ISSUE:

Hello, good morning, I am reviewing topic 13 that you have on the youtube channel about benzene.

In this video about the end you put some examples of the
Huckel's rule with pyridine, pyrrole and furan and a question has arisen.

In the case of pyridine, you mention that it is not necessary to count the free pair of nitrogen, but then in pyrrole you do take it into account. why is this?
When we talk about heterocycles, can we conveniently decide whether to take it into account or not?

All the best!

Good morning!
For heterocycles such as pyridine, pyrrole, thiophene or furan to be aromatic, they require 6 delocalized pi electrons in the ring.
Pyrine has them in its 3 double bonds, while pyrrole, thiophene, and furan only have 4. So they need the lone pair of the heteroatom. In the case of thiophene and furan, only one of the pairs is required, the other will continue as a solitary pair without becoming involved in aromaticity.

All the best!!!

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 Thank you so much!