A chemist uses high-resolution NMR spectroscopy to identify an unknown organic compound. The compound produces three distinct peaks in its ¹H NMR spectrum. Explain how a chemist uses ¹H NMR spectroscopy to determine the structure of an organic compound, including how the number of peaks, chemical shift values, and integration (relative peak areas) each contribute to identifying the compound.

Edexcel A-Level Chemistry (9CH0) — 19.1 NMR and chromatography · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

An unknown organic compound is analysed using ¹H NMR spectroscopy. The spectrum shows three peaks at chemical shift values of 1.2 ppm, 2.4 ppm, and 9.8 ppm, with relative peak areas in the ratio 6:2:1.

Model answer (5 marks)

The chemist first notes that three distinct signals mean there are three different types of hydrogen environments in the molecule. The chemical shift of each signal tells the electronic environment of those hydrogens; 1.2 ppm is typical of alkyl protons, 2.4 ppm indicates protons adjacent to a heteroatom or a carbonyl, and 9.8 ppm is characteristic of an aldehyde proton. Integration gives the relative number of hydrogens in each environment – a 6:2:1 ratio means six protons give the 1.2 ppm signal, two protons give the 2.4 ppm signal and one proton gives the 9.8 ppm signal. Using TMS as the 0 ppm reference, the chemist matches these data to a plausible structure; for example a molecule such as 3‑methylbutanal (CH₃CH₂CH(CH₃)CHO) fits the pattern: the CH₃ groups give the 1.2 ppm signal (6 H), the CH₂ next to the carbonyl gives 2.4 ppm (2 H) and the aldehyde proton gives 9.8 ppm (1 H). Thus, by combining the number of peaks, their chemical shifts and the integration ratios, the chemist can deduce the full structure.

Examiner tips

  • Identify the number of signals to count distinct proton environments;
  • Use the shift values to assign functional groups (alkyl, carbonyl, etc.);
  • Apply integration to determine how many protons belong to each signal;
  • Match the combined data to a plausible structure, citing TMS as 0 ppm.

Common mistakes

  • Confusing integration ratio with absolute proton count;
  • Misassigning the 9.8 ppm signal as a different functional group;
  • Ignoring the reference of TMS and reporting shifts incorrectly.

Mark scheme (5 marks)

  1. The number of peaks indicates the number of different chemical environments of hydrogen atoms (protons) in the molecule.
  2. Chemical shift values indicate the type of chemical environment each set of protons is in, allowing identification of functional groups present.
  3. Integration (relative peak areas) shows the ratio of the number of protons in each environment; a ratio of 6:2:1 indicates 6, 2, and 1 proton(s) respectively.
  4. TMS (tetramethylsilane) is used as the reference standard, set at 0 ppm, against which all chemical shift values are measured.
  5. By combining information from the number of peaks, chemical shift values, and integration, the chemist can deduce the full structure of the molecule; the data here is consistent with a molecule such as 3-methylbutanal (or similar aldehyde with a CH₃CH₂– and (CH₃)₂CH– type environment).

Key terms in this question

¹H NMR spectroscopy · chemical shift · integration

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