Question

In aromatic nitration reactions, which ion is produced by nitric acid?
(A) $N{{O}^{+}}$
(B) $N{{O}_{2}}^{+}$
(C) $N{{O}_{3}}^{-}$
(D) $N{{O}_{2}}^{-}$

Answer 1

Hint: Bear in mind that the nitration of aromatic compounds is an example of electrophilic aromatic substitution reaction. The ion formed from the acid acts as the electrophile and will continue the electrophilic substitution reaction. The protonated nitric acid will generate nitronium ions.

Complete step by step solution:
- Let’s start with the concept of aromatic nitration reactions. The nitration reaction can be defined as the introduction of a nitro group to an organic compound.
- The aromatic nitration reactions are examples of electrophilic aromatic substitution reactions. The acid will activate the electrophile and this electrophile will attack the aromatic compound and by regenerating the acid catalyst, deprotonation will take place to restore the aromaticity.
- The generation of highly reactive electrophile from the nitric acid can be represented as follows
\[HO-N{{O}_{2}}+{{H}_{3}}{{O}^{+}}\rightleftharpoons{{H}_{2}}{{O}^{+}}-N{{O}_{2}}\rightleftharpoons N{{O}_{2}}^{+}\]
-The key to this process is protonation of the hydroxyl (OH) group on the nitric acid, which converts it to ${{H}_{2}}O$. Being a much better-leaving group than the hydroxyl group, ${{H}_{2}}O$ is rapidly lost from nitric acid to produce the nitronium ion $N{{O}_{2}}^{+}$ which is highly reactive and also an electrophile.
- As we can see the $N{{O}_{2}}^{+}$ is generated when water is eliminated from the protonated nitric acid. Only a small amount of $N{{O}_{2}}^{+}$ is present in the pure nitric acid and as a result, nitration with pure nitric acid proceeds very slowly.
- However, in a reasonable period of time, the strongly activated aromatic compounds may also be nitrated with nitric acid. Nonetheless, for nitration of aromatic compounds, a mixture of concentrated sulfuric acid and concentrated nitric acid, usually called nitrating acid, is generally used.
- As we know, Sulfuric acid is a stronger acid than the nitric acid. As a result, sulfuric acid will protonate the nitric acid and will lead to a larger amount of nitronium ion $N{{O}_{2}}^{+}$ in the nitrating acid mixture than in pure nitric acid.
From the above discussions it’s clear that in aromatic nitration reactions, nitronium ion is formed from nitric acid.

Thus the answer is option (B) $N{{O}_{2}}^{+}$.

Note: Keep in mind that the most common example for aromatic nitration reaction is the nitration of benzene. The reaction proceeds through the activation of electrophile $N{{O}_{2}}^{+}$ by acid, followed by the attack of this activated electrophile on benzene and finally deprotonation to restore the aromatic ring which leads to the formation of nitrobenzene.