I was presented with the following compound and asked to comment on the splitting of the proton $\ce{H_a}$ in $\ce{^1H}$ NMR.

Molecule of concern

The question presented today was:

What is the chemical shift (below) and splitting pattern for $\ce{H_a}$ up to and including $\ce{^4J}$?

I relabelled the molecule as shown:

Molecule of concern relabelled

but I was left stuck here. I know that $\ce{H_a}$ will be split by $\ce{H_d}$ and $\ce{H_e}$ to give a doublet of doublets (dd) but I was wondering, since the molecule is asymmetric. Now

are the protons $\ce{H_c}$ and $\ce{H_d}$ chemically equivalent but magnetically inequivalent

to yield the final splitting pattern as dddd?

  • $\begingroup$ And I find the wording of the question by your teacher a bit weird.The interesting point is wether a 4J coupling is large enough to become observable! $\endgroup$ – Karl Dec 30 '19 at 17:07
  • $\begingroup$ @Karl that was how the question was worded to me! I was just wondering if Hb and Hc are magnetically inequivalent, so they split the Ha signal with two different coupling constants? $\endgroup$ – vik1245 Dec 30 '19 at 17:56
  • $\begingroup$ @Karl done. I've shortened the question down to grasp the main point and clarified any grammatical errors. $\endgroup$ – vik1245 Dec 30 '19 at 18:02
  • $\begingroup$ Cool. ;) Ahem, a hint: you notice the methyl group at the bottom. I contains a lot of electrons. How do Hb and Hc feel about that? $\endgroup$ – Karl Dec 30 '19 at 18:11
  • 1
    $\begingroup$ @Karl Hb would feel a stronger magnetic field strength of the methyl then Hc as the methyl group and Hb are in the same plane so they would be at different chemical shifts as a result - That's what I would say is my answer to your question. Is that right? $\endgroup$ – vik1245 Dec 30 '19 at 19:40

The two protons Hb and Hc are also chemically inequivalent, because they have a different chemical environment with the methyl group, i.e. they don´t have the same chemical shift.

In a chiral molecule, the protons in a methylene group are rarely really chemically equivalent. There is usually a preferred orientation.


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