Why pyridine doesn't undergo fridel-crafts acylation and alkylation?

Pyridine is an aromatic heterocyclic compound with a nitrogen atom in the ring. Due to the presence of nitrogen in the ring, pyridine has a basic character and forms a strong bond with hydrogen ions. Pyridine has a different electronic structure compared to benzene, which makes it less reactive towards electrophilic substitution reactions such as Friedel-Crafts acylation and alkylation.

Structural differences between Pyridine and Benzene

The main difference between pyridine and benzene is that pyridine has a nitrogen atom in the ring, whereas benzene has only carbon atoms. The nitrogen atom in pyridine participates in resonance with the π-electron system of the ring, which delocalizes the electrons and makes the ring less electron-rich. This results in a weaker interaction between the ring and electrophiles, which is essential for electrophilic substitution reactions.

Pyridine as a Lewis base

Pyridine is a Lewis base due to the presence of a lone pair of electrons on the nitrogen atom. The lone pair on the nitrogen atom acts as a site for coordination with Lewis acids, which decreases the availability of the nitrogen atom for electrophilic attack. Friedel-Crafts acylation and alkylation reactions require a strong electrophile to attack the ring, which is hindered by the Lewis basicity of pyridine.

Pyridine as a poor leaving group

The nitrogen atom in pyridine is a poor leaving group, which is required for the formation of a carbocation intermediate during Friedel-Crafts reactions. The nitrogen atom in pyridine is not sufficiently electronegative to stabilize a positive charge, which results in the formation of an unstable intermediate. This intermediate is highly reactive and tends to undergo further reactions instead of forming the desired product.

Conclusion

In summary, pyridine does not undergo Friedel-Crafts acylation and alkylation reactions due to its basic character, Lewis basicity, and poor leaving group properties. These factors hinder the formation of a carbocation intermediate, which is essential for the reaction to proceed. Therefore, alternative synthetic routes must be used to introduce functional groups into the pyridine ring.