AQA A-Level Chemistry Paper 3, 2017: Question 29

1 mark · Medium difficulty · Multiple Choice

Determine the number of peaks in the 13C NMR spectrum of benzo[b]thiophene.

Practise this question

Question

Question 29 asks: 'How many peaks does this compound have in its 13C spectrum?' The skeletal formula shows benzo[b]thiophene, consisting of a benzene ring fused to a five-membered ring containing a sulfur atom and a double bond. Four multiple-choice options are provided: A (5), B (6), C (7), and D (8).
Question text

How many peaks does this compound have in its 13C spectrum?

[1 mark]

A 5

B 6

C 7

D 8

Mark scheme

Show the mark scheme Mark scheme table shows that for question 29, the correct answer is D.

29 D

How to answer it

Determining ¹³C NMR Chemical Environments in Fused Heterocycles

What this question tests

This question assesses your ability to deduce the number of unique carbon environments (and thus the number of separate signals in a ¹³C NMR spectrum) for a fused bicyclic aromatic compound (benzo[b]thiophene). It tests molecular symmetry recognition, identifying bridgehead/quaternary carbons, and understanding the electronic influence of heteroatoms (S).

Question 29: Peak Count in ¹³C NMR Spectrum

Multiple Choice Analysis • [1 Mark]

✅ Correct Answer

D — 8

Award 1 mark for selecting D.

💡 Key Knowledge

  • Each unique (non-equivalent) carbon environment gives rise to exactly one signal in a ¹³C NMR spectrum.
  • Symmetry reduces the number of peaks; an asymmetric molecule shows as many peaks as there are carbon atoms.
  • Heteroatoms (such as sulfur) alter electron density unevenly across fused rings, completely destroying potential planes of symmetry.
  • Bridgehead carbons (carbons shared by both rings) are quaternary and count as distinct carbon environments.

📐 Step-by-Step Carbon Environment Audit

Total formula of benzo[b]thiophene is C₈H₆S. Let us systematically verify every carbon:

Carbon Position Type Environment & Chemical Connectivity Equivalence
C2 =CH– Directly bonded to S atom in 5-membered ring Unique (1)
C3 =CH– Bonded to C2 and the bridgehead carbon C3a Unique (2)
C3a >C= (junction) Bridgehead carbon adjacent to C3 (not directly bonded to S) Unique (3)
C7a >C= (junction) Bridgehead carbon directly bonded to electronegative S Unique (4)
C4 =CH– Benzene ring CH, ortho to C3a Unique (5)
C5 =CH– Benzene ring CH, meta to C3a Unique (6)
C6 =CH– Benzene ring CH, meta to C7a Unique (7)
C7 =CH– Benzene ring CH, ortho to C7a (close to S) Unique (8)

Conclusion: Because there is no line or plane of symmetry in benzo[b]thiophene, all 8 carbon atoms are in unique chemical environments → 8 peaks (Option D).

🧠 Exam Technique: Symmetry Check

  • Step 1: Count total carbons first. Here: 6 in the benzene ring + 2 in the thiophene ring = 8 carbons.
  • Step 2: Look for a plane of symmetry. Draw a line straight through the middle horizontally:
    • Top has a sulfur atom ( –S– ).
    • Bottom has a double bond ( –CH=CH– ).
    Since S ≠ CH, there is no horizontal symmetry.
  • Step 3: Draw a vertical line:
    • Left side is a 5-membered ring.
    • Right side is a 6-membered benzene ring.
    Since 5 ≠ 6, there is no vertical symmetry.
  • No symmetry means: Number of peaks = Total number of carbons = 8 .

❌ Common Student Traps

  • Assuming benzene ring symmetry (Picking 5 or 6): Students often falsely assume the 6-membered ring carbons exist in pairs (like typical 1,2-disubstituted benzenes). Because the two attachment points (C3a and C7a) are different, the top and bottom of the benzene ring are NOT equivalent.
  • Forgetting bridgehead carbons: Overlooking the two ring-junction quaternary carbons, counting only CH carbons (giving 6 peaks).
  • Confusing ¹H and ¹³C NMR: Counting proton environments instead of carbon environments. (Note: ¹H spectrum would give 6 peaks, because there are 6 hydrogens!).

Topics

Organic Chemistry · 3.3.15 Nuclear Magnetic Resonance Spectroscopy

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.