Question

How is the electrophile generated in the nitration of benzene?

Answer 1

The nitration of benzene involves sulfuric acid and nitric acid, whose pKas are about `-3` and `-1.3`, respectively. Thus, sulfuric acid is the stronger acid and can protonate nitric acid.

The `"H"_2"O"` leaving group just dissociates on its own. The full reaction is:

`"C"_6"H"_6 + "H"_2"SO"_4 + "HNO"_3 stackrel(k_"obs"" ")(->) "C"_6"H"_5"NO"_2 + "HSO"_4^(-) + "H"_3"O"^(+)`

Water can form because water has a pKa of about `15.7`, and thus its formation in equilibrium is heavily favored in this case.

The electrophile is the bent `\mathbf( :stackrel(..)"O"=stackrel((+))("N")=stackrel(..)"O": )`. Your book may call it the nitronium ion.