Reductions by hyponitrite

Abstract

Hyponitrites, salts of the weak acid HO–NN–OH, hyponitrous acid, react, in aqueous solution (pH 4–8), with an array of inorganic oxidants. With the high potential reagents Br₂ and [Fe(bipy)₃]³⁺ in excess, conversion is mainly to nitrate, whereas with I₂ and [IrCl₆]²⁻, the product is nitrite. Kinetic acidity patterns indicate that nearly all oxidations proceed largely through the conjugate base, HN₂O₂⁻. With I₂, I₃⁻, and Br₂ there is also evidence also for a doubly deprotonated route, possibly involving a halogen–N₂O₂²⁻ complex. The 1e⁻ metal oxidants [Fe(bipy)₃]³⁺ and [IrCl₆]²⁻ probably operate through a series of outer-sphere 1e⁻ steps in which the initial transfer is rate-determining, whereas with I₂ and Br₂, reaction via a halogen bridge, resulting in net transfer of X⁺ (a 2e⁻ change) is more likely. None of the reaction profiles exhibits irregularities attributable to the intervention of an intermediate formed or destroyed on a time scale commensurate with the initial reaction.