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I am working on a straight-forward NMR graph as part of my test prep. I have a pretty limited understanding of NMR, but I thoroughly read the chapter leading up to this practice question and am having difficulty explaining all of the peaks that I am seeing. Let me show the question and my understanding:

Here is the chemical and the trace that results. enter image description here

My understanding of this problem is that there are four groups of equivalent hydrogens, and I numbered them 1-4. The 1st group has 3 equivalent hydrogens, with two adjacent non-equivalent hydrogens (group #2). Using the n+1 rule, the H group #1 should create 2+1=3 peaks in a cluster. Same goes for H group #4, since there are two adjacent hydrogens, so another 3 peaks. When considering H-group #2, the three hydrogens from H-group #1 should be 3+1=4 peaks. Same goes for H-group #3, which has the three hydrogens from H-group #4 so 3+1=4.

So, I am expecting to see 2 groups of 3 peaks and 2 groups of 4 peaks. I see the two groups of 4 peaks, but not the 2 groups of three peaks. Instead, I see 1 group of 6 peaks.

Am I supposed to believe that the 2 groups of 3 are just located so near it appears to be 6? Or am I missing something?

Thanks for any help!

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  • $\begingroup$ AS the answers correctly say, the two methyl peaks overlap. It is worth having a play with NMR simulation tools (many structure drawing packages like ChemDoodle now include NMR simulators). These can help develop a good intuition about what to expect from spectra. $\endgroup$ – matt_black Apr 21 '14 at 13:06
  • $\begingroup$ Sounds like a good idea, thanks for the recommendation! $\endgroup$ – jake9115 Apr 22 '14 at 19:58
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I did a simulation of the peaks (I got the shifts wrong) for 60 MHz and 400 mHz. As you can see, the peaks overlap on the weaker magnet. Ethyl Acetate 60 mHz Ethyl Acetate 400 mHz

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  • $\begingroup$ Wow, thanks for the graphical comparison, that really helps me visualize what is going on $\endgroup$ – jake9115 Apr 21 '14 at 13:31
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You are correct. The chemical shift of methyl protons 1 and 4 are close to one another. You still see the two expected triplets, they just have similar chemical shifts. That's not too surprising since they are both methyl groups attached to a methylene group. We could tell which triplet belongs to which methyl carbon by measuring and comparing the 2 sets of CH2-CH3 coupling constants.

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