The reaction of aniline C6H5NH2 with nitric acid HNO3 to form p-nitroaniline C6H4 NO2 NH2 is an example of electrophilic aromatic substitution, specifically nitration. The mechanism involves the following steps:1. Protonation of nitric acid by sulfuric acid:Sulfuric acid H2SO4 acts as a strong acid and protonates nitric acid, forming the nitronium ion NO2+ , which is a strong electrophile. The reaction is as follows:HNO3 + H2SO4 -> NO2+ + HSO4- + H2O2. Formation of the sigma complex arenium ion :The electrophile, NO2+, attacks the aromatic ring of aniline, resulting in the formation of a sigma complex arenium ion . This step involves the breaking of aromaticity, which is energetically unfavorable. However, the intermediate is stabilized by resonance.3. Proton loss and regeneration of aromaticity:The sigma complex loses a proton to the HSO4- ion, which regenerates sulfuric acid and restores the aromaticity of the ring, forming the p-nitroaniline product.The role of sulfuric acid in this reaction is twofold:1. It protonates nitric acid to generate the nitronium ion NO2+ , the electrophile needed for the reaction.2. It acts as a catalyst, being regenerated at the end of the reaction, and thus is not consumed in the overall process.It is important to note that aniline is an activating group, which directs electrophilic substitution to the ortho and para positions. However, due to steric hindrance, the para position is favored, and p-nitroaniline is the major product.