Kinetic study of an autocatalytic reaction: nitrosation of formamidine disulfide

Abstract

The reaction kinetics for the acid nitrosation of formamidine disulfide (FDS) show an autocatalytic behavior that arises from the fact that the thiocyanate ion formed as a product acts as a powerful catalyst for the nitrosation reaction. In the presence of added nucleophiles the suppression of the autocatalytic route results from competition for the nitrous acid between the added halides and the thiocyanate anion, which is formed as a reaction product. Analysis of the kinetic data enabled extraction of the bimolecular rate constants, k_NO⁺ = (3.2 ± 1.8) × 10¹⁰ M⁻¹ s⁻¹; k_NOSCN = (2.1 ± 0.2) × 10⁵ M⁻¹ s⁻¹; k_NOBr = (9.4 ± 0.2) × 10⁶ M⁻¹ s¹ and k_NOCl = (4.0 ± 0.2) × 10⁷ M⁻¹ s⁻¹, for the pathways catalyzed by SCN⁻, Br⁻ and Cl⁻, respectively. Kinetic results are consistent with the attack on the nitrosating agent as the rate limiting step, i.e., the nitrosation of FDS behaves in a similar manner to the nitrosation of an amine. Rather different behavior is found for other substrates with an imino moiety adjacent to an amino nitrogen, such as the guanidines, which react by a mechanism in which the rate limiting step is the reorganization of the nitrosated substrate.