The preparation of p-nitroaniline from aniline and nitric acid involves an electrophilic aromatic substitution reaction. Here's the step-by-step mechanism:1. Protonation of nitric acid: Nitric acid HNO3 reacts with a strong acid, usually sulfuric acid H2SO4 , to form the nitronium ion NO2+ , which is a strong electrophile. This reaction is facilitated by the protonation of nitric acid by sulfuric acid.HNO3 + H2SO4 NO2+ + H3O+ + HSO4-2. Formation of the sigma complex: The electrophile, nitronium ion NO2+ , attacks the aniline molecule at the para-position due to the activating effect of the amino group on the aromatic ring. This leads to the formation of a sigma complex, which is a resonance-stabilized carbocation intermediate.Aniline + NO2+ Sigma complex p-nitroaniline carbocation 3. Deprotonation: The sigma complex loses a proton H+ to restore the aromaticity of the ring. This step is facilitated by the conjugate base of the strong acid used in the first step HSO4- in this case .Sigma complex p-Nitroaniline + HSO4-The final product is p-nitroaniline. It is important to note that aniline is usually acetylated before the nitration process to protect the amino group from further reactions. After the nitration, the acetyl group is removed by hydrolysis to obtain the p-nitroaniline.