NMR H bonding

[chiddler]

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"An alcohol is distinguishable from an ether by its broad peak between 1.0 ppm and 5.0 ppm...The peak is broad due to hydrogen bonding within the alcohol."

If it's relevant, this is comparing pentanol and ethyl propyl ether.

I don't remember this from ochem class. Why is this the case? I thought alcohol protons form a tiny little bump downfield in an NMR spec don't they?

 

[MT Headed]

Nmr tries to analyze intramolecular hydrogens. Unfortunately when two molecules have -OH groups, they start (hydrogen) bonding with each other, making broad smears in the spectrum. The loose H is sorta bonded to two molecules (one via a sigma bond, one via hydrogen bond).

Carboxylic acids are worse: each OH group can hydrogen bond with the carbonyl of one other acid and vice versa. The damn things practically create dimers in solution.

 

[Erythropoietin]

Yes you observe a small peak but even though it is small, it is broad as well (you may not realize it). But if you have taken orgo 2 lab, you probably worked with HNMR's and you do see a broad peak for OH groups. The broadness of the IR spectra for alcohols is also due to hydrogen bonding. To differentiate between 1-pentanol and ethyl propyl ether, realize that ethyl propyl ether has only one multiplet, whereas 1-pentanol has three multiplets on carbons number 2,3, and 4.