A nucleophilic aromatic substitution is a substitution reaction in organic chemistry in which the nucleophile displaces a good leaving group, such as a halide, on an aromatic ring.There are 6 nucleophilic substitution mechanisms encountered with aromatic systems: 1  the SNAr (addition-elimination) mechanism.ryl halides cannot undergo SN2 reaction .The C–Br bond is in the plane of the ring as the carbon atom is trigonal. To attack from the back, the nucleophile would have to appear inside the benzene ring and invert the carbon atom in an absurd way. This reaction is not possible.