spectroscopy quiz: on aromaticity (huckel's rule)

NS_Tutor_Mathias
Posts: 616
Joined: Sat Mar 30, 2019 8:39 pm

Re: spectroscopy quiz: on aromaticity (huckel's rule)

Post by NS_Tutor_Mathias » Wed Jan 29, 2020 2:59 am

Sorry, my typo: Cyclobutadiene is not aromatic. Cyclobutene is. Refer to the annotated image.

This is a common misconception in applying Hueckel's rule. You can plug in any number you desire for n, this value only exists to produce a series of possible pi-electron counts for aromaticity. So: 2, 6, 10, 14, 18, 22, 26, 30, 34 etc.

Therefore regular cyclobutene is aromatic: 2 is in that series.
But cyclobutadiene is not, because 4 is not within that series.

Regarding the challenge:
I think you're onto it, but be aware that both the nitrogen and carbonyl carbon are sp2, meaning together they form a fixed plane - that the only point of free rotation is the alpha carbon itself, as is illustrated here: https://3.bp.blogspot.com/_DZH2cmCoois/ ... e+4-06.jpg
lotus0618
Posts: 182
Joined: Sun Dec 29, 2019 5:47 pm

Re: spectroscopy quiz: on aromaticity (huckel's rule)

Post by lotus0618 » Thu Jan 30, 2020 2:33 am

Yeah, the cyclobutadiene is definitely not aromatic

regarding the peptide bond, how is the N sp2 since it also has a lone pair aside from the 3 bonds connected to the C of carbonyl, the other C, and a H?
NS_Tutor_Mathias
Posts: 616
Joined: Sat Mar 30, 2019 8:39 pm

Re: spectroscopy quiz: on aromaticity (huckel's rule)

Post by NS_Tutor_Mathias » Thu Jan 30, 2020 2:21 pm

Resonance of the peptide bond. That one little lone pair matters:
https://upload.wikimedia.org/wikipedia/ ... onance.jpg
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