[baum0372]

I am wondering why some proton NMR's show the alcohol hydrogen and similar NMR's by classmates do not.  Does it have to do with the pH of the solution during the extraction?  Curious to know.  Thanks.

[Dan]

OH groups are acidic enough that they can undergo proton exchange with water, alcohols and other relatively acidic molecules. So that H on the OH is not in one place but is constantly moving around, getting passed from molecule to molecule. This causes peak broadening of the OH signal, and if the exchange process is sufficiently fast on the timescale of NMR the peak may be so broad it is indistinguishable from the baseline.

In the context of this problem, the main factor that speeds up the exchange process is the presence of other OH groups in high concentration. The presence of contaminant water (or other relatively acidic molecules) will cause broadening - possibly to obscurity. NMRs run using D₂O or ROD as the solvent will not show the OH peaks because the H exchanges for D.