aromaticity question

[DDSoneday283]

Hey guys..I have been studying ochem....and I am still confused when cyclic structures involving "N" and "O" groups on them. How do I know which ones I should include in my electron-counting to see if they meet huckels rule or not? Please help....it's super frustrating.

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[CyborgNinjaX]

I'm guessing you're a little confused with pyrole and pyridine. Pyridine has the lone pair on the outside of the structure and doesn't count, while pyrole has the lone pair on the inside so it does count.

Pyridine: 6 pi electrons. 4n+2 ---> n=1
Pyrole: 6 pi---> n=1

I read anything about oxygen in the Kaplan book...

 

[Lonely Sol]

I'm guessing you're a little confused with pyrole and pyridine. Pyridine has the lone pair on the outside of the structure and doesn't count, while pyrole has the lone pair on the inside so it does count.

Pyridine: 6 pi electrons. 4n+2 ---> n=1
Pyrole: 6 pi---> n=1

I read anything about oxygen in the Kaplan book...

The reason pyrole has lone pair inside the ring is because there is a hydrogen bond on the outside, which forces the 2 remaining electrons to the inside, therefore the ring has 6pi electrons and meets huckle's rule. While the pyridine doesn't have a hydrogen attached to the nitrogen so the electrons prefer to stay outside rather than inside, therefore doesnt meet the hucke's rule (4 pi)

As far as I can remember, O always constitute 2 e- inside the ring and 2e- outside (since O has 6e-). For example, furan, which is aromatic because there are 2e- inside (always) from O and 4 e- from the two double bonds, there meets huckle rule.


Hope this helps!