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In an aldol condensation reaction, I synthesized a molecule out of two unknown reactants. These reactants I found to be 4-chlorobenzaldehyde and acetophenone which made 4-chlorochalcone. I was given an H NMR spectrum to identify and label the hydrogens for the molecule but am having trouble reading it!

There is a quintuplet with an integration of 6 but no where in the molecule am I seeing 6 hydrogens that share a chemical environment (I counted 7 environments). Is this possible or could I have come up with the wrong product structure?

Any help deciphering this would be great!

HNMR

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    $\begingroup$ The region corresponding to 6 hydrogens doesn't have to be 6 equivalent hydrogens. It could be 6 hydrogens that are in a similar chemical environment. So that could be two different hydrogen types in a 1:5 ratio, or a 2:4 ratio, or a 3:3 ratio. Or it could be three hydrogen types in a 1:1:4, or a 1:2:3 ratio, or a 2:2:2 ratio. And so forth up to six hydrogen types in a 1:1:1:1:1:1 ratio. $\endgroup$ – jerepierre Mar 29 '16 at 21:42
  • $\begingroup$ Based on the structure, you have already determined correctly that there are 7 hydrogen types (environments). How many of each type do you expect? What multiplicity do you expect for each? What chemical shift makes the most sense for each environment? Assign the peaks as best you can for your prediction, and whatever is left is likely in that multiplet with integration 6. $\endgroup$ – jerepierre Mar 29 '16 at 21:43
  • $\begingroup$ Interesting... But I don't see 6 similar hydrogens. I see 5 hydrogens on the phenyl on the left that could be similar. But where is the sixth? I'll try assigning them and see where it gets me! Thank you! $\endgroup$ – Salma Mar 29 '16 at 21:47
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    $\begingroup$ The integrals in a quintuplett should be 1:4:6:4:1. Pascals triangle. Not here. The dubletts at 7.4 and 7.77 both look suspiciously like they are part of an AB coupling each. $\endgroup$ – Karl Mar 29 '16 at 23:55
  • $\begingroup$ Could it possibly be an overlapping doublet triplet doublet? What would that mean for the structure? $\endgroup$ – Salma Mar 30 '16 at 14:22
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As with all NMR analysis problems like this, it is good to have a strategy to interpret data. Perhaps rather than looking at your spectrum and trying to rationalise your structure, look at your structure and predict what you might expect to see in your spectrum. You'll find it much easier to rationalise the appearance of your peaks, especially in the case of overlapping peaks as you have here. So, in your molecule, what splitting would you expect?

You have 11 proton environments, so your integrals look fine. Your chloroform doublet is a bit of a worry though. Perhaps your sample needs to be mixed a little better? You've got 3 distinct spin systems that will give characteristic splittings:

  • trans double bond. These two peaks will appear as distinct doublets, with couplings of around 12-17Hz. I can see one very clearly. The other, which we know to be upfield of the one at 7.77 because of the tenting is probably under the mess of stuff between 7.5-7.6. Being next to the carbonyl will have strong influence on the chemical shifts of these, and through a bit of electron-arrow pushing we can show that the proton closest to the carbonyl is shielded, and the other deshielded. Hence, the signal at 7.77 is likely to be your proton β to the carbonyl group (and closest to the para-substituted benzene ring)
  • para-substituted benzene ring. Again a distinct splitting pattern; these will look like a pair of doublets (actually a second order pattern) with coupling about 7.5Hz. I can see one probably at 7.4. Its partner will also be under the mess between 7.5 and 7.6
  • a mono-substituted benzene ring, which will have three sets of signal, with ratio 2:2:1. A carbonyl group is strongly electron withdrawing, deshielding the ortho protons, which is probably the signal up at about 8. The others.... under the mess between 7.5-7.6.

This is clearly a homework/lab problem, so this should be enough to keep you going until the next question.

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