Assuming this image is correct, why is the Carbon-3 more deshielded than Carbon-2 and Carbon-1? From what I understand, deshielding is the greatest when the Carbon atom is closer to electron withdrawer and it appears Carbon-3 is the farthest.
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asked Jun 12, 2014 at 1:05
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1$\begingroup$ Do a search of 'electron withdrawing groups' and 'electron donating groups'. You'll see several cases where resonance forms are drawn out to illustrate which carbons are most or least shielded depending on the classification of the group attached to the aromatic ring. $\endgroup$– Ari Ben CanaanJun 12, 2014 at 7:27
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1 Answer
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Resonance effects come into play along with the inductive effects. I've drawn a few of the resonance structures for benzaldehyde below, notice how resonance effects place some positive charge on carbons 2, 3 and 5 which results in their deshielding. I might have guessed that the combination of resonance and inductive effects would place C2 below C3, but that's not the case. At least C5 is at lowest field and C2 and C3 are close and both are below C1. The interplay of inductive and resonance effects is complex and estimating the magnitudes of these effects can get tricky.
