Is concentrated H2SO4 a catalyst in benzene nitration?

[cyt91]

In the nitration of benzene using conc. H2SO4 and HNO3 at 55 degrees celsius, is concentrated H2SO4 a catalyst for the reaction?

 

[Borek]

This is a border case for me.

Write reaction equation.

You start with concentrated acid, does it stay concentrated?

 

[cyt91]

Hi, it is a catalyst. Check out this link : http://www.chemguide.co.uk/mechanisms/elsub/nitration.html

 

[Borek]

This is a little bit more complicated.

Concentrated sulfuric acid serves two purposes. First, as an acid it catalyzes the reaction. However, it also removes water from the HNO₃/benzene pair, shifting reaction equilibrium to the right. In the process it gets more and more diluted and at some moment it is no longer able to catalyze the process. By definition catalyst is not consumed, obviously in this case it is not entirely true.

 

[cyt91]

But in the mechanism...isn't sulphuric acid regenerated? So sulphuric acid is not consumed.

 

[Borek]

In the mechanism - yes. But if you run the reaction acid gets diluted, so if it wasn't in a large excess reaction will stop. There is no error in calling it a catalyst when applying simple and rigid definition of catalyst. Unfortunately, chemistry hates simple and rigid definitions, and you can be sure almost always you will find a system that doesn't fit.