Pyridine, an Odd Aromatic Compound
Today we talked about pyridine, a somewhat strange aromatic compound. Pyridine is essentially a benzene ring with an sp2-hybridized nitrogen replacing one of the carbon atoms. With six ring electrons, the system is aromatic but the lone pair of electrons on the nitrogen is NOT part of the aromatic system. This gives pyridine more basicity than aromatic amines. However, since the orbital holding the lone pair is an sp2, with more s-character than the sp3-hybridized orbitals of aliphatic amines, pyridines hold the lone pair closer to the nuclei and are therefore less basic than aliphatic amines.It is possible to perform EAS reactions on pyridine, with the electrophile ending up on the carbon with the largest partial negative charge (two carbons from the nitrogen, you can think of it as a sort of "meta" position). The reaction is typically of poor yield, as the nitrogen lone pair has a tendency to interact with the electrophile, not giving it a good opportunity to react with the ring.
A better reaction is the NAS (Nucleophilic Aromatic Substitution) of 2-halo pyridines. The electronegativity of the nitrogen allows for stabilization of the anionic intermediate, making the process viable.
posted by DCChem @ 9:07 AM
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