Question

Why SNAr reactions are not considered either SN1 or SN2: Explain considering chlorobenzene as an example.


Answer 1

If we try and draw parallels between nucleophilic substitution in aliphatic systems and that in aromatic systems, we would quickly realize that the typical SN1 and SN2 mechanisms are not feasible in aromatic systems. One of the major reasons is that the p-electrons in aromatic systems are in conjugation. More over, back side attack (as in SN2) and inversion is precluded by the geometry of the ring. On the other hand, SN1 type of substitution would require formation of the phenyl cation which is less stable than a primary carbocation.Obviously, the reaction has to proceed with a totally different type of mechanisms. There are two mechanisms which are possible and they are: a) Addition-Elimination Mechanism b) Elimination-Addition Mechanism