Pyridine like benzene, has six pi -electrons in delocalized pi -orbitals, but unlike benzene the orbitals will be deformed as electrons are more attracted towards the nitrogen atom because of its high electro negativity. This is reflected in the dipole of pyridine which has the negative end on N and the positive end on the nucleus. We can compare the respective properties of benzene, pyridine, pyrrole, furan and thiophene, which very frequently give aromatic electrophilic and nucleophilic substitution reactions in different positions. Nitration, chlorination, bromination of pyridine is very difficult at normal conditions. Why?

Pyridine like benzene, has six pi -electrons in delocalized pi -orbita

1.	Pyridine like benzene, has six pi -electrons in delocalized pi -orbitals, but unlike benzene the orbitals will be deformed as electrons are more attracted towards the nitrogen atom because of its high electro negativity. This is reflected in the dipole of pyridine which has the negative end on N and the positive end on the nucleus. We can compare the respective properties of benzene, pyridine, pyrrole, furan and thiophene, which very frequently give aromatic electrophilic and nucleophilic substitution reactions in different positions. Nitration, chlorination, bromination of pyridine is very difficult at normal conditions. Why?
Answer» High aromaticity of pyridine than benzene Pyridine has an LONE PAIR of e-s on NITROGEN so it protonates pyridine has 5 CARBON atoms pyridine is cyclic compound. Solution :In pyridine .N. is highly EN. So it protonates and can behave as LEWIS base.