what are difference between aromatic elecrophilic substitution and aromatic nucleophilic substitution ?

Electrophilic aromatic substitution (EAS) invovlves the addition of an electrophile to an aromatic ring while nucleophilic aromatic substitution (NAS) involves the addition of a nucleophile to an aromatic ring. In EAS reactions, a aromatic hydrogen is replaced by the incoming electrophile. The most important reactions of this type that take place are aromatic nitration, aromatic halogenation, aromatic sulfonation, and acylation and alkylating Friedel-Crafts reactions. NAS reactions, on the other hand, are substitution reactions in which the incoming nucleophile displaces a good leaving group, such as a halide, on an aromatic ring. There are 6 nucleophilic substitution mechanisms encountered with aromatic systems including the SNAr, SN1, benzyne, free radical SNR1, ANRORC, and vicarious nucleophilic substition mechanisms. The most important of these is the SNAr mechanism, where electron withdrawing groups activate the ring towards nucleophilic attack, for example if there are nitro functional groups positioned ortho or para to the halide leaving group. Similarly, EAS reactions are enhanced by the presense of electon donating groups on the aromatic ring. This finding is in concert with the general rule that 'opposites attract'...

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