Question

Explain the mechanism of nitration of benzene.

Answer 1

Benzene is treated with a mixture of concentrated nitric acid and concentrated sulphuric acid at a temperature not exceeding ${50}^{\circ }C$. As temperature increases, there is a greater chance of getting more than one nitro group, $N{O}_2$, substituted onto the ring. Nitrobenzene is formed.
$C_6{H}_6+HN{O}_3\to C_6{H}_{5}N{O}_2+H_{2}O$
or:

The concentrated sulphuric acid acts as a catalyst.
The formation of the electrophile:
The electrophile is the "nitronium ion" or the "nitryl cation", $N{O}_2^{+}$. This is formed by reaction between the nitric acid and the sulphuric acid.
$HN{O}_3+2H_{2}S{O}_4\to N{O}_2^{+}+2H_{2}S{O}_4^{-}+H_3{O}^{+}$
The electrophilic substitution mechanism
Stage one

Stage two