The nitronium ion \(\left({ }^{+} \mathrm{NO}_{2}\right)\) is known and can react with simple benzenes to give nitrobenzenes. Write an arrow formalism mechanism for this reaction.

The nitronium ion, \(\text{NO}_2^+\), is a powerful electrophile used in many aromatic substitution reactions. It is composed of a nitrogen atom double-bonded to two oxygen atoms, carrying a formal positive charge on the nitrogen. This ion is typically generated in situ by mixing nitric acid (HNO₃) with sulfuric acid (H₂SO₄), the latter acting as a dehydrating agent. The equation for this generation is:
\[ \text{HNO}_3 + 2 \text{H}_2\text{SO}_4 \to \text{NO}_2^+ + \text{HSO}_4^- + \text{H}_3\text{O}^+ \] This reaction provides the nitronium ions necessary for reactions like nitration of benzene. In this process, the nitronium ion is highly electrophilic due to the positive charge on nitrogen, making it an effective agent to attack electron-rich centers like the benzene ring.

Benzene (\(\text{C}_6\text{H}_6\)) is a hydrocarbon with a unique structure characterized by a ring of six carbon atoms, all connected by alternating single and double bonds. This arrangement is often represented by a hexagon with a circle inside, indicating resonance stabilization. Benzene's \(\text{sp}^2\) hybridized carbons create a planar ring with overlapping \(\text{p}\) orbitals above and below the plane, forming a delocalized \(\text{π}\) electron cloud.
This delocalization grants benzene exceptional stability, known as aromaticity, making it less reactive towards addition reactions. Instead, it readily participates in substitution reactions where its aromaticity is momentarily disrupted but ultimately restored after the reaction, maintaining the ring's stability.

When the electrophilic nitronium ion attacks the benzene ring, it forms a sigma complex, also known as an arenium ion. This intermediate is characterized by a carbocation (positively charged carbon) within the benzene ring, which temporarily loses its aromaticity during the reaction.
The formation of the sigma complex involves the interaction of the \(\text{π}\) electrons of benzene with the nitronium ion, resulting in a \(\text{σ}\) bond between the benzene ring and the nitronium ion. The resulting structure disrupts the benzene's \(\text{π}\) electron system, causing a positive charge to be localized on the ring.

• This step is crucial as it sets the stage for the re-establishment of aromaticity through a proton loss.
• The positive charge is delocalized over the ring carbons, stabilizing the intermediate.

The sigma complex formed during the nitration of benzene features a carbocation intermediate. This carbocation is a highly reactive species characterized by a carbon atom bearing a positive charge due to the loss of \(\text{π}\) electron density.
Carbocation intermediates in aromatic substitution reactions are stabilized by resonance. In the case of the sigma complex, the positive charge is delocalized over three carbon atoms in the benzene ring through resonance structures:
\[ \begin{array}{c} \text{C}_6\text{H}_5\text{NO}_2^+ \to \text{C}_6\text{H}_5\text{NO}_2 + \text{H}^+ \end{array} \] The stability provided by resonance ensures the carbocation does not break apart before the reaction completes. This intermediate is a key step in the mechanism, leading to the restoration of aromaticity after the proton loss (deprotonation).

Aromaticity is a significant property of benzene, contributing to its stability and reactivity. A molecule is considered aromatic if it is cyclic, planar, fully conjugated (alternating single and double bonds), and follows Hückel's rule, having \((4n + 2)\) \(\text{π}\) electrons (where \(n\) is a non-negative integer).
The benzene ring in this reaction maintains its aromaticity throughout the process. Initially, benzene is aromatic, but the formation of the sigma complex interrupts this stability. However, the final product, nitrobenzene, regains aromaticity:

• The electron pair from the former \(\text{C-H}\) bond migrates back to the ring, re-establishing the delocalized \(\text{π}\) system.
• This step is critical to restore the stability of the ring after the substitution of hydrogen for the nitro group.

The ability to regain aromaticity is what makes electrophilic aromatic substitution reactions favorable for compounds like benzene.