Radical, Not Ionic: The Mechanochemical Zincke Nitration of Phenols

Nitrite-mediated nitration of phenols is synthetically valuable, yet the mechanism of the classical Zincke nitration has remained insufficiently defined. Here, we show that nitrous acid, generated in situ from NaNO₂ and NaHSO₄ under liquid-assisted vibrational mechanochemical conditions, enables both direct nitration of phenols and ipso nitration of halogenated phenols. The protocol applies to simple, substituted, electron-rich, polycyclic and biologically relevant phenols while reproducing the halogen-displacement reactivity historically associated with the Zincke nitration. Across 50 phenolic substrates, with isolated yields up to 98%, this mechanochemical platform extends Zincke-type halogen-displacement chemistry and makes phenoxyl-radical oxidative pathways experimentally accessible under a solvent-minimised regime. Atmosphere-controlled experiments, NO_x-transfer studies, anisole controls, EPR detection of phenoxyl-type radicals and hybrid DFT-D3 calculations support a pathway involving O-nitrosation, aryl nitrite fragmentation, aerobic NO-to-NO₂ oxidation and radical coupling. These results recast the Zincke nitration as an open-shell nitration manifold and show that mechanochemistry can render experimentally readable mechanistic features that are difficult to resolve under conventional solution conditions.