Basicity of Nitrogen-Containing compounds

[ModernAlchemist]

I saw this on a practice test and couldn't really get my head around it. Here is the structure being considered:

The question asks which nitrogen is most basic out of 2, 1, 7, or 9. I thought that the bond with more s-character increased in acidity, but the answer is supposed to be 7. I understand that the lone pairs of the Nitrogens on the left ring are tied up in resonance and on the right are tied up in aromaticity, but I still don't see how you can factor all that in to answer the question.

Can someone enlighten me? Thank you in advance.

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[BenZq]

Wow, this is a tough problem. I can't say with certainty, but my best guess would be that the answer has something to do with N-7 not having a Hydrogen to donate, whereas N-9 does, so by the Bronsted-Lowry definition, it would make N-9 a better acid, and thus a weaker base. As for the other Nitrogens, the conjugate base of N-1 is resonant stabilized so it would be more acidic, and C-2 is not a nitrogen.

Also, there may be a component of resonance stability involved with the conjugate base of N-9, but I can't really put my finger on it.

 

[TwoPaddles]

I saw this on a practice test and couldn't really get my head around it. Here is the structure being considered:

The question asks which nitrogen is most basic out of 2, 1, 7, or 9. I thought that the bond with more s-character increased in acidity, but the answer is supposed to be 7. I understand that the lone pairs of the Nitrogens on the left ring are tied up in resonance and on the right are tied up in aromaticity, but I still don't see how you can factor all that in to answer the question.

Can someone enlighten me? Thank you in advance.

hmm..I can think of two things:
1) resonance and aromaticity are intramolecular forces.. and nitrogen #7 is not involved in it..so maybe this could be why
or..
2) nitrogen #7's lone pairs are not involved in aromaticity whereas nitrogen #9's lone pairs are involved. so #7 is more willing to donate..?..and #1 & #2 are just tied up in resonance...

either way, I'm not sure

 

[enTropeeeeeeeee]

Basicity of nitrogenous compounds increase when more electron density is given to that nitrogen. In other words, anything that takes away the electron density of a nitrogen will decrease the basicity, or its ability to behave as a Bronsted-Lowry base. Nitrogen #7 is the only Nitrogen that doesn't allow his lone pair to wander off and play with the other atoms. In the other choices, the lone pair is either off playing the aromaticity or resonance game.

 

[SeeEll]

Sorry for my complete lack of understanding, but why aren't #7's lone pair involved in aromaticity? Is it because it cannot form a double bond to its left and the preexisting double bond to its right? I would be grateful if someone could just spell it out for me.

 

[Rabolisk]

If both #7 and #9's lone pairs were involved in aromaticity, then this would violate Huckel's rule. Remember that the double bond between carbon 4 and carbon 5 is part of the ring. With 2 double bonds in the ring, 2 lone pairs make it 8 electrons. It also does not make sense for the lone pair of #7 to be involved with neither of the lone pairs of #9. Aromaticity results from delocalization of the pi system, this would be localizing it to 7 rather than 9.

 

[Lily20]

Just came across this question in BR CBT1, I'm wondering why #9's lone pair is more likely to be involved in aromaticity than #7?

Thank you