[Dynamo22]

What would be the reason as to why there would be an unusually large dipole towards the 5 membered ring in this structure? I'm not sure if aromitacity would play a role in that. It doesn't seem that the 5 membered ring is aromatic unless the pi bond between the two rings would contribute its electrons to the 5 membered ring to make it aromatic. Any help would be appreciated!

[orgopete]

Hint, this is an aromaticity question.

[Dynamo22]

thanks for the reply, orgopete. I see that the 6 membered ring does have aromaticity due to the nitrogen's lone pair in the p orbital making 6 pi electrons in the ring. I don't have a clue as to what role the double bond between the rings play. I am not sure if that is one of the factors contributing to the dipole or how it would affect aromaticity within the 5 membered ring. Can't find it any where in his book, either.

[Dynamo22]

Hint, this is an aromaticity question.

Okay, correct me of I'm wrong or let me know if I'm on the right path. The double bond between the ring will flow its electrons into the 5 membered ring which can easily spread the negative charge about itself. The nitrogen, which is also an electron donating group, will only bear the positive charge, which creates an even larger dipole within the five membered ring. I feel I may be missing some other things, let me know if this is a true statement. Thanks!

[fledarmus]

Think about the p orbitals that are forming the pi bond between the two rings. This pi bond contains two electrons. Which carbon(s) will they need to be on to make both rings aromatic?

Hint - what is the definition of aromaticity?

[Dynamo22]

Rules: Flat, 2, 6, 10, 14, etc. pi electrons, fully conjugated. It appears the carbon in the 5 membered ring would house the pi electrons to make it an aromatic ring. I guess I'm just confused as to why it would create a greater dipole in its direction, like I said its a concept we never touched on. I'm not sure if my explanation is sufficient, either.

[orgopete]

I think you have it. Just draw out the aromatic structures on a piece of paper, look at it, and compare it with the uncharged version. Can you find a reason for a large dipole to exist?

[Dynamo22]

As I drew the various resonance structures, its easy to see the dipole now. The 5 membered ring has quite a bit of resonance and spreads that negative charge about freely. The 6 memebred ring has little resonance and the positive charge stays put on the nitrogen, which makes that that ring very positive in charge. Thanks for the help guys, every time I come here I always get great responses to get the wheels turning!