Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: Addison on June 15, 2007, 07:49:44 PM
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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?
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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.
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see, if you look close you can see the dq structure (blue = one q, green the other):
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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?
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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.
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Thanks for the explanation, I understand it a lot better now.