OCR A-Level Chemistry Synthesis and analytical techniques (02), November 2021: Question 13

Analyzing 13C NMR Spectra Isomers

What this question tests:
This question assesses your ability to determine the number of distinct carbon environments in organic molecules using molecular symmetry. You must translate structural formulas (including cyclic ketones, aliphatic ketones, and substituted aromatic rings) into expected peak counts on a carbon-13 ( 13 C) NMR spectrum.

Question 13

Exam Breakdown & Correct Answer

✅ Correct Answer: Option B (Only 1 and 2)

Compound 1 (cyclohexanone) and Compound 2 (butan-2-one) both produce exactly four peaks in their respective 13 C NMR spectra due to molecular symmetry.

🧠 Exam Technique

Do not just count carbons! Instead, look for planes of symmetry or rotational axes. Equivalent carbon environments share identical local chemical environments and collapse into a single peak.

Mark Scheme Allocation: 1 mark for selecting B. (AO2.1 - Application of knowledge and understanding of analytical techniques).

Detailed Analysis by Compound

Compound 1: Cyclohexanone

Contains a plane of symmetry splitting the ring vertically through the carbonyl carbon (C=O) and the opposite CH₂ group.
Carbon environments: C=O (1), C-2/C-6 equivalent (2), C-3/C-5 equivalent (3), C-4 (4).
Total 13 C peaks = 4. (Matches target!)

Compound 2: Butan-2-one ( CH₃CH₂COCH₃ )

Contains four structurally distinct carbon positions with no internal symmetry.
Carbon environments: Terminal methyl (C1), methylene (C2), carbonyl carbon (C3), and methyl attached to carbonyl (C4).
Total 13 C peaks = 4. (Matches target!)

Compound 3: 1,4-dimethylbenzene (p-xylene)

Highly symmetrical aromatic ring with two perpendicular planes of symmetry.
Carbon environments: Two methyl carbons (equivalent), four equivalent ring carbons bonded to hydrogen, and two equivalent ring carbons bonded to methyl groups.
Total 13 C peaks = 3. (Incorrect — does not match target of 4).

Common Pitfalls to Avoid

❌ Where Students Lost Marks

Confusing 1H NMR with 13C NMR: Students sometimes try to apply splitting patterns or integration rules to 13 C spectra. Remember: 13 C spectra consist of single peaks (singlets due to proton decoupling, ignoring 13 C- 13 C coupling rarity). You are solely counting environments!
Miscounting cyclic carbons: For cyclic molecules like cyclohexanone, failing to trace symmetry lines leads to overcounting peaks. Always fold the molecule along its line of symmetry.
Rushing aromatic rings: Students often assume substituted benzenes have 6 ring peaks. High symmetry in para-substituted benzenes dramatically reduces this to just 2 ring carbon environments + 1 substituent environment = 3 peaks total.