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Topic: chemical shift  (Read 4180 times)

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Offline rleung

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chemical shift
« on: August 05, 2005, 10:45:14 AM »
Hi,

I am confused as to why the chemical shift of the c hydrogen is 7.1 while the chemical shift of the b hydrogen is 5.8. I would assume that the shift of the b hydrogen would be larger since it is more deshielded by the presence of a carboxyl group.  Thanks.

Ryan
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Offline Winga

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Re:chemical shift
« Reply #1 on: August 09, 2005, 11:00:35 AM »
Mesomeric effect.

Try to draw the resonance structure starting from e- withdrawing C=O group.
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Offline rleung

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Re:chemical shift
« Reply #2 on: August 09, 2005, 01:27:50 PM »
Hi,

Here are the resonance structures that I come up with.  I am still not uncertain what you mean by mesomerism.  The chapter in my book does not talk about how mesomerism may affect chemical shifting.  Thanks.

Ryan
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Offline Winga

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Re:chemical shift
« Reply #3 on: August 09, 2005, 01:48:25 PM »
The terms, mesomeric effect and resonance effect are compatible.
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Offline movies

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Re:chemical shift
« Reply #4 on: August 09, 2005, 04:44:07 PM »
The beta hydrogen is a "vinylogous" aldehyde proton.  The resonance structure drawn above shows the the beta carbon is electronically similar to an aldehyde.
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