Chapter 22, Problem 22.8P

Interpretation Introduction

Interpretation:

Reason for the preferential of nitration of pyridine at 3^rd position has to be given and also resonance contributors for the intermediate formed by attack of ${\text{NO}}_{\text{2}}^{\text{+}}$ at the 2, 3, 4 positions of the conjugate acid of pyridine has to be given.

Explanation of Solution

Pyridine is a base, so in the presence of acid it will be protonated.  For nitration at the 3^rd position, the additional positive charge is delocalized on three carbon atoms of the pyridine ring.  In this case, none of the resonance structures has a +2 charge on the same atom.

For nitration at the 2^nd position or 4^th position, the additional positive charge on the cation intermediate is also delocalized on three atoms of the pyridine ring, but one of the resonance structure has a +2 charge on nitrogen atom.  This situation is less stable than that happens for nitration at the 3^rd position.