This page gives you the facts and a simple, uncluttered mechanism for the electrophilic substitution reaction between benzene and a mixture of concentrated nitric acid and concentrated sulfuric acid.
The electrophilic substitution reaction between benzene and nitric acid
Benzene is treated with a mixture of concentrated nitric acid and concentrated sulfuric acid at a temperature not exceeding 50°C. As temperature increases there is a greater chance of getting more than one nitro group, -NO2, substituted onto the ring. Nitrobenzene is formed:
\[ C_6H_6 + HNO_3 \rightarrow C_6H_5NO_2 + H_2O\]
The concentrated sulfuric acid is acting as a catalyst.
The formation of the electrophile
The electrophile is the "nitronium ion" or the "nitryl cation", \(NO_2^+\). This is formed by reaction between the nitric acid and the sulphuric acid.
\[ HNO_3 + 2H_2SO_4 \rightarrow NO_2^+ 2HSO_4^- + H_3O^+\]
The electrophilic substitution mechanism
Jim Clark (Chemguide.co.uk)