Question

Propose a mechanism for nitration of pyridine at the 4-position, and show why this orientation is not observed.

Short answer

This orientation is not observed because electrophilic substitution reaction at 4-position is not stable. The intermediate formed by electrophilic addition at 4-position is less stable than that of 3-position.

Step-by-step solution

Introduction

In above mechanism, addition of an electrophile to pyridine in the 4 position results in the formation of three resonance structures of the intermediate form.

Justification

Electronic substitution at 4th position is not stable because 4-position carbon is not the most electron rich C atom in the pyridine ring. Whereas 3-position carbon is the most electron-rich C-atom, therefore pyridine undergoes electrophilic substitution at 3 position rather than 4 position.