Zipzap

QUESTION: Describe all the protons that are chemically equivalent to each other, and magnetically to each other. Then, give the Pople notation describing the proton spin system(s) in the molecule. Full marks for proper justification http://imgur.com/VXhQl ATTEMPT TO SOLVE THIS: For the methyl that's 'dashed', I know that the 3 protons on it are chemically and magnetically equivalent. For the sake of naming it, I assigned it as A3. However, I forgot 'why' it is that this is the case. Can somebody remind me of the reasoning behind this? For the wedged proton, I figured that it just stood on its own. Since it's the only proton on that carbon, I called it B. I seem to be having great trouble assigned the cis protons, though. While I know that there exists a symmetrical axis between the two, I also know that "cis"-coupling is close to ~11 Hz. If I go by the symmetry argument, they are chemically equivalent. Following up on that, they would not be magnetically equivalent due to the different coupling paths they would take towards the wedged proton. If this was the case, I would assign them as MM'. On the other hand, my gut (for a lack of better words) tells me that they are not chemically equivalent due to the fact that "coupling is never observed between chemical shift equivalent nuclei, be it from symmetry or by accident, not because the Bn field disappears but because spin transitions that would reveal the coupling are forbidden by symmetry" (taken from http://orgchem.color.../splitting.html). ^It's possible that my justification for either option is lacking, which is why I'm also posting this question here for feedback. If described by Pople Notation, I either have two options: A3BMM' if I use the argument by symmetry, or A3BMX if I use the argument by coupling. What are your thoughts, fellow forum members?

You are amazing, man. The analysis makes total sense! Help with the carbon spectrum would be appreciated, and it seems like it would be a lot easier to come up with given that the peaks are simpler and more obvious.

So far, me and my fellow classmates have come to this structure: http://www.sigmaaldrich.com/catalog/product/aldrich/247634?lang=en&region=CA Based on having read its NMR, I can conclude that this is the structure I have. However, it's the explanation of the shifts that's really throwing me off.

Yeah, it's most definitely only 3 oxygens. I'll post up the bicyclic structures I tried in a few hours, it's midnight where I live. =P

So far, I have experimented with bicyclic compounds as a possibility for my NMR (i.e. two connected rings). I spoke with my professor today, and she said that while I haven't gotten it right yet, I'm definitely on the right track and getting closer to the answer. A helpful hint that I got was "consider the possibility that you could have more than two rings for your compound". I'm currently working on tricyclic compounds and getting much closer to an answer, but I'm still struggling a bit. I'll wait until I see what you've posted, and that'll probably help make a more useful discussion. I also wanted to take the time to say how much I appreciate your help with this, my brain is almost fried from trying to solve it!

UPDATE: Turns out that I was given the wrong molecular formula for that spectra, it should be C9H8O3. Would that make things easier? I'm now getting closer to the answer, but I'm still stuck.

Here's the images of my spectra: http://imgur.com/m02Qr,yIDM0 Well, the only sort of bonds I could see with those DBEs would be a ring structure (not phenyl though), carbonyl, and a double bond. I was considering the possibility of a triple bond, but I don't think that the 13C NMR would agree with me. All I know is that a CH2 HAS to be right beside two deshielding groups, otherwise I wouldn't get the signal at 6.31. Am I on the right track? EDIT: After taking a second look at my proton NMR, what does 'sc' mean (it's assigned to 3.60)?

Thanks for replying, hypervalent_iodine! The peaks are put in there twice because I copied them straight out of the raw data that we were given. If it would help you more, I can post a picture of the actual NMRs we were given. I had the same thought you did about the phenyl ring being unlikely, and your input made me realize that there's most likely not a phenyl ring. However, I need to somehow create that deshielding at 6.31. My ideas have been to put a CH2 right next to two double bonds, or right next to two esters (I could play around with attaching it to the oxygen or the carbonyl itself in different combinations). I guess what I'm having most trouble with is the individual shifts at the lower ppm's. They all only have one hydrogen, along with having doublets. It doesn't really help. =( Did you have any particular hints that you could lend me so that I can figure it out for myself?

Hey guys, I'm having trouble determining the structure of a compound based on some NMR that I have been given. Here is what I've been given: Molecular Formula: C8H6O4 1H NMR: 1.57 (d, J=8.66 Hz, 1 H) 1.77 (d, J=8.66 Hz, 1 H) 3.51 (d, J=3.00 Hz, 1 H) 3.51 (d, J=3.00 Hz, 1 H) 3.60 (sc,J=7.56, 3.00 Hz, 1 H) 3.60 (sc, J=7.56, 3.00 Hz, 1 H) 6.31 (s, 2 H) 13C NMR: 46.10 (s, 1 C) 46.10 (s, 1 C) 47.20 (s, 1 C) 47.20 (s, 1 C) 52.80 (s, 1 C) 135.60 (s, 1 C) 135.60 (s, 1 C) 171.50 (s, 1 C) 171.50 (s, 1 C) So, here's what I've been able to figure out thus far: I decided to first find the degree of unsaturation in this molecule, which comes out as 6. From that, my first thought was that I could get 4 degrees from a benzene ring, and 2 degrees from 2 carbonyls (somehow) attached to the ring. Based on the highest signal of the carbon NMR, I know that one of the carbons is going to be some sort of R(CO)X derivative, and thus I won't have any hydroxyls within my final structure. Given the highest signal on my proton NMR, I figured that there were only going to be 2 symmetrical hydrogen atoms (somewhere) on my ring. Either that, or the remote possibility of putting a CH2 right between two esters. My biggest problem comes from the fact that I have two proton signals in the 1.57-1.77 range, and the 3.51-3.60 range. This tells me that I have some carbons NOT involved in the ring, but at the same time I have carbons that are adjacent to either side of the ester groups. I've tried designing a few structures, but they always keep falling short of either the molecular formula, the integration of each shift, or the group I tried adding doesn't match with the observed deshielding. What are your thoughts, guys? All feedback is greatly appreciated!