Reactions

First, the sulfuric acid protonates the hydroxyl group of the nitric acid, forming a good leaving group which then departs to generate a highly electrophilic nitronium ion. Then, the 𝜋 electrons of the aromatic ring attack the electrophilic nitrogen of the nitronium ion, forming an arenium ion (also called Wheland intermediate). At this stage, aromaticity is lost and the carbon bearing the nitro group becomes sp³ hybridized. A base (HSO₄^- or H₂O) abstracts the proton from the sp³ carbon, restoring aromaticity and yielding the nitroarene product.