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Topic: Proton NMR - unexpected peak from alpha-beta unsat. ester?  (Read 5493 times)

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Offline Addison

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Proton NMR - unexpected peak from alpha-beta unsat. ester?
« on: June 15, 2007, 07:49:44 PM »
I've been trying to identify an unknown compound and I'm nearly certain that I've succeeded, but there is one part of the HNMR that has confused me. I've included the IR and H NMR data below. NMR peaks are close estimates because some of the peaks didn't display a value, for some reason.

IR:
Ester peak at 1725, C=C at 1662

H NMR:
4 signals
doublet at ~1.9 ppm
singlet at 3.7 ppm
doublet at ~5.8 ppm
sextet at ~6.9 ppm

The unknown appears to be methyl crotonate, but I wouldn't expect to see a sextet in that compound's H NMR. Is there some principle of NMR that I'm forgetting that explains this? The H in question has only 4 three bond neighbors.

Do the s-trans and s-cis conformations affect this somehow?


Offline kiwi

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Re: Proton NMR - unexpected peak from alpha-beta unsat. ester?
« Reply #1 on: June 15, 2007, 08:06:25 PM »
you are right, the unknown is methyl crotonate. The "sextet" isn't a sextet, but a complex doublet of quartets, resulting from two different sets of coupling to the C3 alkene H, one coupling to the methyl H's (giving a quartet), which is split again by the coupling across the alkene to the C2 H.

Offline kiwi

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Re: Proton NMR - unexpected peak from alpha-beta unsat. ester?
« Reply #2 on: June 15, 2007, 08:19:23 PM »
see, if you look close you can see the dq structure (blue = one q, green the other):

Offline Addison

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Re: Proton NMR - unexpected peak from alpha-beta unsat. ester?
« Reply #3 on: June 15, 2007, 08:52:59 PM »
So, are we also looking at a similar effect in the coupling of other H's near the C=C? Doublet of doublets for the methyl H's and another doublet of quartets for the other alkenic H?

Offline kiwi

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Re: Proton NMR - unexpected peak from alpha-beta unsat. ester?
« Reply #4 on: June 16, 2007, 04:16:52 AM »
The origin of the dq effect isn't that all the spins are coupled, but that both the Me and H2 spins are independently coupled to H3 (i.e Me <-> H3 <-> H2). so you wouldn't expect the same effect to give rise to complex fine structure for the methyl group and H2, but, these peaks may have fine structure from other effects.You'd have to take a close look at the spectrum you have. The C2 H will have a strong coupling to the C3 H, and may also have a weak coupling to C4 (making it a doublet of doublets, dd), but the long distance coupling will be weak, and whether you see it or not will be dependent on field strength.

PS. and while we're brushing up on NMR, the peak i posted before also is a good example of second order effects - note the deviation in intensity from the expected 1:2:2:1:1:2:2:1 pattern for a dq.

Offline Addison

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Re: Proton NMR - unexpected peak from alpha-beta unsat. ester?
« Reply #5 on: June 16, 2007, 09:15:49 AM »
Thanks for the explanation, I understand it a lot better now.

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