[rleung]

Hi,

Sorry to be such a bother, but I have yet another chemical shift problem that is related to my other post on the topic.  In the top structure, I do not understand why proton b has a chemical shift of 6-7, which is greater than the proton shift of proton c (5-6).  I would think that proton shift would hvae the greater chemical shift since it is surrounded by a double bond on one side and a -COH group on the other.  

I drew a resonance structure of the top structure and put it underneeath, assuming "mesomerism" is responsible for this, but I do not really even know what mesomerism is and why it gives proton b a higher chemical shift.

Thank you so much.

Ryan

[Winga]

Generally, mesomeric effect = resonance effect. (mesomeric is an old term)

The reason is, the +ve charged C makes Hb to become less shielding due to resonance effect.

The inductive effect decreases with the no. of intervening bonds.
Hc is not direct bonded to C=O group.

In reality, there are even more factors should be counted in.
Overall, the mesomeric effect is greater.