[Mammal]

Hi there, part of my coursework assignment involves predicting the H-NMR for 4-Methylphenyl 2-methylpropanoate.

I predicted there would be two doublets in the 7ppm~ range relating to the benzene ring hydrogen atoms, but when I have checked with this (http://www.nmrdb.org/new_predictor/index.shtml?v=v2.34.2) H-NMR predictor there are two quartets.

I've no idea how these occur as each H atom on the benzene ring has only one J3 coupled H atom (should get two doublets with integral two?).

Any help would be appreciated!
Below is the smiles if you want to copy that into the predictor.


Cc1ccc(OC(=O)C(C)C)cc1

[Babcock_Hall]

I would not call them quartets; they look like doublets of doublets.  There is at least one complicating factor here, and there may be two (I am not sure about the second one).  This molecule has a para-type (1,4) substitution pattern; therefore, the two hydrogens that are ortho to the methyl group are not magnetically equivalent.  Likewise the two hydrogens that are ortho to the carbonyl group are not magnetically equivalent.  In order to be magnetically equivalent, the nuclei must have the same chemical shift and be coupled identically to other nuclei.  This issue is often ignored in introductory organic textbooks (which is perfectly reasonable).  However, nuclei that are not magnetically equivalent do not follow first-order coupling rules.
EDT
I made a mistake in my drawing of this compound when I examined the calculated spectrum.  The actual compound is a substituted phenol, but I reversed the position of the carbonyl group and the oxygen atom.  When I recalculated the spectrum, it became much more complex.

[jeffmoonchop]

its been ages since I've done NMR but the two quartets would come from the ipso SN3 hydrogens and the hydrogens on the other side of the molecule. Im guessing the 3D structure would mean both of the dimethyl groups would be electronically identical however there may be little shift. In addition the Hydrogens in the benzene ring would show 2 doublets slightly shifted from each other. when split down the centre the 2 nearest the methyl group is in a nonpolar region and vise versa for the 2 on the right side. However I may be totally wrong because as I say its been ages. So in conclusion I think youre right about the doublets but it has the quartets too.

[Irlanur]

However, nuclei that are not magnetically equivalent do not follow first-order coupling rules.

I would still guess that you don't see the coupling between the chemically equivalent protons. I've never seen a spectrum where this coupling is clearly resolved. However, I have not seen every spectrum in the world.

[Babcock_Hall]

I have not seen a para-disubstituted benzene derivative that produced a spectrum that is identical to the predicted spectrum of this compound.  Most of the ones that I have seen are in fact close to what one would predict with simple first-order rules.  However, occasionally one sees something else.  Scroll down to page 5-HMR-15.7 at this link for two examples:  http://www.chem.wisc.edu/areas/reich/nmr/notes-5-hmr-14-15-A2B2.pdf
EDT
The chemical shifts of the two sets of protons is not very large.  I am beginning to think that this system is AA'BB'.  If the chemical shift difference were greater, it would be AA'XX'.

[Mammal]

Thanks for the discussion guys, AA'BB' is what I decided on, we haven't covered this but it should impress if he is just expecting us to say two doublets.

[Irlanur]

AA'BB' is what I decided on

But then you should know what it means;)

[Mammal]

Hi there Irlanur, I also asked this question on a chemistry subreddit and was pointd to the above linked chem.wise page, I had a read up and while I do not completely grasp the different effects that are happening I have got enough information to predict the peaks and answer the question. Thanks everyone again

[Babcock_Hall]

Hi there Irlanur, I also asked this question on a chemistry subreddit and was pointd to the above linked chem.wise page, I had a read up and while I do not completely grasp the different effects that are happening I have got enough information to predict the peaks and answer the question. Thanks everyone again

With all due respect, I have to wonder about what you have just written.  How does one "predict the peaks" of an AA'BB' system?

[Mammal]

Well I just need a general idea of the chemical shift and then the shape of the peaks.