(N/A) . Chemical reaction for nitration: When benzene is treated with a mixture of concentrated nitric acid $(HNO_{3})$ and concentrated sulfuric acid $(H_{2}SO_{4})$ at $323 \ K$ to $333 \ K$,it undergoes nitration to form nitrobenzene.
$C_{6}H_{6} + HNO_{3} \xrightarrow{conc. H_{2}SO_{4}, 323-333 \ K} C_{6}H_{5}NO_{2} + H_{2}O$
$b$. Mechanism of formation of the nitronium ion $(NO_{2}^{+})$:
Step-$I$:
$H_{2}SO_{4} + HNO_{3} \rightleftharpoons H_{2}NO_{3}^{+} + HSO_{4}^{-}$
Step-$II$:
$H_{2}NO_{3}^{+} \rightleftharpoons H_{2}O + NO_{2}^{+}$
Overall reaction for the generation of the electrophile:
$H_{2}SO_{4} + HNO_{3} \rightleftharpoons NO_{2}^{+} + HSO_{4}^{-} + H_{2}O$
In this reaction,$H_{2}SO_{4}$ acts as a proton donor (acid) and $HNO_{3}$ acts as a proton acceptor (base),facilitating the formation of the electrophilic nitronium ion $(NO_{2}^{+})$.