Chapter 23, Problem 23.63P

Interpretation Introduction

Interpretation:

Why the arenium ion formed when en electrophile attaches to the 1 position of $\text{2-methylnaphthalene}$ is more stable than the one formed when it attaches to the 3 position is to be explained.

Concept introduction:

Aromatic compounds undergo electrophilic aromatic substitution reactions that involve an arenium cation. An arenium cation is one in which a ring carbon adjacent to the one where the electrophile attaches bears a positive charge. If there are two (or more) possible arenium ions, the major product is determined by the stability of each arenium ion. The most stable arenium ion determines the major product, the position where the electrophile attaches.

The stability of an arenium ion depends on the number of resonance forms it can have. If the alternative cations have the same number of resonance forms, the stability depends on the number of resonance structures in which the aromaticity is preserved. This applies to substrates containing two or more (conjugated) aromatic rings.