This reaction is analogous to carbonyl addition elimination, in that it is a two step process where the negative charge is accomodated by an electron withdrawing group. To emphasize the simularity, this example uses a ketone:
The negative charge on the intermediate is spread out into the benzene ring and onto the ketone by resonance. The resonance form with the charge on oxygen looks very much like the intermediate in carbonyl addition elimination, except that there is a benzene ring between the nucleophile and the carbonyl.
The initial attack is usually the rate limiting step. Fluorine works better than other halogens as a leaving group because it is small (low steric hindrance), and very electronegative (causing a strong partial positive charge on the carbon bearing F, making it more attractive to the incoming nucleophile). This is contrary to SN2 reactions, which are concerted, and in which fluorine is a poor choice.
The comparison of aromatic nucleophilic substitution to carbonyl addition-elimination is an example of "vinylogy," which means the action of a functional group is transmitted through a conjugated pi-bonded framework to a remote location.
Reference:
Back to Polymers by Aromatic Nucleophilic
Substitution