[Compaq]

Hi again. Firstly, I'm sorry to bombard this place with structure elucidation problems. It's just that it's Easter holidays, and my parents aren't exactly able to help me

Anyway, the structure I was supposed to find (and actually did conclude on) is this:



It was the ¹H-NMR that confused me - I didn't not expect such a messy spectrum:



Here's zoomed in to region around 6,4 ppm:



When I first saw this, it reminded me of aromatic protons. Around the same region, and messy. Considering the actual structure, that's obviously impossible. I didn't know the structure when I looked at this, and I wasn't really able to tell much from this.

I based my structure on the summation formula C₃H₄O, DBE=2 and that no symmetry was found in the ¹³C-spectrum. It could not be a cycloproan, not a ketone (low field absorption in proton NMR) => I was left with the allyl aldehyde

BUT, would allylic coupling + coupling to the C2-proton give that coupling pattern? That just wasn't what I expected (not sure what I expected, but not that).

I hope I've explained my problem properly I thank you for any guidance!

[Babcock_Hall]

Your second drawing looks like the carbon spectrum, not a zoomed-in H-1 spectrum.  Assuming that your proposed structure is correct, can you explain what you expected the hydrogen on C-2 to look like?

[fledarmus]

Your second drawing looks like the carbon spectrum, not a zoomed-in H-1 spectrum.  Assuming that your proposed structure is correct, can you explain what you expected the hydrogen on C-2 to look like?

And what you expected the hydrogens on C-1 to look like. Consider chemical shifts and the J-coupling you would expect from geminal coupling, cis vicinal coupling, and trans vicinal coupling

[Compaq]

Sorry, wrong image indeed!




I've just I'm starting to solve problems, and must not have thought through how cis/trans protons would affect neighboring  protons. I just didn't think it would look that messy.. more interpretable multiplicities.

But I should have seen that the CH-methine proton should couple with both the cis and trans proton, and that thecis/trans coupling would occur - and that that would be rather messy.

But, should I be able to interpret that? What usable information should I draw out from that?

[Compaq]

And what's the absorption at around 2 ppm?

[fledarmus]

Hmm, now I'm seeing a blowup of a quartet between 4 and 4.1 in your first post - if that is the same spectrum, it is quite possible your peak at ~2 is because you're seeing some ethyl acetate contaminating your sample. Do you also see a triplet at ~1.2?

As for the splitting pattern, what I see when I look at your blowup is a double of doublets centered at about 6.55 with J values of ~12 and 4 (assuming this is on a 400 MHz instrument?). There is also a doublet of doublets centered on ~6.315, with J values of around 12 and 16, and one centered at ~6.26 with J values of around 16 and 3, the right group of which is seriously attenuated for some reason. Very rough guesses on the values, but you could get better results from the peak list. There is additional fine splitting and some additional peaks, possibly due to impurities, but possibly due to the some restricted rotation about the carbonyl to alkenyl carbon single bond either showing an enol form or a resonance form which gives some double bond character to that single bond.