Question

nucleophiles sun. in aryl halides is facilated by electron withdrawing group while electophilic su. reaction is facilated by electron releasing group. why?

Answer 1

Electron withdrawing groups takes away the electron density fron the benzene ring.

This increase the nucleophilic character of the ring and hence the substitution reaction gets facilitated.

On the other hand the electron donating groups increases the electron density of the benzene ring. This decreases the tendency of molecule to undergo neucleophilic substitution reaction and facilitates the electrophilic substitution reaction.