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NMR Spectrum
#1 Posted : Sunday, July 12, 2020 5:42:19 AM
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I am wondering how we can infer what functional group a molecule will have in regards to NMR spectra? Not sure if this is specific enough of a question...
For example, on page 260 in the chemistry CC, I am not sure how we know what the molecule will look like from the chemical formula and the NMR spectra.
I just kind of confused as to what we are really looking for? There are a couple examples in the CC of MCAT questions but I am wondering what NMR questions the MCAT would test?

Thank you!
Hopefully this isn't too confusing!
#2 Posted : Sunday, July 12, 2020 5:45:21 PM
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Hey Keshav!

NMR is actually a really powerful technique, and there is a way you can determine the functional groups that are present in the spectra.

By looking at their shifts, you can have a good idea of the groups that are present.

For example, if you see a proton NMR with a shift of about 12 ppms, you are most likely looking at a proton from the carboxylic acid group.

For a full list of functional groups and their locations, check out this link:
Proton NMR Regions (Scroll down, its about mid-page)

If you would like to generalize the trend, usually, protons near lots of electron withdrawing groups (due to high electronegativity) will have higher ppms (such as carbocylic acids, with two oxygens), whereas protons near atoms that are not very electron withdrawing will have lower ppms (such as carbons from the alkyl chain, or CH3-CH2-CH3).

For your second question, NMR can be quite extensive, and take some time to solve. I would be surprised if you would be asked to solve an NMR, because its so time-consuming. But, it is still fair game, so definitely brush up on it. I would say, that the most likely question you will come across, would be something along the line of you being asked to identify a possible reaction by looking at an NMR, which will tell you what functional groups might have changed. For example, if there was an oxidation of a carboxylic acid to an alcohohol, you will see that the NMR peak at 12 ppm will be present in the reactant, but not the product.

Hope this helps, happy studying! :)
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