Identifying O-H and N-H Protons with D2O

Protons in O-H (alcohols, carboxylic acids) and N-H (amines, amides) groups are hard to identify from a ¹H NMR spectrum:
• their chemical shift varies with solvent, concentration and hydrogen bonding, so the data sheet gives a wide range (for example R-OH 0.5-6.0 ppm)
• they usually appear as a broad singlet, because they exchange rapidly between molecules and so do not split, or get split by, neighbouring protonsTo confirm which peak is O-H or N-H, the spectrum is run twice:
1. Record the ¹H NMR spectrum as normal.
2. Add a few drops of deuterium oxide, D₂O, shake, and record the spectrum again.The O-H and N-H protons exchange with deuterium:R-OH + D₂O → R-OD + HODR-NH₂ + 2D₂O → R-ND₂ + 2HODDeuterium does not absorb in the range of a ¹H NMR spectrum, so the O-H or N-H peak disappears. C-H protons do not exchange, so all other peaks stay the same.Worked example: the spectrum of ethanol shows peaks at about 1.2 ppm (triplet, 3H), 3.7 ppm (quartet, 2H) and 2.6 ppm (broad singlet, 1H). After shaking with D₂O, the peak at 2.6 ppm has gone, so this peak is the O-H proton.