Benzene on nitration gives nitrobenzene in presence of HNO3 and H2SO4 mixture, where:

Step 1: Mechanism of Nitration

In nitration, the active electrophile is $\ce{NO2^+}$ (nitronium ion): Step 2: Acid-Base Roles

In this reaction:

• $\ce{H2SO4}$ (stronger acid, pKa is -9): donates a proton to $\ce{HNO3}$ (pKa is -1.4) = acts as an acid
• $\ce{HNO3}$ (weaker acid in this context, pKa is -1.4): accepts a proton from $\ce{H2SO4}$ = acts as a base

So: $\ce{HNO3}$ = base (proton acceptor); $\ce{H2SO4}$ = acid (proton donor).

Key Points:

• $\ce{H2SO4}$ is a stronger acid than $\ce{HNO3}$, so $\ce{H2SO4}$ protonates $\ce{HNO3}$
• $\ce{HNO3}$ acts as a base relative to $\ce{H2SO4}$
• The protonated nitric acid loses water to give $\ce{NO2^+}$ (electrophile)
• This is a Bronsted acid-base interaction