Quinoline and isoquinoline react with electrophiles through the benzene ring (carbocycle), due to its higher electronic richness, compared to the pyridine ring. The most favored positions are 5 and 8.

electrophile substitution 01

The explanation that positions 5 and 8 are more favorable than 6 and 7 lies in the stability of the cationic intermediate formed.

electrophile substitution 02

Intermediate with two canonical structures that delocalize the charge. I do not represent the structures that break the aromaticity of the pyridine ring.

electrophile substitution 03

This last intermediate has only one structure, being therefore less stable.

electrophile substitution 04

With isoquinoline the results are similar.