region (below 1500 cm⁻¹).
🧠 Exam Technique: 6-Mark Levels of Response
- Use headings: Subdivide your answer clearly into the three required headings (Boiling points, ¹³C NMR, and IR).
- Level 3 (5–6 marks): Requires all 3 stages to be virtually complete and chemically sound.
- Quote exact ranges: Always transcribe data ranges straight from the Data Booklet (e.g., write 1680–1750 cm⁻¹, do not just guess a single value like 1700 cm⁻¹).
- Specify "between molecules": Always state intermolecular forces operate between molecules.
❌ Common Errors & Misconceptions
- The "Covalent Bond" Trap: Saying that "covalent bonds break when boiling" immediately forfeits the mark for energy comparison! Boiling only overcomes intermolecular forces.
- Counting C Environments Wrong: Believing Y has 5 peaks. Look closely at the symmetry: the vertical mirror plane down the C–OH carbon makes the two C=C carbons identical and the two CH₂ carbons identical (3 environments total!).
- Vague IMF Descriptions: Stating "X has Van der Waals while Y has hydrogen bonds" without crediting permanent dipole-dipole forces for the ketone carbonyl group.
- Level 3 (5–6 marks): All 3 stages covered, correct, and complete. Clear logical structure with correct terminology.
- Level 2 (3–4 marks): All 3 stages covered with minor omissions/errors, OR 2 stages virtually complete.
- Level 1 (1–2 marks): 2 stages attempted with omissions, OR only 1 stage complete.
Topics
Organic Chemistry · Physical Chemistry · 3.3.1 Introduction to Organic Chemistry · 3.3.8 Aldehydes and Ketones · 3.3.6 Organic Analysis · 3.3.15 Nuclear Magnetic Resonance Spectroscopy · 3.1.3 Bonding
Question and mark scheme from the AQA A-Level Chemistry examination, Paper 2, 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.