A Kinetic Study of S‐Nitrosothiol Decomposition

Under anaerobic conditions S-nitrosothiols 1 a–e undergo thermal decomposition by homolytic cleavage of the S−N bond; the reaction leads to nitric oxide and sulfanyl radicals formed in a reversible manner. The rate constants, k1, have been determined at different temperatures from kinetic measurements performed in refluxing alkane solvents. The tertiary nitrosothiols 1 c (k1(69 °C)=13×10−3 min−1) and 1 d (k1(69 °C)=91×10−3 min−1) decomposed faster than the primary nitrosothiols 1 a (k1(69 °C)=3.0×10−3 min−1) and 1 b (k1(69 °C)=6.5×10−3 min−1). The activation energies (E#=20.5–22.8 Kcal mol−1) have been calculated from the Arrhenius equation. Under aerobic conditions the decay of S-nitrosothiols 1 a–e takes place by an autocatalytic chain-decomposition process catalyzed by N2O3. The latter is formed by reaction of dioxygen with endogenous and/or exogenous nitric oxide. The autocatalytic decomposition is strongly inhibited by removing the endogenous nitric oxide or by the presence of antioxidants, such as p-cresol, β-styrene, and BHT. The rate of the chain reaction is independent of the RSNO concentration and decreases with increasing bulkiness of the alkyl group; this shows that steric effects are crucial in the propagation step.