# Nitration Mechanism of Aromatics: Lessons from Born–Oppenheimer Molecular Dynamics

## TL;DR

This study uses molecular dynamics to explore how toluene is nitrated, revealing new insights into the reaction mechanisms and pathways.

## Contribution

The study provides new mechanistic insights into aromatic nitration via molecular dynamics simulations and identifies novel oxygen transfer pathways.

## Key findings

- Nitronium ion (NO2+) forms spontaneously via double protonation of HNO3 by H2SO4.
- Four distinct reaction outcomes were observed, including nitration at ortho and para positions and oxygen transfer pathways.
- Single-electron transfer (SET) is confirmed as the key step in all reaction pathways.

## Abstract

The nitration of aromatic compounds is a fundamental
transformation
in organic chemistry, traditionally understood through the Ingold–Hughes
polar mechanism and, more recently, via single-electron transfer (SET)
pathways. In this work, Born–Oppenheimer molecular dynamics
(BOMD) simulations were employed to explore the mechanistic features
of toluene nitration in a protic polar medium, specifically a concentrated
sulfonitric mixture (HNO3/H2SO4).
Simulations at 423 K revealed the spontaneous formation of the nitronium
ion (NO2
+) via double protonation of HNO3 by H2SO4. Several BOMD trajectories
were analyzed for the reaction between toluene and NO2
+ at 300 K, leading to four different reaction outcomes: (i)
no reaction, highlighting nucleophilic rather than protic solvation
of NO2
+; (ii) nitration at the positions ortho and para via a V-shaped [NO2·ArH]+ SET complex evolving into a σ-complex
and ultimately the o- or p-nitrotoluene
after deprotonation; (iii) oxygen transfer resulting in o-cresol and NO, initiated from a Λ-shaped [NO2·ArH]+ SET complex; and (iv) the formation of a cyclohexadienone–NO
complex via 1,2-hydride shift, also proceeding through a Λ-shaped
[NO2·ArH]+ intermediate. Electronic structure
analyses (HOMO/LUMO, spin density, Bader charges) confirmed SET as
the key step in all reacting pathways. No evidence of superelectrophilic
solvation was observed under BOMD conditions. These results reinforce
the role of SET in electrophilic aromatic nitration under strongly
acidic conditions and reveal new oxygen transfer pathways dependent
on the spatial orientation of the NO2
+ relative
to the aromatic ring.