3
$\begingroup$

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?

$\endgroup$
  • $\begingroup$ 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
3
$\begingroup$

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.

| improve this answer | |
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.