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Hydroxyl group on HNMR spectra+++

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miss_molecule:
Hi,

I am doing a synthesis of 1.1-diphenylbutane-1.3-diol. I am supposed to discuss the NMR-spectra, but it seems quit complicated!

This is what I have done so far:

chemical shift - multiplicity - hydrogens - coupling constants - assignment
1.2ppm - doublet - 3 - 6.4 - CH3...
2.3ppm - Quartet - 1 - 8.4 - ...
2.5ppm - quartet (or two doublets?) - 1 - 5.3 - ...
3.1ppm - doublet - 1 - 3.2 - ...
3..8-3.9ppm - multiplet- 1 - * - ...
4.8ppm - singlet - 1 - * - ...
7.2-7.5 - multiplet - 10 - * - H-Ar
+some impurities

I feel really lost here...

Why are the two hydroxyl groups so different? One is a singlet, and the other one is about 14 signals! I am supposed to draw a coupling tree for this one... And why does the CH2 result in two different peaks? Does it have something to do with cis/trans? And what about the coupling constants?? They really seem wrong...
I don't have to discuss the aromatic region though, and I am happy for that:)

thanks a lot if you choose to help me!

StarvinMarvin:
Hmmm, just a wild guess: are you sure you are dealing with one enantiomer? Because maybe if you are dealing with a racemate, some of the signals would actually be doubled. What is more, the CH2 protons might also appear separately (like in sugars).

Squirmy:

--- Quote from: miss_molecule on May 03, 2009, 02:51:04 AM ---Why are the two hydroxyl groups so different? One is a singlet, and the other one is about 14 signals!
--- End quote ---

Are you sure the one with 14 signals is an OH? I think it's something else.


--- Quote ---And why does the CH2 result in two different peaks? Does it have something to do with cis/trans?
--- End quote ---

Those two H's are diastereotopic. I believe I linked a wiki page about topicity before, but here it is again. If there's anything in it that you need cleared up, please let us know:
http://en.wikipedia.org/wiki/Homotopic_groups


--- Quote ---And what about the coupling constants?? They really seem wrong...
--- End quote ---

It's hard to say from what you've posted, because I'm not sure how you obtained them. Could you show how you obtained them?

Any chance you could scan in a copy of the NMR?

miss_molecule:
Hi again..

I will try to scan my spectra tomorrow, and post my coupling constant calculations.

I see now that it can't be the OH causing the multiplet, but then i really don't know what it is...:(

So the two Hydrogen atoms on the same carbon causing two different  signals, are different in R/S not cis/trans? I remember having learned about this some years ago... And I believe that this was a part that I really understood. Thanks!

Is a racemate the mix of two enantiomers?

StarvinMarvin:
racemate is a 50:50 mixture of two enantiomers. (it's easy to check really, run an optical rotation experiment - racemates are optically inactive). However, NMR can't show you the difference between enantiomers (you'd have to add chiral shift reagents to see it). Diastereomeric protons, though, can be seen in the NMR as different peaks with different chemical shifts and coupling constants.

Scan the spectrum (if you have a 13C spectrum it would also help a bit) and post it.

Perhaps COSY spectrum would tell you more about the relationships between your mysterious peaks

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