Question

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. uncharged
• C. doubled
• D. faster

Answer 1

The Correct Option is A

Nitration of benzene involves the formation of the nitronium ion \((NO_2^+)\), which is the actual electrophile in the reaction. The reaction is: 
\(HNO_3 + H_2SO_4 \rightarrow NO_2^+ + HSO_4^- + H_2O\)  

The presence of excess KHSO₄ (which is acidic) increases the concentration of HSO₄⁻ in the medium, making the solution more acidic but less reactive toward nitronium ion formation. This suppresses the equilibrium that produces the active electrophile:

• More HSO₄⁻ shifts equilibrium backwards.
• This reduces the formation of NO₂⁺, the key reactive species.

Therefore, the rate of nitration will decrease.