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 + HOD R-NH2 + 2D2O → R-ND2 + 2HOD Deuterium 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.
• 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: