Aromaticity

[JDHK]

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I'm trying to figure out why A is aromatic..(Question is which is aromatic and answer is A). I feel like the nitrogen is an sp3 isn't it?? including the lone electrons...i must be grossly mistaken...the answer says it's aromatic.

 

[ChrisM07]

The nitrogen is contributing a lone pair of electrons to the ring so therefore it abides by the 4n+2 rule (3 pi bonds). Now, if the methyl wasnt there and it was just the nitrogen, there would be two lone pairs of electrons around nitrogen BUT only one of those will contribute to the conjugation of the ring. It would still be aromatic, but only one pair goes into the ring.

 

[sakthivs]

this is one of those scenarios that Chad explains...I dont really understand it fully but just know that in this case the nitrogen may appear as sp3 but in actuality is sp2 because the lone pairs can participate in resonance with the conjugated system. So in this case we have a total of 6pi electrons and thus it is indeed aromatic.

 

[JDHK]

Right right. Thank you guys!

 

[denttal]

exactly what they responded!

Also, it is good to note this structure for basicity reasons. B/c that lone pair is part of the conjugated system it is not available to act like a base and extract a hydrogen... So if you had the structure like ChrisM07 noted: there are 2 lone pairs on Nitrogen, this structure will be basic b/c it still has a lone pair with which to extract a Hydrogen.