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3.2.11Additional lessons

Peak Splitting in Proton NMR (HL) — IB Chemistry HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 68 of 172

📚 IB Chemistry HL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 5The n + 1 Rule

Rule
  • Rule A signal is split into n + 1 peaks by n hydrogens on the adjacent carbon atoms, provided they are in a different environment.
Singlet
  • Singlet A hydrogen group with no neighbouring hydrogens gives one peak, as for the CH₃ in ethanoate groups.
Doublet
  • Doublet One adjacent hydrogen gives two peaks of equal height in a ratio 1 : 1.
Triplet
  • Triplet Two adjacent hydrogens give three peaks in a ratio 1 : 2 : 1.
Quartet
  • Quartet Three adjacent hydrogens give four peaks in a ratio 1 : 3 : 3 : 1.
Neighbours (n)PeaksNameRatio
01Singlet1
12Doublet1 : 1
23Triplet1 : 2 : 1
34Quartet1 : 3 : 3 : 1

Part 2 of 5Applying the Rule

1
Identify each hydrogen environment, then count the hydrogens on carbon atoms directly attached to that carbon.
2
Hydrogens of the same environment do not split each other, so a CH₃ group does not split itself.
3
The OH proton usually appears as a singlet because rapid exchange averages out its coupling.
4
In an ethyl group, CH₃ is a triplet (next to CH₂) and CH₂ is a quartet (next to CH₃).
5
Of two coupled signals, the one nearer an electronegative atom, such as the OCH₂ quartet, appears at the higher chemical shift.

Part 3 of 5Worked Patterns

Worked Patterns
Ethanol
  • Ethanol CH₃ is a triplet, CH₂ is a quartet and OH is a singlet, with areas 3 : 2 : 1.
Ethyl ethanoate
  • Ethyl ethanoate CH₃CO gives a singlet, OCH₂ gives a quartet and CH₃ gives a triplet, with areas 3 : 2 : 3.
Propane
  • Propane The two CH₃ groups are equivalent and form a triplet, while CH₂ is split into a septet.
Propan-2-ol
  • Propan-2-ol The two CH₃ groups give a doublet and the single CH gives a septet.

Part 4 of 5Worked Example

Question Predict the ¹H NMR spectrum of ethyl ethanoate, CH₃COOCH₂CH₃.

  1. There are three hydrogen environmentsCH₃CO, OCH₂ and CH₂CH₃.
  2. CH₃CO has no hydrogen neighbours, so it is a singlet of area 3.
  3. OCH₂ has three neighbouring hydrogens, so it is a quartet of area 2.
  4. The terminal CH₃ has two neighbours, so it is a triplet of area 3.

Answer Singlet (3H), quartet (2H), triplet (3H)

Part 5 of 5Exam Checklist & Tips

3.2.11 – Key Points for the Exam

  • Splitting The division of a signal into several peaks by neighbouring hydrogens.
  • Multiplet A signal with several peaks, named by the number of peaks.
  • Equivalent hydrogens Hydrogens in identical chemical environments that do not split each other.
  • Coupling Interaction between the spins of hydrogens on neighbouring carbons.

Common Mistakes

  • Counting hydrogens on the same carbon when applying n + 1.
  • Splitting a signal because of hydrogens two carbons away.
  • Reversing the triplet and quartet in an ethyl group.
  • Forgetting that equivalent hydrogens do not cause splitting.

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