[Goala]

Hi guys,

I have this structure shown below and I put this structure in the H-NMR Predictor. It popped out with the H-NMR shown below.

I showed an arrow to each hydrogen and to its corresponding chemical shift. I was wondering why the hydrogen closest to the oxygen was not the one most downfield (highest ppm) but instead the hydrogen right next to it was most downfield?

If someone could explain this to me it would be great!

Thanks!

[fledarmus]

Look at the polarization of your a,b-unsaturated ketone. Your oxygen is electron withdrawing and will be partially negative, while the carbonyl carbon will be partially positive. This will further polarize the alkene so that the a carbon is partially negative, and the b carbon partially positive. The hydrogen on the partially negative carbon will be shielded and move slightly upfield, the one on the partially positive carbon will be deshielded and move slightly downfield.

[Honclbrif]

I'm going with the resonance argument on this one.

[Goala]

Hmmm, interesting! Thanks guys!

I never looked at a molecule in the manner you did fledarmus.. I really like it! A new tool in my bag of tricks

For the two hydrogens (protons) which are adjacent to the chiral center carbon (chlorine and methyl attached to the chiral carbon)... those two hydrogens are chemically distinct because they are adjacent to a chiral center, right?

Those two hydrogens are chemically distinct (different chemical shifts) because they are adjacent to a chiral center...; am I correct in assuming that it is impossible to know what each of those hydrogen's chemical shifts are based on the NMR spectra that is shown. I know their chemical shift is between 2.75-3 but I don't know which hydrogen corresponds to which peaks on the 2.75-3 range because they can't unambiguously be distinguished.

Are my above-mentioned assumptions/statements correct?

[fledarmus]

I'm going with the resonance argument on this one.

You're right, my method is a shorthand I've used for too many years. I should have presented it in resonance terms - that's the explanation for why my method works anyway.

[fledarmus]

Those two hydrogens are chemically distinct (different chemical shifts) because they are adjacent to a chiral center...; am I correct in assuming that it is impossible to know what each of those hydrogen's chemical shifts are based on the NMR spectra that is shown. I know their chemical shift is between 2.75-3 but I don't know which hydrogen corresponds to which peaks on the 2.75-3 range because they can't unambiguously be distinguished.

Are my above-mentioned assumptions/statements correct?

Yes they are distinct because they are adjacent to a chiral center. No it isn't impossible to know which is which, but it is very difficult. You have to look very closely at the electron density on the chlorine side of the ring (chlorine being a large atom with 3 lone-pair electrons situated at a considerable distance from the nucleus) and on the methyl side (carbon being a small atom with three protons in pretty close). I've seen some good hand-waving arguments for which proton should be where, but they all seem to occur after the spectrum has been taken and the hydrogens unambiguously identified by other methods such as through-space interactions or coupling constants. It would be easier if the Cl was a phenyl ring or another aromatic system - it is fairly easy to predict the anisotropic effect on chemical shift.

If I had to make a prediction, I would guess that the hydrogen on the same side of the ring as the chlorine would be slightly shielded by the electron cloud on the chlorine and thus slightly upfield of the other one. And I would give myself about a 50% chance of being right.

[Goala]

Thanks a lot

Makes great sense!