H NMR Question

[Spiker]

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How many discrete signals will be present in the proton NMR spectrum of 1,3-propanediol?

I always thought only Hydrogen attached to the C counts in NMR is that wrong?

 

[Hemichordate]

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[capn jazz]

This is an AAMC question and you cannot post this without an AAMC warning. You should post it IN the specific thread for this test. Do not ruin AAMC tests for the other people on this forum!!

 

[amine2086]

This is an AAMC question and you cannot post this without an AAMC warning. You should post it IN the specific thread for this test. Do not ruin AAMC tests for the other people on this forum!!

Please post this in the appropriate forum. Also, I know this question has been asked before so if you do a search you should be able to find the answer.

 

[Spiker]

there I changed the question

 

[sleepy425]

I always thought only Hydrogen attached to the C counts in NMR is that wrong?

That's sort of wrong. It depends on the deuterated solvent you use and how exchangeable the proton is. If you dissolved 1,3-propanediol in D2O (deuterated water), the alcohol protons will exchange rapidly with the protons from the D2O so you wouldn't see them in the NMR spectrum. However, if you dissolved the 1,3-propanediol in CDCl3 (deuterated chloroform), you would see all protons because the solvent does not have exchangeable protons.

Sometimes even protons attached to carbons (in your analyte) can exchage. A good example of this is oxaloacetate. If you look up the structure of oxaloacetate, the only protons it has are two protons in the alpha position of a ketone and a carboxylic acid. (Oxaloacetic acid, the conjugate acid, also has acidic carboxylic acid protons, but we already know these exchange rapidly). Anyway, those alpha protons I mentioned are very acidic because they are in between a carbonyl and a carboxyl group. The NMR signal of oxaloacetic acid in water (D2O) shows no protons, because those alpha protons exchange, and the carboxylic acid protons also exchange. In deuterated DMSO (a non exchanging deuterated solvent), you see multiple peaks (in reality, the spectrum is a bit complicated since it exists partially in its enol form) but the point is, changing solvent can change your spectrum significantly if you have exchangeable protons.

If they don't specify the solvent, always assume that exchange does NOT occur.