A2 · Practice questions · Proton NMR spectroscopy

Read the spectrum.

Six original Cambridge-style questions on proton NMR spectroscopy.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Peaks, shift, area, splitting.

Number of peaks = number of H environments.
Shift (Data Booklet) = type of H; TMS is 0.
Area ratio = number of H in each environment.
n+1 rule: n H on adjacent carbons give n+1 lines; O–H vanishes with D₂O.

Recall

[3 marks]

Give three reasons why tetramethylsilane, Si(CH₃)₄, is used as the reference in proton NMR.

All 12 hydrogens are equivalent, so it gives a single strong peak, defined as δ = 0. ✓
Its peak is upfield of (to the right of) almost all organic protons, so it does not overlap them. ✓
It is inert, non-toxic and volatile, so it does not react with the sample and is easily removed. ✓

Analysis

[4 marks]

Predict the high-resolution proton NMR spectrum of ethyl ethanoate, CH₃COOCH₂CH₃: number of peaks, approximate δ, relative areas and splitting.

Three environments, so three peaks in the ratio 3 : 2 : 3. ✓
CH₃–C=O: about 2.0, 3H, singlet (no H on the adjacent carbon). ✓
O–CH₂: about 4.1, 2H, quartet (3 H on the adjacent CH₃). ✓
CH₃ of the ethyl group: about 1.3, 3H, triplet (2 H on the adjacent CH₂). ✓

Explain

[2 marks]

Explain why the spectrum is recorded in CDCl₃ rather than CHCl₃.

CHCl₃ contains a hydrogen, which would give its own peak and could hide sample peaks. ✓
Deuterium does not absorb in the proton NMR region, so CDCl₃ gives no signal. ✓

Analysis

[3 marks]

Compound Y, C₃H₇Cl, gives two peaks: a doublet at 1.5 (6H) and a seven-line peak at 4.1 (1H). Identify Y and explain the splitting.

Y is 2-chloropropane, CH₃CHClCH₃: two equivalent CH₃ groups (6H) and one CH (1H). ✓
Each CH₃ is next to the CH with 1 H: 1 + 1 = 2 lines, a doublet. ✓
The CH is next to 6 equivalent H: 6 + 1 = 7 lines; it is shifted to 4.1 by the Cl on the same carbon. ✓

Analysis

[3 marks]

Describe how proton NMR distinguishes propanone, CH₃COCH₃, from propanal, CH₃CH₂CHO.

Propanone: one peak, a singlet at about 2.1 (6H). ✓
Propanal: three peaks in the ratio 3 : 2 : 1, including the –CHO proton at about 9.8. ✓
Propanal also shows splitting: CH₃ triplet, CH₃CH₂CHO multiplet, CHO triplet. ✓

Explain

[2 marks]

The spectrum of an unknown compound has a broad singlet at 3.0. Describe a test, using NMR, to show whether it is an O–H proton.

Add a few drops of D₂O, shake, and record the spectrum again. ✓
If the peak at 3.0 disappears, it was an O–H (or N–H) proton, exchanged for D, which gives no proton signal. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.