Question

Mononitration can be easily achieved on benzene at room temperature using H2SO4 as a catalyst, while bromination, which often requires a stronger catalyst such as FeBr3 and increased temperatures, is more likely to give a mixture of mono and dibrominated products. How can this result be rationalized (hint, consider the relative electrophilicity)?

Answer 1

Solution:

The nitration of benzene in presence of H2SO4 occurs even at room temperature because the nitrating reagent (HNO3) is more reactive and highly electrophilic in nature than the bromination reagent (Br2). The presence of H2SO4 (a strong dehydrating agent) increases the electrophilicity of the HNO3 by removing the OH- ion from HNO3.

H+  + HNO3 = +NO2 ( nitronium ion) + H2O

Thus, the electrophilicity of HNO3 increases rapidly in the presence of H2SO4. Therefore, reaction occurs at room temperature.

In the case of Br2, a stronger catalyst is required to form a electrophile (+Br) due to mild electrophilicity of Br2.

FeBr3 + Br2 = FeBr4- + Br+ (electrophile)

Therefore, to form Br+ electrophile, a higher temperature is required.