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

The nitration of benzene involves the formation of the electrophile, the nitronium ion (${NO_2}^+$), which is crucial for the reaction to proceed. The reaction is typically facilitated by a mixture of concentrated sulfuric acid (${H_2SO_4}$) and nitric acid (${HNO_3}$). Here, sulfuric acid acts as a catalyst, helping generate the nitronium ion from nitric acid.

When ${KHSO_4}$ (potassium hydrogen sulfate) is added to the reaction mixture, it affects the concentration of sulfuric acid because:

1. ${KHSO_4}$ is a strong electrolyte that dissociates to furnish ${H^+}$ ions, competing with sulfuric acid for maintaining the acidity of the medium.
2. This leads to a dilution of the sulfuric acid's concentration as some of the sulfuric acid is converted into its bisulfate form, ${HSO_4}^-$, due to the common ion effect.
3. The reduced catalytic effectiveness of sulfuric acid results in a slower rate of nitronium ion generation.

As the formation of the nitronium ion becomes less efficient, the overall rate of nitration of benzene is reduced.

Conclusion: By adding a large amount of ${KHSO_4}$, the concentration and effectiveness of ${H_2SO_4}$ are reduced, slowing down the reaction. Therefore, the rate of nitration will be slower.