NMR spectra of -OH and -NH protons 94-95 Flashcards
Why can it be difficult to identify -OH and -NH protons?
- peaks can appear over a wide range of different chemical shift values, depending on the solvent used and the concentration of the sample. - the signals are often broad. - there is usually no splitting pattern.
What does deuterium oxide, D2O, not produce an NMR signal.
D2O is the same chemical compound as water but with the isotope of hydrogen ,2H or D,in place of 1H, the common isotope of hydrogen. This is why the common name for deuterium oxide is heavy water. Unlike 1H, deuterium does not produce an NMR signal because an odd number of nucleons is needed for an NMR.
Explain how D2O is used in stage of an NMR scan.
D2O is used by following these stages:
- First a proton NMR spectrum is run
- a small amount of D2O is added to the sample solution and the mixture is shaken
- then a second proton NMR spectrum is run, and any peak due to -OH or -NH protons disappears.
Why does D2O remove any peak due to -OH or -NH?
This works because the deuterium in D2O exchanges with H present in -OH and -NH.
For example, when D2O is added to ethanol, CH3CH2OH, deuterium exchanges with the -OH proton:
CH3CH2OH + D2O ⇔CH3CH2OD + HOD
When the second NMR spectrum is run, the material being analysed is now CH3CH2OH. In the absene of the -OH proton there will obviously be no -OH signal.
Why are the -OH and -NH NMR signals broad?
It is difficult to get solvents that are absolutely dry - traces of water in the solvent form hydrogen bonds with -OH and -NH protons in the compound being analysed. This results in the broadening of -OH and -NH signals.
What are to properties of -OH and -NH signals?
As a general rule, NMR peaks for -OH or -NH protons are not split. An -OH or -NH peak usually shows as a singlet , which may be broad.
- Protons on adjacent carbon atoms are not split by the -OH or -NH; neither is the -OH or -NH proton itself.
