[girlsailor]

Hi there! I'm working on determining the structures of two compounds used in an aldol condensation reaction. I'm pretty bad at NMR and am having trouble.

First compound: C₈H₈O
I believe this is a ketone, based on the shift of three hydrogens at ~2.6 ppm. Here is a photo of the spectrum. On the top right corner, I've drawn what I believe the structure is - if anyone could confirm or let me know a problem area, I would appreciate it!




Second compound: C₈H₆O₃
I think this one is the aldehyde, based on the shift of one hydrogen at ~9.8 ppm. My first instinct was a benzaldehyde with 2 alcohol substituents... but this doesn't seem to be working. I am just kind of lost. If anyone could point me in the right direction, I would appreciate it.

[discodermolide]

The first is indeed the ketone.
The second spectrum is an aldehyde but what is the 2H signal at 6ppm, can you figure it out? How many aromatic protons do you have?

[girlsailor]

Thanks for confirming the first one!

For the second.. here's what I've got from fiddling around.



I think the two protons on the left (labeled E, I don't know how well you can see that) are at 6ppm. And the proton labeled A on the right is ~10ppm, the aldehyde proton. What I'm struggling with now is figuring out how the 3 other protons correspond to the spectrum, like which one is which.. Is the peak at 7.33 a real doublet? or a singlet?

[discodermolide]

Yes, that is the structure I got as well. The signal at 7.33 is a doublet it is one of the aromatic protons, which one?

[girlsailor]

Thanks for your response!

Hm, I'm a little confused about the splitting. I thought that a doublet looked like the signal at ~6.94. What does it mean when the signal does not split all the way, but only partially at the top, like at 7.33?

I was thinking that if it were a singlet, the signal at 7.33 would correspond to the proton on the top of the aromatic ring (in the picture I posted) since that is not within 3 bond lengths of any other protons -- my instructor has told me that I should only consider splitting between protons that are within 3 bond lengths' distance from each other.

That would mean that the other two aromatic protons are at 7.41 and 6.94. Those would be the two other aromatic protons that are next to each other on the ring, hence the doublet splitting. I would guess that the proton at 7.41 would be the one on the left, closer to the oxygen, since this might shift it further downfield.

However.. if the signal at 7.33 is considered a doublet, is the signal at 7.41 a doublet of a doublet? Would this mean that the aromatic proton on the bottom right (farthest from the O) would be more likely?

[discodermolide]

This aromatic region is not a first order spectrum it is an ABX spin system. Have a look here:
http://www.chem.wisc.edu/areas/reich/nmr/05-hmr-12-abx.htm