Hnmr ?

[MedPR]

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From mcat-review

How does the hydrogen on OH affect the CH2 peaks? I thought the number of peaks = the number of adjacent hydrogens + 1? So shouldnt there be 4 peaks.. 3 from CH3, 1 from OH, and +1?

I'm also not really sure why the OH hydrogen only has 1 weak even though it has 2 adjacent hydrogens? Really the only one that makes sense to me is the peak at 1ppm..

 

[milski]

I think splitting happens only between hydrogens attached to C atoms. So you'll have 4 peaks because of the 3 hydrogens in CH3 and 3 peaks for the 3 hydrogens in CH3 because of the CH2 group. The H in OH is not going to be split because it's not connected to a C atom. That's ochem though, I might be incorrect.

 

[milski]

Since I'm reviewing NMR tonight anyway, here is a quote from my textbook:

"The -OH proton of a primary alcohol RCH2OH is vicinal to two protons, and its signal would be expected to be split into a triplet. Under certain conditions signal splitting of alcohol protons is observed, but usually it is not. ... The reason that splitting of the hydroxyl proton is not observed is that it is involved in rapid exchange reactions with other alcohol molecules."

 

[SaintJude]

Since I'm reviewing NMR tonight anyway, here is a quote from my textbook:

"The -OH proton of a primary alcohol RCH2OH is vicinal to two protons, and its signal would be expected to be split into a triplet. Under certain conditions signal splitting of alcohol protons is observed, but usually it is not. ... The reason that splitting of the hydroxyl proton is not observed is that it is involved in rapid exchange reactions with other alcohol molecules."

Thanks milski! That's good to know. Could see that as a good passage based question asking for the curious absence of the the hydroxyl proton.