Nitrosothiosulphate ion (S2O3NO–) as a nitrosating species

Abstract

Analysis of rate data obtained for the nitrosation of N-methylaniline in the presence of thiosulphate ion indicates that the nitrosothiosulphate ion is formed and that it acts as a nitrosating species. The bimolecular rate constant for reaction in water at 25 °C is evaluated as 1.2 × 10⁴ l mol^–1 s^–1 from the variation of the observed rate constant with [N-methylaniline] and also as 1.2 × 10⁴ l mol^–1 s^–1 from the variation of the observed rate constant with [thiosulphate ion]. Results are also reported, for comparison purposes, for the corresponding reactions of nitrosyl bromide and nitrosyl thiocyanate. Nitrosyl bromide (in its reaction with N-methylaniline) is more reactive than the nitrosothiosulphate ion by a factor of ca. 4 × 10⁵, and nitrosyl thiocyanate is more reactive than the nitrosothiosulphate ion by a factor of ca. 1.5 × 10⁴. There is also kinetic evidence of nitrosation by the nitrosothiosulphate ion of methanol, hydrazine, and a thiol. The decomposition of S₂O₃NO^– follows the rate law –d[S₂O₃NO^–]/dt=k₁[S₂O₃NO^–]+k₂[S₂O₃NO^–]². This is interpreted in terms of two concurrent reaction pathways, the first involving the formation of the radical anion S₂O₃^–˙ in a rate-limiting homolysis (followed by rapid dimerisation) and the second a bimolecular reaction of two S₂O₃NO^– ions with concurrent S–N bond fission and S–S bond formation yielding the tetrathionate ion S₄O₆^2–.