Question

Why the nitration of nitrobenzene is much slower than the nitration of toluene?

Answer 1

Because the nitro-group is a classic DEACTIVATING group....

Explanation:

Benzene, and more so `pi-"excessive"` heterocycles are eletron-rich materials, and typically react faciley with electrophiles. To prepare a `NO_2^+` electrophile, typically, nitric acid is treated with sulfuric acid in a so-called nitrating mixture... The sulfuric acid protonates the nitronium centre, and delivers an `NO_2^(+)` electrophile.

`H_2SO_4(aq) + HNO_3(aq) rarr HSO_4^(-) + HO-stackrel(+)N(=O)OH`

And of course the parent nitric acid, has formal charge separation in its Lewis representation...

`HO-stackrel+N(=O)O^(-)stackrel(H^+)rarrHO-stackrel+N(=O)OH`

And this protonated nitric acid undergoes elimination of water to give `NO_2^+`:

`HO-stackrel+N(=O)OHrarrNO_2^+ + H_2O`

And `NO_2^+` is A POTENT ELECTROPHILE, which undergoes reaction with aromatic species...

On the other hand, substitution of the phenyl ring by a methyl group activates the ring towards electrophilic substitution. And so toluene should be more reactive towards electrophilic substitution than unsubstituted benzene.

Please note that substitution of toluene by a nitrating mixture, is something that you should do ONLY under controlled circumstances, with backup and advice from an experienced punter. You do not do the reaction on much more than gram scales. THIS IS VERY IMPORTANT....why so...because no great mistake on your part could blow off your fingers....or spray you with acid.