In the preceding sections on Electrophilic aromatic substitution, we observed benzene behaving like a nucleophile in electrophilic aromatic substitution reactions. However, some specially substituted benzenes undergo Nucleophilic aromatic substitution (NAS). In such reactions, the aromatic ring behaves as an electrophile.
For an NAS to occur the following three conditions must be met.
The NAS reaction above proceeds as follows. The aromatic ring is deficient in electrons because of the -NO2 group and is an electrophile. The nucleophilic hydroxide ion (OH-) attacks at the carbon attached to the leaving group in an addition reaction. This forms a cyclohexadienyl anion (Meisenheimer complex). The Meisenheimer complex is not aromatic. There are 4 resonance structure for this complex, I've drawn two of them, can you draw A and B? The Meisenheimer complex then eliminates the Cl ion.
I once had to perform the following reaction for a pharmaceutical intermediate. The compound on the left is chloropyrimidine and it undergoes an NAS reaction. It is a bit more reactive than benzene, especially considering the Cl is attached to the carbon between the two nitrogens.