91Ó°ÊÓ

In nucleophilic aromatic substitution reactions, the ring has a leaving group that is replaced by a nucleophile.Nucleophilic aromatic substitution of pyridine takes place at C-2 or C-4 because addition to these positions leads to the most stable intermediate.

Only when addition occurs to these positions, a resonance contributor obtained that has the greatest electron density on nitrogen, the most electronegative elementof the ring atoms.

It involves two steps:

1. The nucleophile added to the ring carbon that is attached to the leaving group.
2. The leaving group is eliminated in the last step of the reaction.

The mechanism for the given reaction (a) is shown in the figure below:

(a) In this reaction, hydronium ion attacks at the 2-position of the pyridine ring due to which N atoms gets a lone pair of charge on it. To stabilize this charge, hydrolysis occurs in the reaction we get our desired product as shown below:

Mechanism of the reaction (a)

(b) In this reaction, nucleophilic addition of hydroxide ion can take place at 4^th position as well as the second position due to which formation of 4-pyridone takes place as shown in the figure given below:

Reaction product