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Unread 01-09-2005, 01:40 AM
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Default Answered: HNMR chemical shift characteristic?

Hi, I'm trying to justify why that specific proton has such chemical shift characteristic.The molecule I'm working with is cyclopentadiene. I know that two protons attached on one of five carbon has 2.89ppm and the proton attached to the next carbon has 6.46ppm and the one on next carbon has 6.57ppm. I can't seem to justify the reason for that pattern.Another molecule is anisole. The methiyl group attached to the ester oxygen has 3.79ppm and the proton attached to carbon that is at ortho position with oxygen has 6.88ppm and the next proton(meta position) is 7.29ppm and the next one (para position with oxygen) is 6.92ppm. The pattern also doesn't make sense to me.Third molecule is acetophenone and the pattern of chemical shift seems to make sense to me since further the proton from the ketone oxygen decrease in chemical shift. can you please help me with explaining why?
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Best Answer - Posted by Supertramp_82
I'm not sure what is confusing you about cyclopentadiene. The methine (CH) protons are part of alkenes and the methylene (CH2) is doubly allylic, next to two alkenes. Anisole is an electron-rich aromatic molecule. By resonance, electron density is donated to the ortho and para positions shifting those protons upfield. The meta positions don't have much of a different electron density than benzene, so they are near 7.27. The opposite effect should be observed in acetophenone, since the ketone will withdraw electron density from the meta positions, and not really from the ortho/para positions. Hope that helps. Something else that you might consider about cyclopentadiene is the fact that the cyclopentadienyl anion is aromatic, so the bonds to the methylene protons may be adjusted so there is partial negative charge on that carbon.

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Default HNMR chemical shift characteristic?

I'm not sure what is confusing you about cyclopentadiene. The methine (CH) protons are part of alkenes and the methylene (CH2) is doubly allylic, next to two alkenes. Anisole is an electron-rich aromatic molecule. By resonance, electron density is donated to the ortho and para positions shifting those protons upfield. The meta positions don't have much of a different electron density than benzene, so they are near 7.27. The opposite effect should be observed in acetophenone, since the ketone will withdraw electron density from the meta positions, and not really from the ortho/para positions. Hope that helps. Something else that you might consider about cyclopentadiene is the fact that the cyclopentadienyl anion is aromatic, so the bonds to the methylene protons may be adjusted so there is partial negative charge on that carbon.

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