Matching a ¹H NMR spectrum to a structure
From a structure you can predict its spectrum: how many signals, which integral, which splitting and roughly which position. If you can do that, you can also go the other way.
Rules and values
- Count signals and integrals first, then check the splitting
- Position: alkyl 0.9–1.7 ppm; next to C=O or a ring 2.0–3.0; on O or a halogen 3.3–4.5; ester OCH 3.7–5.0
- Aromatic H 6.8–8.1 ppm, aldehyde H 9.5–10.1 ppm, carboxylic acid H 10.5–12.5 ppm
- OH and NH: broad, position varies
- Groups one carbon away shift a little more (CH₃ in ethanol at 1.2 instead of 0.9 ppm)
Your turn
Three problems on this topic. Type whole numbers, choose splitting and structures.
Task 1 of 3Match the spectrum
Task
Which ¹H NMR spectrum belongs to N-methylethanamine?
Spectrum
Worked example
Which ¹H NMR spectrum belongs to benzyl methyl ether?
Solution
- 1.Expected signals:
- 2.δ ≈ 7.2–7.4 ppm, m, 5 H: Ar–H (5 equivalent) protons on the benzene ring
- 3.δ ≈ 4.6 ppm, s, 2 H: CH₂ on the ether oxygen and on the benzene ring, 0 neighbouring H
- 4.δ ≈ 3.3 ppm, s, 3 H: CH₃ on the ether oxygen, 0 neighbouring H
- 5.A has 4 instead of 3 signals (1-phenylethanol).
- 6.C has 4 instead of 3 signals (2-phenylethanol).
- 7.D contains a quartet with 2 H that does not occur here (phenetole (ethoxybenzene)).
Typical mistake: Only looking at the number of signals. Integral, splitting and position (next to O, C=O or a ring) must all fit.
Common questions
How exactly do I need to know the shifts?
For the exam the ranges are usually enough. What matters is whether a signal sits next to O, C=O, N, a halogen or on a ring.