pyridine

[akimhaneul]

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Why can't the lone pair on nitrogen in pyridine be part of the pi system delocalization?

I feel like it can be when you draw the resonance structures so I'm confused.

 

[theonlytycrane]

Based on the 4n + 2 rule, 6 electrons is a nice number to have in the pi system, with 4n + 2 = 6.

If the lone pair electrons were in, it would be a 4n = 8 system (anti-aromatic = bad).

 

[aldol16]

It's a simpler answer than that. That nitrogen is already using one p-orbital to pi-bond (that's how pi-bonding works - sideways overlap between co-planar p-orbitals). So the lone pair must be in another p orbital that is orthogonal to the one already being used for pi-bonding. Therefore, it cannot delocalize because it does not have the right geometry, i.e. it's not co-planar with the p-orbitals of the pi system.