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 S4O62–.