THE SIMILARITY, DIFFERENCE AND INTERRELATION OF SET AND POLAR MECHANISMS OF REACTIONS OF AROMATIC NUCLEOPHILIC-SUBSTITUTION

The questions, concerning the possible mechanisms of chemical bond formation between nucleophile and electrophile are discussed on example of reactions of aromatic compounds with nucleophiles. It is suggested to consider: 1) as a SET-type processes - the reactions, that proceed during either transition states (intermediates) having ET biradical electronic configuration with nearly complete decoupling of alpha- and beta-electrons, or a radical pair; 2) as heterolitic processes - the reactions, which are characterized by complete pairing of alpha- and beta-electrons on every step. The reactions which proceed during transition states and/or intermediates with partial unpairing of electrons may be considered as pseudo-SET or pseudo-heterolitic depending on the degree of such unpairing. From this point of view a critical analyses of the role of SET-steps in reactions of aromatic nucleophilic substitution is given.