S-Nitrosocaptopril formation in ionic self-aggregation systems: effects of counterions

Abstract

The reaction of the formation of S-nitrosocaptopril (NOcap) was studied in aqueous alkaline medium of surfactants resulting in the formation of micelles. The NOcap is formed in the nitroso group (NO) transfer from tert-butyl nitrite (tBN) to captopril (cap). Kinetics of first order in both tBN and cap concentration were observed and the reaction rate is independent of [OH− ]. These kinetic features indicate that the rate-limiting step is the nucleophilic attack of the thiolate ion of cap to the nitroso group of tBN. Anionic micelles of sodium dodecyl sulphate inhibit the reaction throughout all the concentration range, due to the reagents separation: tBN associates with micelles by hydrophobic effect, whereas the cap–thiolate anions are repelled from the micellar surface by electrostatic effect. Cationic micelles catalyse the reaction at surfactant concentrations just above the critical micelle concentration, but a further increase in the concentration of the surfactant inhibits the reaction. In this case, both reagents bind to the cationic micelles, but the reaction rate in the micellar pseudo phase is slower than that in the aqueous pseudo phase. In spite of the cap molecule existing in alkaline medium as a doubly charged anion, the results cannot be explained on the basis of the pseudophase ion exchange model, in part due to the high polarisability of the cap anion.