[qw098]

Hi guys,

I have this HNMR of my benzil product, and I am asked to use a coupling diagram to explain the appearance of the multiplet (triplet I think in my case) between 7.675ppm and 7.75ppm. The part of the question that gets me.. is that it says "I will need to account for more than three bond coupling".

I have drawn my coupling diagram on the top right of the document attached... but I have the feeling it might be wrong because I haven't accounted for three bond coupling... though I am not sure how I would do so for a coupling diagram!

Any help would greatly be appreciated!
Thanks!

[fledarmus]

It seems to me that between 7.675 and 7.75 you have a little more than just a triplet. At the risk of giving up too much information, I would call it a triplet of triplets. How might you account for that, given the structure of your compound?

[qw098]

I would call it a triplet of triplets.

Ah, ok. I was thinking it might be a doublet of doublets but not a triplet of triplets.

When I labeled each hydrogen in my benzil molecule, I determined that the peaks from 7.675-7.75 were due to hydrogen b), 8-8.05 due to hydrogen a), and 7.55-7.60 due to hydrogen c). From Hydrogen b), when I looked at the structure of benzil, hydrogen B was coupling with two chemically distinct protons (Ha and Hc) so wouldn't that make it a doublet of doublets? Why a triplet of triplets?

Please find the picture of benzil with my labeled hydrogens.

Thanks!

[fledarmus]

I think the three bond coupling would change your assignment.

If you look at proton c, you see a single proton being split by two protons b in identical magnetic environments. This would give you a triplet. Looking out an additional carbon, you could see that it could be further split by two additional protons in identical magnetic environments - each peak of the triplet caused by b would be further split into a triplet by a. Being three bond coupling, the coupling constants a-c would be far smaller than b-c, and you would see a clear triplet of triplets.

Consider proton b - it is being split into a doublet by the adjacent a and a doublet by the adjacent c, but since the splitting constants are approximately the same for a and c (both being aromatic protons the same distance away), you get something that looks more like a triplet, where one peak from each large doublet lie almost on top of each other. There is additional splitting from the other b proton, and a very tiny amount of splitting from the other a proton. So, something that looks like a very messy triplet.

Finally, proton a - it is split by a single adjacent proton b to give a doublet. That doublet is further split into doublets by both c and the opposite a, giving something which might be sort of a doublet of triplets, or possibly a doublet of very tight doublet-of-doublets. Finally, each of those peaks is double very slightly by the opposite b proton.

How does this line up with your spectrum?

[qw098]

Wow, this is all interesting stuff, but unfortunately I was never taught such things!

So based on what you have just said, the peaks from 7.675-7.75 were due to hydrogen c), 8-8.05 due to hydrogen a), and 7.55-7.60 due to hydrogen b). Hydrogen b) and hydrogen c) were initially mislabeled by me then?

Based on what you said this makes perfect sense! The coupling diagram should not be that hard now considering I know it's a triplet of triplets now . I just hope that I labeled everything correctly... as in my chemical shifts are assigned to the correct hydrogens!

Thanks

[qw098]

After working really hard, and looking through many textbooks/internet I came up with this coupling diagram.

I was wondering if anyone can tell me if this is correct. I have put a lot of effort in trying to understand this concept as I have JUST been introduced that the "n+1" is not fully correct and I have to use advanced techniques such as "w" coupling.

Please look at the attached document.

Thank you all!

[fledarmus]

Don't forget the B-B' couplings - meta couplings are usually larger than para couplings.

[qw098]

Ah, ok! Great! Thanks, I modified my picture and it looks like this now:

I hope my diagram makes sense as I really feel as though I understand this very well now! This is awesome!

Now to calculate the coupling constants of each.. uh-oh!

PS: The top hydrogen is labeled HA and the bottom coupling constant is labelled JBA'. It got cut off when I scanned