Animated Solution for Chemistry - Organic Chemistry: Benzene on nitration gives nitrobenzene in presence of ${\text{HNO}}_3$ and ${\text{H}}_2{\text{SO}}_4$ mixture, where

A Battle of Acids

When we talk about the nitration of benzene, we are looking at a classic example of electrophilic aromatic substitution. But before the benzene ring can even get involved, a crucial battle takes place in the reaction flask. We mix two powerful acids: concentrated sulfuric acid (${\text{H}}_2{\text{SO}}_4$) and concentrated nitric acid (${\text{HNO}}_3$). This combination is famously known as the nitrating mixture.

Now, you might wonder, if both are acids, how do they react with each other? This is where the concept of relative acid strength comes into play. Sulfuric acid is a significantly stronger acid than nitric acid. In the world of chemistry, the stronger acid dictates the terms.

According to the Bronsted-Lowry theory, an acid is a proton (${\text{H}}^{+}$) donor, and a base is a proton acceptor. Because ${\text{H}}_2{\text{SO}}_4$ is the stronger acid, it forces ${\text{HNO}}_3$ to step out of its usual character and act as a base.

Sulfuric acid donates a proton:

Nitric acid, acting as the base, accepts this proton on one of its oxygen atoms:

This protonation step is the key to unlocking the entire reaction. By accepting the proton, ${\text{HNO}}_3$ acts as a base, and by donating it, ${\text{H}}_2{\text{SO}}_4$ acts as an acid.

The protonated nitric acid molecule, ${\text{H}}_2{\text{NO}}_3^{+}$, is highly unstable. It contains a fantastic leaving group: a neutral water molecule (${\text{H}}_2\text{O}$). The molecule quickly decomposes, shedding water to form the true hero of our story:

This ${\text{NO}}_2^{+}$ species is the nitronium ion. It is a powerful, positively charged electrophile, perfectly primed to attack the electron-rich $\pi$-cloud of the benzene ring. Without sulfuric acid acting as the stronger acid to protonate nitric acid, we would never generate a sufficient concentration of this vital electrophile.

So, the next time you see a nitrating mixture, remember: it's not just a mix of two acids; it's a carefully orchestrated acid-base reaction designed to unleash the nitronium ion!