Can nitric acid be protonated?

Sep 8, 2016

In highly acidic media, nitric acid can be protonated.

Explanation:

This is the basis for the nitrating mixture used to prepare nitro-arenes:

${H}_{2} S {O}_{4} \left(a q\right) + H N {O}_{3} \left(a q\right) \rightarrow H S {O}_{4}^{-} + {H}_{2} N {O}_{3}^{+}$

${H}_{2} N {O}_{3}^{+} \rightarrow N {O}_{2}^{+} + {H}_{2} O$

The nitronium ion is a very potent electrophile because of the quaternized nitrogen centre in $O = {N}^{+} = O$. As to how to quantify this basicity, you have got me. We use a highly electrophilc (i.e. acidic) medium to produce the nitronium ion.

The nitronium ion is produced by these means in explosives manufacture when toluene is nitrated, to produce $\text{trinitrotoluene}$. I have no doubt it is safe but it is something I would not like to do.