Kinetics and mechanism of the formation and reactions of S-nitroso derivatives of some heterocyclic thiones

Abstract

Rate and equilibrium measurements have been obtained for the nitrosation (using nitrous acid in dilute acid aqueous solution) of the following thione–thiol nitrogen heterocyclic species and for the decomposition reactions of the formed S-NO⁺ ions: 4-mercaptopyridine (1), 2-mercaptopyridine 1-oxide (2), 2-mercaptopyrimidine (3) and 2-mercapto-1H-imidazole (4). The nitrosation reactions were all rapid and rate constants were obtained by stopped-flow spectrophotometry. In all cases kinetic analysis showed that the halide ion-catalysed and the uncatalysed reactions occurred at or close to the diffusion-controlled limit and the reactive form of the substrate is its thione form. In some cases at high [nitrous acid] reaction occurred via N₂O₃ where the rate-limiting step is that of N₂O₃ formation. Similarly reaction of 1 and 3 in the presence of thiocyanate showed that reaction takes place via rate-limiting formation of ONSCN. All of the nitrosation reactions were somewhat reversible, with large equilibrium constants of 2.0 × 10⁷, ∼4 × 10⁵, 1.7 × 10³ and 1.5 × 10⁵ dm⁶ mol⁻² respectively for 1, 2, 3, and 4. All of the S-NO⁺ ions decomposed rapidly in solution generating the disulfides and nitric oxide. This reaction was monitored only for the S-NO⁺ derived from 1 and 2 because of stability problems. The deprotonated form of the S-nitroso product from 1 was identified from measurements at pH 7.4 and the pK_a value of the protonated form estimated as 4.5. It decomposed fairly rapidly to give the thione. We report some results where thiones show considerable catalytic activity in nitrosation reactions of N-methylaniline and ascorbic acid.