Biochem related question: arrow pushing on pyridine systems

[Majumatata]

Hey all,

I am a little confused as to why the lone pair on the nitrogen molecule in pyridine is considered localized instead of a delocalized. I have it drawn where the pi electrons are pushed on nitrogen, and the lone pair from nitrogen pushed into the ring/pi system, hence stating the lone pair is delocalized. This seems more of a organic arrow pushing related question but it's bugging the crap out of me...

...is the rate of pi electron conjugation above and below the ring supersede the lone pair contributing to resonance?

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[Czarcasm]

Hey all,

I am a little confused as to why the lone pair on the nitrogen molecule in pyridine is considered localized instead of a delocalized. I have it drawn where the pi electrons are pushed on nitrogen, and the lone pair from nitrogen pushed into the ring/pi system, hence stating the lone pair is delocalized. This seems more of a organic arrow pushing related question but it's bugging the crap out of me...

...is the rate of pi electron conjugation above and below the ring supersede the lone pair contributing to resonance?
View attachment 184500

It has to do with the hybridization. Remember that the nitrogen is sp2 hybridized (3 sp2 orbitals, 1 p orbital). The pi bond between the carbon is using up that spare p-orbital (recall all pi-bonds are p-p orbital bonds). The localized lone pair is in an sp2 orbital that is not involved in resonance (also, two other sp2 orbitals are in a bond with carbon atoms).

 

[justadream]

Just want to confirm: the fact that the lone pair is not involved in the aromatic resonance makes pyridine a good base right? What about as a nucleophile?

 

[Thoroughbred_Med]

Just want to confirm: the fact that the lone pair is not involved in the aromatic resonance makes pyridine a good base right? What about as a nucleophile?

Definitely a good "base", where it can abstract hydrogens. I think it's too big to be a nucleophile though.... trying to squeeze in and attack carbons

 

[Teleologist]

...is the rate of pi electron conjugation above and below the ring supersede the lone pair contributing to resonance?

Wait, what? Why are we bringing in kinetics? What do rates of anything have to do with this question?

It has to do with the hybridization. Remember that the nitrogen is sp2 hybridized (3 sp2 orbitals, 1 p orbital). The pi bond between the carbon is using up that spare p-orbital (recall all pi-bonds are p-p orbital bonds). The localized lone pair is in an sp2 orbital that is not involved in resonance (also, two other sp2 orbitals are in a bond with carbon atoms).

This doesn't answer the question. You're just restating what the OP knows - that the lone pair on nitrogen in pyridine doesn't participate in resonance. Riddle me this: below is imidazole. How can you rationalize why one lone pair participates in resonance while the other doesn't?

Yes, both nitrogens are sp2 hybridized. But one nitrogen's lone pair isn't participating in resonance.

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OP: the real reason has nothing to do with the stuff posted above. Consider: if the nitrogen's lone pair were delocalized - i.e. participating in resonance - then the hybridization of the nitrogen would be best characterized as what? Let the chemistry talk to you. Screw all the massive book-memorization and testing techniques pushed by review books. The answer lies in your successfully answering my question.