Question

How to decide it is electrophilic aromatic substitution in naphthalene?

Answer 1

Electrophilic Substitution of Disubstituted Benzene Rings. When a benzene ring has two substituent groups, each exerts an influence on subsequent substitution reactions. The activation or deactivation of the ring can be predicted more or less by the sum of the individual effects of these substituents.

Answer 2

Their lack of reactivity toward addition reactions is due to the great stability of the ring systems that result from complete π electron delocalization (resonance). Aromatic compounds react by electrophilic aromatic substitution reactions, in which the aromaticity of the ring system is preserved.