Nitration of Benzene:
The nitration of benzene involves the substitution of a hydrogen atom with a nitro group (-NO2) in the presence of a nitrating mixture. This reaction is an important step in the synthesis of various organic compounds, including aromatic nitro compounds and explosives like TNT (trinitrotoluene).

Role of HNO3:
During the nitration of benzene, HNO3 acts as a base. Let's understand why HNO3 acts as a base in this reaction:

1. Protonation of Nitric Acid:
In the nitrating mixture, HNO3 is protonated by the presence of strong sulfuric acid (H2SO4). The reaction can be represented as follows:
HNO3 + H2SO4 → [H3O]+ + [NO3]-

2. Formation of Nitronium Ion:
The protonated nitric acid, [H3O]+, then reacts with the sulfuric acid to form a nitronium ion (NO2+). The reaction can be represented as follows:
[H3O]+ + H2SO4 → H3SO4+ + H2O
H3SO4+ + HNO3 → H2SO4 + NO2+ + H2O

3. Nitration of Benzene:
The nitronium ion (NO2+) is the electrophile that attacks the benzene ring. The nitronium ion accepts a pair of electrons from the benzene ring, resulting in the substitution of a hydrogen atom with a nitro group (-NO2). The reaction can be represented as follows:
NO2+ + C6H6 → C6H5NO2 + H+

Why HNO3 acts as a base?
HNO3 acts as a base during the nitration of benzene because it donates a proton (H+) to form the nitronium ion. The protonation of HNO3 is necessary to generate the nitronium ion, which is the key electrophile in the reaction. Without the formation of the nitronium ion, the nitration of benzene cannot proceed.

In summary, HNO3 acts as a base during the nitration of benzene because it donates a proton to form the nitronium ion. This ion acts as an electrophile and substitutes a hydrogen atom on the benzene ring, resulting in the formation of aromatic nitro compounds.