Explanation:
Part a
Pyridine is an aromatic heterocyclic compound and undergoes nucleophilic aromatic substitution reaction at 2 and 4 positions.
This is because, when a nucleophile attack pyridine at 2 and 4 positions, the anoin formed is stabilized. In the anion formed is tabilized by resonance and also negative charge is present at electronegative atom, nitrogen.
If the nucleophile attack at 3 position, the anion formed is not as stable as anions formed when nucleophile attach at 2 and 4 positions. This is because negative charge is not present at the nitrogen atom.
Part b:
Tautomers are structural isomers of each others formed by migration atom within the molecule. Tautomers exist in equilibrium with each other.
Tautomers of 2-Hydroxypyridine and 4-Hydroxypyrine are given in the attachment.
