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Aida_5230
#1 Posted : Sunday, July 05, 2020 5:44:18 PM
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Hi, I just have a few questions about aromaticity. Firstly, why is a cyclopentadienyl anion considered aromatic? I understand that the pi electrons fit huckel's rule, however isn't the carbanion sp3 hybridized because it is attached to 4 electrons groups (the lone pair, the hydrogen, and both carbons)? However, online sources tell me it is sp2 hybridized and I don't understand why...

As well, I'm having trouble understanding why bases are aromatic or not... For example, for cytosine, the internet tells me it has 6 pi electrons because you include the ones on the NH, however why doesn't it count the lone pair electrons on the double bonded N (shouldn't it be 8 pi electrons using that logic)?

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INSTR_Katerina_102
#2 Posted : Sunday, July 05, 2020 6:00:38 PM
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Firstly, why is a cyclopentadienyl anion considered aromatic? I understand that the pi electrons fit huckel's rule, however isn't the carbanion sp3 hybridized because it is attached to 4 electrons groups (the lone pair, the hydrogen, and both carbons)? However, online sources tell me it is sp2 hybridized and I don't understand why...

Recall that a lone pair that participates in resonance must be in a p orbital - that forces your geometry to be sp2 as a result. Without this resonance, you are correct and a carbanion would a tetrahedral parent shape. This is very similar to the amide bond, where the nitrogen may look like an sp3 hybridized atom, but has in fact significant sp2 character.


As well, I'm having trouble understanding why bases are aromatic or not... For example, for cytosine, the internet tells me it has 6 pi electrons because you include the ones on the NH, however why doesn't it count the lone pair electrons on the double bonded N (shouldn't it be 8 pi electrons using that logic)?


https://ibb.co/dgWrwWZ

I've drawn the main tautomers where you can clearly see the aromaticity of nucleotides. This is what you'd look at when you are considering what to leave in and leave out for counting aromaticity.

Please let know if this is unclear.
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