The latter protonates the former to generate nitronium ions which are the actual nitrating species.
The lower para-selectivity of HZSM-5 may be due to its hydrophobic pores that resist the diffusion of aqueous nitric acid resulting in fewer acidic sites available to generate required nitronium ions.
One could envisage a situation whereby exposure of a heterocyclic substrate to the highly acidic medium of the nitronium
species could be minimized, with the nitrated product free to emerge from the flow reactor before it becomes polymerised.
2)]*) or heterolytic cleavage to form the nitronium
A very early application of the use of superacids in inorganic chemistry was the demonstration that the nitronium
ion is formed in a solution of nitric acid in sulfuric acid