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Consider the nitration of benzene using mixed conc. $H_2SO_4$ and $HNO_3$. If a large amount of $KHSO_4$ is added to the mixture, the rate of nitration will be
A
slower.
B
unchanged.
C
doubled.
D
faster.
Explanation
Common ion $HSO_4^-$ suppresses $NO_2^+$ formation.
Detailed Solution
Nitration of benzene is an electrophilic substitution in which the electrophile is the nitronium ion ($NO_2^+$).
$2H_2SO_4 + HNO_3 \rightarrow 2HSO_4^- + NO_2^+ + H_3O^+$
When a large amount of $KHSO_4$ is added, the common ion effect of $HSO_4^-$ lowers the concentration of $NO_2^+$, so the rate of nitration decreases.
$2H_2SO_4 + HNO_3 \rightarrow 2HSO_4^- + NO_2^+ + H_3O^+$
When a large amount of $KHSO_4$ is added, the common ion effect of $HSO_4^-$ lowers the concentration of $NO_2^+$, so the rate of nitration decreases.