CIE A Level Chemistry (9701) exams from 2022

Revision Notes

8.1.7 Deuterated Solvents in Proton NMR

Deuterated Solvents in Proton NMR

  • When samples are analysed through NMR spectroscopy, they must be dissolved in a solvent
  • Tetramethylsilane (TMS) is a commonly used solvent in NMR
  • Despite TMS showing one sharp reference peak on NMR spectra, the proton atoms can still interfere with peaks of a sample compound
  • To avoid this interference, solvents containing Deuterium can be used instead
    • For example CDCl3
    • Deuterium (2H) is an isotope of hydrogen (1H)
  • Deuterium nuclei absorb radio waves in a different region to the protons analysed in organic compounds
  • Therefore, the reference solvent peak will not interfere with those of the sample

Use of D20 in Identifying O-H & N-H Protons

Identifying -OH & -NH signals on a 1H NMR spectrum

  • The proton in -OH is not affected by its neighbouring molecular environments
  • As a result, the hydrogen atom of -OH group appears as a singlet
  • This is due to the hydrogen atom readily exchanging with hydrogens atoms of water molecules or any acid that may be present
  • When interpreting 1H NMR spectra of amines and amides, the same exchanging phenomenon can be seen
  • Protons of these functional groups exchanging leads to changes in their chemical shift ranges
  • This table shows the range of chemical shifts for -OH and -NH- protons
  • Their surrounding molecular environment has a direct impact on this range

The range of chemical shifts for -OH & -NH- protons table

Using D2O

  • Deuterium oxide (D2O) can be added to correctly identify -OH and -NH- protons
  • Adding a small quantity of this solvent ‘removes’ the peaks from the spectrum

 

The -OH proton and the -NH proton both undergo the same exchanging process as seen before. This time with a deuterium atom (D) of D2O

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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