Peroxynitrous acid: controversy and consensus surrounding an enigmatic oxidant

Abstract

The isomerisation of ONOOH to NO₃⁻ and H⁺, some oxidations and all hydroxylations and nitrations of aromatic compounds are first-order in ONOOH and zero-order in the compounds that are modified. These reactions are widely believed to proceed via homolysis of ONOOH into HO˙ and NO₂˙ to an extent of ca. 30%. We review the evidence pro and contra homolysis in studies that involve (1) thermochemical considerations, (2) isomerisation to NO₃⁻ and H⁺, (3) decomposition to NO₂⁻ and O₂, (4) HO˙ scavenger studies, (5) deuterium isotope effects, (6) ¹⁸O-scrambling studies, (7) electrochemistry, (8) CIDNP NMR, and (9) photolysis. Our conclusion is that homolysis may be involved to a minor extent of ca. 5%. The initiation of ONOOH isomerisation may be visualised as an out-of-plane vibration of the terminal HO-group relative to the nitrogen. At ONOO⁻ concentrations exceeding 0.1 mM and near neutral pH, disproportionation to NO₂⁻ and O₂ occurs; such disproportionations are typical for peroxy acids. For oxidation and nitration of organic substrates, we favour a mechanism involving initial formation of an adduct between the compound to be oxidised or nitrated and ONOOH.