Underappreciated and Complex Role of Nitrous Acid in Aromatic Nitration under Mild Environmental Conditions: The Case of Activated Methoxyphenols

Many
ambiguities surround the possible mechanisms of colored and
toxic nitrophenols formation in natural systems. Nitration of a biologically
and environmentally relevant aromatic compound, guaiacol (2-methoxyphenol),
under mild aqueous-phase conditions (ambient temperatures, pH 4.5)
was investigated by a temperature-dependent experimental modeling
coupled to extensive ab initio calculations to obtain the activation
energies of the modeled reaction pathways. The importance of dark
nonradical reactions is emphasized, involving nitrous (HNO2) and peroxynitrous (HOONO) acids. Oxidation by HOONO is shown to
proceed via a nonradical pathway, possibly involving the nitronium
ion (NO2+) formation. Using quantum chemical
calculations at the MP2/6-31++g­(d,p) level, NO2• is shown capable of abstracting a hydrogen atom from the phenolic
group on the aromatic ring. In a protic solvent, the corresponding
aryl radical can combine with HNO2 to yield OH• and, after a subsequent oxidation step, nitrated aromatic products.
The demonstrated chemistry is especially important for understanding
the aging of nighttime atmospheric deliquesced aerosol. The relevance
should be further investigated in the atmospheric gaseous phase. The
results of this study have direct implications for accurate modeling
of the burden of toxic nitroaromatic pollutants, and the formation
of atmospheric brown carbon and its associated influence on Earth’s
albedo and climate forcing.