1. Purpose

The signals of OH protons of alcohols as well as of NH protons of amines are usually assigned by their broadness, by exchange with D₂O, or by their solvent- and temperature-dependent chemical shifts. This behavior is due to exchange processes. Therefore, coupling between OH protons and adjacent CH protons is usually not observed. In this experiment we demonstrate the occurrence of OH multiplets when dimethylsulfoxide (DMSO) is used as solvent. In DMSO the exchange processes are so slow that one can observe the OH group of a primary alcohol RCH₂OH as a triplet or the OH group of a secondary alcohol R₂CHOH as a doublet. Thus, these two types of alcohols can be easily distinguished. In addition, one may add a few drops of D₂O or CD₃OD regardless of the solvent used, to exchange the OH proton for deuterium to confirm the assignment.

2. Literature

K. K. M. Sanders, B. K. Hunter, Modern NMR Spectroscopy, 2nd Edition, Oxford University Press, Oxford, 1993, 220–221.
D. Martin, A. Weise, H.-J. Niclas, Angew. Chem. Int. Ed. Engl. 1967, 6, 318–334.

3. Pulse Scheme and Phase Cycle

OH proton and H/D exchange experiment pulse sequence and phase cycle

4. Acquisition

Time requirement: 10 min

Sample: 5% glycerol in [D₆]DMSO.

Record a standard ¹H NMR spectrum according to Experiment 3.1. Remove the tube from the magnet, add a drop of D₂O, and shake the NMR tube thoroughly. Record again an ¹H NMR spectrum.