Aromaticity Of Pyridine

The structure of pyridine is very similar to benzene. According to modern physical methods, the length of carbon-carbon bond in pyridine molecule is 139pm, between C-N single bond (147pm) and C=N double bond (128pm), and the bond length value of carbon-carbon bond and carbon-nitrogen bond is similar, and the bond Angle is about 120°, which indicates that the average degree of the bond on the pyridine ring is high. But not as complete as benzene.
The carbon and nitrogen atoms in the pyridine ring overlap each other with sp2 hybrid orbitals to form a σ bond, forming a planar six-membered ring. Each atom has a p orbital perpendicular to the ring plane, each p orbital has an electron, and these p orbitals overlap on the sides to form a closed large π bond with 6 π electrons, following the 4n+2 rule, similar to the benzene ring. Therefore, pyridine has certain aromatic properties. There is also an sp2 hybrid orbital on the nitrogen atom that is not involved in bonding and is occupied by a lone pair of electrons, making pyridine alkaline. The electronegativity of the nitrogen atom on the pyridine ring is large, which has a great effect on the density distribution of the electron cloud on the ring, making the π electron cloud shift towards the nitrogen atom, and the electron cloud density is high around the nitrogen atom, while the electron cloud density in other parts of the ring is decreased, especially in the neighboring and para-position. So pyridine is less aromatic than benzene.
In the pyridine molecule, the role of nitrogen atoms is similar to the nitro of nitrobenzene, so that the density of the electron cloud on the adjacent and para-position is reduced than that of the benzene ring, and the meta is similar to the benzene ring, so the electron cloud density of the carbon atom on the ring is much less than that of benzene, so the aromatic heterocyclic ring like pyridine is also known as the "π-deficient" heterocyclic ring. The chemical properties of this kind of heterocyclic ring are electrophilic substitution reaction becomes difficult, nucleophilic substitution reaction becomes easy, oxidation reaction becomes difficult, reduction reaction becomes easy.