[souzami4]

Hello,

I'm trying to complete OWL (online homework) exercises and cannot understand how to calculate chemical shifts for a given molecule in order to label shielded and de-shielded hydrogens.

The OWL question: The chemical environment of protons can be deduced form their chemical shifts. What are the relative chemical shifts of the protons at the labeled positions. [Protons that are highly shielded have low delta (chemical shift) values.]. The problem is attached as an image file.

How would I go about figuring out this problem? I thought that you first identify the more electronegative atom and then find the neighboring hyrdrogens. The hydrogens closes to the more EN atom are the most de-shielded and those furthest away are more shielded. I do not know where chemical shifts come into the analysis.

A step-by-step breakdown of how you would go about this type of problem would be helpful.

Thank you!

[Vidya]

Most shielded proton will have high chemical shift value.If H is bonded to more electronegative C atom than it is more deshielded and shifts to down-field.So you have to check the electronegativity or electron withdrawing nature of the atoms closer to H .Electron withdrawing may be due to - resonance,hybridization,electronegativity of atoms

[souzami4]

We haven't gotten to benzene rings yet.  Why are the ring protons more deshielded then the proton nearest oxygen?  So there's no calculation needed? Just look at the rings, bonds and location of hydrogens near EN atoms?
-Thanks!

[Vidya]

No calculations are required
Ring H is more deshielded due to sp2 nature of C atom and due pi electrons movement in the ring.C next to ether is less deshielded due lone pairs on O atom which it can push in the direction of the C atom to which protons are bonded.Ether O is electron donating and hence protons near to ether O are more shielded in comparison to others.

[souzami4]

Great! Thanks for the *delete me*

[marcal]

We haven't gotten to benzene rings yet.  Why are the ring protons more deshielded then the proton nearest oxygen?  So there's no calculation needed? Just look at the rings, bonds and location of hydrogens near EN atoms?
-Thanks!

it is called anysotropic affect (I´m not sure that the translation is fully OK). It happens in aromatic rings because of the particular interaction of the aromatic electrons and the magnetic field. It produces the opposite effect you would expect (on your level, as I understand). Probably you´ve been told that if a proton is surrounded of C´s and H´s it´ll be more shielded and therefore be at higher field (that is to say to the right of the spectrum). That is mostly true but in aromatic rings the effect is, as I said, opposite. I could go on here, but you´ll probably be covering this soon enough and with diagrams and schemes will make much more sense that me trying to explain.

I hope it helps!

[souzami4]

Thank you! Yes, your explanation was helpful.