AQA A-Level Chemistry: Analytical Techniques & Structure Determination

High-resolution proton (¹H) NMR spectroscopy is one of the most powerful tools available for determining the structure of an organic molecule.

Describe and explain how a high-resolution ¹H NMR spectrum is used to deduce the structure of an organic compound. In your answer you should refer to the information provided by the number of peaks, the chemical shift ($\delta$δ, in ppm), the integration trace, and the spin-spin splitting of each signal. You should also state the purpose of tetramethylsilane (TMS) and of using a deuterated solvent.

(6 marks)

An unknown organic compound Z contains only carbon, hydrogen and oxygen. It was analysed by mass spectrometry, infrared spectroscopy and ¹H NMR spectroscopy. The data are shown below.

Mass spectrum (selected peaks)

m/z	Relative abundance / %	Note
74	22	molecular ion, M⁺·
45	40	fragment
29	100	base peak

Infrared spectrum (selected absorptions)

Wavenumber / cm⁻¹	Absorption
2500–3300	very broad
1710	strong, sharp

¹H NMR spectrum

$\delta$δ / ppm	Integration ratio	Splitting
11.8	1	singlet
2.4	2	quartet
1.1	3	triplet

(a) Show that the molecular formula of Z is consistent with C₃H₆O₂, and calculate its degree of unsaturation. (2 marks)

(b) Deduce the structure of Z, using all three sets of data to justify your answer. Identify the functional group present and assign the key peaks. (4 marks)

A compound Q has the molecular formula C₄H₈O₂ ($M_r = 88$Mr​=88). Its high-resolution ¹H NMR spectrum is summarised below.

$\delta$δ / ppm	Integration ratio	Splitting
4.1	2	quartet
2.0	3	singlet
1.3	3	triplet

There is no broad absorption between 9 and 12 ppm.

Interpret this spectrum to deduce the structure of Q. Assign each of the three signals and explain how the splitting patterns establish the connectivity. (5 marks)

The mass spectrum of an ester E (molecular formula C₄H₈O₂) shows the molecular ion and several fragment-ion peaks.

m/z	Relative abundance / %
88	18
59	65
57	100
29	80

(a) State the species responsible for the molecular ion at m/z 88 and explain what it tells you. (1 mark)

(b) For the peaks at m/z 57, m/z 59 and m/z 29, deduce the neutral fragment lost from the molecular ion and identify the ion responsible for each peak. (3 marks)

(c) Hence deduce the structure of E. (1 mark)

Two compounds, propanal (CH₃CH₂CHO) and propanone (CH₃COCH₃), are isomers with the molecular formula C₃H₆O. Both show a strong C=O absorption near 1715 cm⁻¹ in their infrared spectra, so IR alone cannot distinguish them.

Choose ONE named technique and explain exactly what difference would be observed that lets you tell propanal from propanone. (4 marks)

A small amount of tetramethylsilane (TMS), Si(CH₃)₄, is added to a sample before recording its NMR spectrum.

State what role TMS plays in NMR spectroscopy and give two reasons why it is a suitable choice of reference standard. (3 marks)