January 21, 2022, 05:26:46 AM
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Topic: Methoxybenzene C13 spectrum - Why are the ortho's the most shielded?  (Read 9594 times)

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Offline raging_nucleus

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        The aromatic carbons in the methoxybenzene spectrum give a shift pattern where the meta carbons are the most deshielded (129.5ppm), followed by the para carbon (120.8ppm), and then the ortho carbons (114.1ppm), where a typical benzene carbon chemical shift is 128.5ppm. There are other peaks, but I am most concerned with these three. I know that the pi-bonds within the benzene ring structure can resonate slightly with oxygen to form a negative charge on the ortho and para carbons, but why would the ortho carbons be more shielded than the para bond if both can resonate? It would make sense that the oxygen would withdraw electron density and make the ortho's more deshielded, but I am having trouble coming up with a reason (that doesn't come from my a**) as to why the ortho's are indeed less deshielded than the para. I was thinking that the oxygen would not resonate very often as doing so would create a positive charge on it, so it would more likely form a negative charge on the nearest carbon and resonate right back, leaving the para carbon less negatively charged. Any help or insight would be appreciated, thank you.

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