Question

Why ${\mathrm{H}}_2{\mathrm{SO}}_4$ is used in nitration?

Answer 1

Sulphuric acid:

1. Sulphuric acid is a mineral acid that is composed of elements such as sulfur, oxygen, and hydrogen.
2. It is a colorless, odorless, viscous liquid that gets miscible with water.

The steps involved in Nitration are:

Step 1: Formation of the electrophile ${\mathrm{NO}}_2^{+}$ ion:

$$\begin{gathered} {\mathrm{H}}_2{\mathrm{SO}}_4+{\mathrm{HNO}}_3⟶{\mathrm{HSO}}_4^{\ominus }+{\mathrm{H}}_2{\mathrm{NO}}_3^{\oplus } \\ {\mathrm{H}}_2{\mathrm{NO}}_3^{\oplus }⟶{\mathrm{H}}_2\mathrm{O}+{\mathrm{NO}}_2^{\oplus } \\ {\mathrm{H}}_2{\mathrm{SO}}_4+{\mathrm{H}}_2\mathrm{O}⟶{\mathrm{HSO}}_4^{\ominus }+{\mathrm{H}}_3{\mathrm{O}}^{\oplus } \end{gathered}$$

${\mathrm{HNO}}_3+2{\mathrm{H}}_2{\mathrm{SO}}_4\to {\mathrm{NO}}_2^{+}+{\mathrm{H}}_3{\mathrm{O}}^{+}+2{\mathrm{HSO}}_4^{-}$

Step 2: Loss of proton, ${\mathrm{H}}^{+}$ ion

Step 3: The electrophile produced in the first step attaches itself to the carbocation and forms the complex.

Role of Sulphuric acid:

Sulphuric acid performs two major functions in the reaction.

1. Firstly, it removes the water produced as a by-product during nitration.
2. Also, it is a stronger acid than nitric acid, hence, it protonates the nitric acid to form a nitryl ion, which is a strong nitrating agent.