The infrared absorbtion peak for a carbonyl group is very sharp while that for an OH group is quite broad. Explain the origin of this difference.
Any help would be great.
Any help would be great.
IR Spec Question
- Thread starter JDAD
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actually it has NOTHING to do w/ the polarity and everything to do with the fact that the OH bond is a hydrogen bond donor and acceptor. Hence, it can absorb a very wide range of frequencies depending on the orientation (distance) between it's H-bonding partner.
it's not polarity. It is due to hydrogen bonding. That cuases a wide variety of differences in the distances and strengths of O-H bonds, both within the same molecule, and the O-H bond between two separate molecules.
C=O peaks are fun and bouncy, but don't really interact with much, so it's goign to have pretty much the same energy regardless of position relative to other molecules.
C=O peaks are fun and bouncy, but don't really interact with much, so it's goign to have pretty much the same energy regardless of position relative to other molecules.
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interesting side note that has helped me on many spectroscopy problems:
if you DO get a non-broad OH peak, it usually means a highly nonpolar, high molecular weight molecule. i remember we once got some spectra and had to figure out a large nonpolar alcohol (dodecanol i think, could be totally misspelled) and that was huge because nobody could assign the OH peak because it looked like a normal, non broad absorbance
if you DO get a non-broad OH peak, it usually means a highly nonpolar, high molecular weight molecule. i remember we once got some spectra and had to figure out a large nonpolar alcohol (dodecanol i think, could be totally misspelled) and that was huge because nobody could assign the OH peak because it looked like a normal, non broad absorbance
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Hydrogen bonding occurs due to the polarity of the O-H bond. You cannot say that something has to do with H-bonding but not with polarity - they are intimately associated.
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you can get sharp OH peaks in tertiary alcohols or alcohols that are sterically hindered.
While it is true that there is a close association between polarity and hydrogen bonding, not all highly polar species can hydrogen bond (as someone mentioned, tertiary alcohols would have a hard time doing so). The stretched out O-H peak is not an effect of polarity, as polarity in itself cannot cause a wide range of energies for a bond. Hydrogen bonding can.
