How Does Nitration of Benzene Work? (HL)

Benzene undergoes nitration by electrophilic substitution: a hydrogen atom on the ring is replaced by a nitro group, $\text{NO}_2$. Concentrated nitric acid provides the nitro group, while concentrated sulfuric acid generates the electrophile and acts as a catalyst.

This is Additional Higher Level (AHL) content in Reactivity 3.4, electron-pair sharing reactions.

Overall Reaction

The overall equation is:

$\text{C}_6\text{H}_6(l) + \text{HNO}_3(l) \rightarrow \text{C}_6\text{H}_5\text{NO}_2(l) + \text{H}_2\text{O}(l)$

The mixture is usually warmed to approximately $50$ to $55\,^{\circ}\text{C}$. Higher temperatures increase the likelihood of further substitution.

The Mechanism

1. Formation of the electrophile: Concentrated sulfuric acid protonates nitric acid. The protonated nitric acid then loses water, producing the nitronium ion, $\text{NO}_2^+$. At this stage, sulfuric acid behaves as a Brønsted and Lowry acid and nitric acid as a base. Protonation converts an OH group into water, a much better leaving group, allowing the electron-deficient nitronium ion to form. The ring temporarily loses aromatic stabilization, making the sigma complex relatively high in energy. Regeneration of sulfuric acid confirms its catalytic role.

2. Electrophilic attack: Benzene’s delocalized $\pi$ electrons attract $\text{NO}_2^+$. A curly arrow must begin at a bond and point to the nitrogen atom of . A carbon bonded to nitrogen bond forms, temporarily disrupting the delocalized electron system and producing a positively charged . Its positive charge is delocalized around the ring.

A common misconception is that nitration is an addition reaction. It is substitution because loss of $\text{H}^+$ restores the stable delocalized electron system of benzene.

Exam Technique

For an explain or mechanism question, identify $\text{NO}_2^+$ as the electrophile, show both curly arrows starting from electron pairs, include the sigma complex, and state that aromaticity is restored. Do not draw a curly arrow starting from the electrophile.